From 505c19580e0f43fe5224431459cacb7c21edd93d Mon Sep 17 00:00:00 2001 From: Ondřej Surý Date: Fri, 6 Apr 2012 15:14:11 +0200 Subject: Imported Upstream version 1 --- doc/articles/c_go_cgo.html | 180 +++++++ doc/articles/concurrency_patterns.html | 79 +++ doc/articles/defer_panic_recover.html | 197 +++++++ doc/articles/error_handling.html | 316 +++++++++++ doc/articles/go_command.html | 265 ++++++++++ doc/articles/gobs_of_data.html | 315 +++++++++++ doc/articles/godoc_documenting_go_code.html | 139 +++++ doc/articles/gos_declaration_syntax.html | 348 +++++++++++++ doc/articles/image-20.png | Bin 0 -> 95383 bytes doc/articles/image-2a.png | Bin 0 -> 3625 bytes doc/articles/image-2b.png | Bin 0 -> 95423 bytes doc/articles/image-2c.png | Bin 0 -> 60552 bytes doc/articles/image-2d.png | Bin 0 -> 68314 bytes doc/articles/image-2e.png | Bin 0 -> 96721 bytes doc/articles/image-2f.png | Bin 0 -> 62662 bytes doc/articles/image-package-01.png | Bin 0 -> 1393 bytes 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+Cgo lets Go packages call C code. Given a Go source file written with some +special features, cgo outputs Go and C files that can be combined into a +single Go package. +

+ +

+To lead with an example, here's a Go package that provides two functions - +Random and Seed - that wrap C's random +and srandom functions. +

+ +{{code "/doc/progs/cgo1.go" `/package rand/` `/END/`}} + +

+Let's look at what's happening here, starting with the import statement. +

+ +

+The rand package imports "C", but you'll find there's +no such package in the standard Go library. That's because C is a +"pseudo-package", a special name interpreted by cgo as a reference to C's +name space. +

+ +

+The rand package contains four references to the C +package: the calls to C.random and C.srandom, the +conversion C.uint(i), and the import statement. +

+ +

+The Random function calls the standard C library's random +function and returns the result. In C, random returns a value of the +C type long, which cgo represents as the type C.long. +It must be converted to a Go type before it can be used by Go code outside this +package, using an ordinary Go type conversion: +

+ +{{code "/doc/progs/cgo1.go" `/func Random/` `/STOP/`}} + +

+Here's an equivalent function that uses a temporary variable to illustrate +the type conversion more explicitly: +

+ +{{code "/doc/progs/cgo2.go" `/func Random/` `/STOP/`}} + +

+The Seed function does the reverse, in a way. It takes a +regular Go int, converts it to the C unsigned int +type, and passes it to the C function srandom. +

+ +{{code "/doc/progs/cgo1.go" `/func Seed/` `/END/`}} + +

+Note that cgo knows the unsigned int type as C.uint; +see the cgo documentation for a complete list of +these numeric type names. +

+ +

+The one detail of this example we haven't examined yet is the comment +above the import statement. +

+ +{{code "/doc/progs/cgo1.go" `/\/\*/` `/STOP/`}} + +

+Cgo recognizes this comment. Any lines starting +with #cgo +followed +by a space character are removed; these become directives for cgo. +The remaining lines are used as a header when compiling the C parts of +the package. In this case those lines are just a +single #include +statement, but they can be almost any C code. The #cgo +directives are +used to provide flags for the compiler and linker when building the C +parts of the package. +

+ +

+There is a limitation: if your program uses any //export +directives, then the C code in the comment may only include declarations +(extern int f();), not definitions (int f() { +return 1; }). You can use //export directives to +make Go functions accessible to C code. +

+ +

+The #cgo and //export directives are +documented in +the cgo documentation. +

+ +

+Strings and things +

+ +

+Unlike Go, C doesn't have an explicit string type. Strings in C are +represented by a zero-terminated array of chars. +

+ +

+Conversion between Go and C strings is done with the +C.CString, C.GoString, and +C.GoStringN functions. These conversions make a copy of the +string data. +

+ +

+This next example implements a Print function that writes a +string to standard output using C's fputs function from the +stdio library: +

+ +{{code "/doc/progs/cgo3.go" `/package print/` `/END/`}} + +

+Memory allocations made by C code are not known to Go's memory manager. +When you create a C string with C.CString (or any C memory +allocation) you must remember to free the memory when you're done with it +by calling C.free. +

+ +

+The call to C.CString returns a pointer to the start of the +char array, so before the function exits we convert it to an +unsafe.Pointer and release +the memory allocation with C.free. A common idiom in cgo programs +is to defer +the free immediately after allocating (especially when the code that follows +is more complex than a single function call), as in this rewrite of +Print: +

+ +{{code "/doc/progs/cgo4.go" `/func Print/` `/END/`}} + +

+Building cgo packages +

+ +

+To build cgo packages, just use " +go build" or +"go install +" as usual. The go tool recognizes the special "C" import and automatically +uses cgo for those files. +

+ +

+More cgo resources +

+ +

+The cgo command documentation has more detail about +the C pseudo-package and the build process. The cgo examples +in the Go tree demonstrate more advanced concepts. +

+ +

+For a simple, idiomatic example of a cgo-based package, see Russ Cox's gosqlite. +Also, the Go Project Dashboard lists several other +cgo packages. +

+ +

+Finally, if you're curious as to how all this works internally, take a look +at the introductory comment of the runtime package's cgocall.c. +

diff --git a/doc/articles/concurrency_patterns.html b/doc/articles/concurrency_patterns.html new file mode 100644 index 000000000..63c8cd59e --- /dev/null +++ b/doc/articles/concurrency_patterns.html @@ -0,0 +1,79 @@ + + +

+Concurrent programming has its own idioms. A good example is timeouts. Although +Go's channels do not support them directly, they are easy to implement. Say we +want to receive from the channel ch, but want to wait at most one +second for the value to arrive. We would start by creating a signalling channel +and launching a goroutine that sleeps before sending on the channel: +

+ +{{code "/doc/progs/timeout1.go" `/timeout :=/` `/STOP/`}} + +

+We can then use a select statement to receive from either +ch or timeout. If nothing arrives on ch +after one second, the timeout case is selected and the attempt to read from +ch is abandoned. +

+ +{{code "/doc/progs/timeout1.go" `/select {/` `/STOP/`}} + +

+The timeout channel is buffered with space for 1 value, allowing +the timeout goroutine to send to the channel and then exit. The goroutine +doesn't know (or care) whether the value is received. This means the goroutine +won't hang around forever if the ch receive happens before the +timeout is reached. The timeout channel will eventually be +deallocated by the garbage collector. +

+ +

+(In this example we used time.Sleep to demonstrate the mechanics +of goroutines and channels. In real programs you should use +time.After, a function that returns +a channel and sends on that channel after the specified duration.) +

+ +

+Let's look at another variation of this pattern. In this example we have a +program that reads from multiple replicated databases simultaneously. The +program needs only one of the answers, and it should accept the answer that +arrives first. +

+ +

+The function Query takes a slice of database connections and a +query string. It queries each of the databases in parallel and +returns the first response it receives: +

+ +{{code "/doc/progs/timeout2.go" `/func Query/` `/STOP/`}} + +

+In this example, the closure does a non-blocking send, which it achieves by +using the send operation in select statement with a +default case. If the send cannot go through immediately the +default case will be selected. Making the send non-blocking guarantees that +none of the goroutines launched in the loop will hang around. However, if the +result arrives before the main function has made it to the receive, the send +could fail since no one is ready. +

+ +

+This problem is a textbook of example of what is known as a +race condition, but +the fix is trivial. We just make sure to buffer the channel ch (by +adding the buffer length as the second argument to make), +guaranteeing that the first send has a place to put the value. This ensures the +send will always succeed, and the first value to arrive will be retrieved +regardless of the order of execution. +

+ +

+These two examples demonstrate the simplicity with which Go can express complex +interactions between goroutines. +

diff --git a/doc/articles/defer_panic_recover.html b/doc/articles/defer_panic_recover.html new file mode 100644 index 000000000..c964cd368 --- /dev/null +++ b/doc/articles/defer_panic_recover.html @@ -0,0 +1,197 @@ + + +

+Go has the usual mechanisms for control flow: if, for, switch, goto. It also +has the go statement to run code in a separate goroutine. Here I'd like to +discuss some of the less common ones: defer, panic, and recover. +

+ +

+A defer statement pushes a function call onto a list. The list of saved +calls is executed after the surrounding function returns. Defer is commonly +used to simplify functions that perform various clean-up actions. +

+ +

+For example, let's look at a function that opens two files and copies the +contents of one file to the other: +

+ +{{code "/doc/progs/defer.go" `/func CopyFile/` `/STOP/`}} + +

+This works, but there is a bug. If the call to os.Create fails, the +function will return without closing the source file. This can be easily +remedied by putting a call to src.Close before the second return statement, +but if the function were more complex the problem might not be so easily +noticed and resolved. By introducing defer statements we can ensure that the +files are always closed: +

+ +{{code "/doc/progs/defer2.go" `/func CopyFile/` `/STOP/`}} + +

+Defer statements allow us to think about closing each file right after opening +it, guaranteeing that, regardless of the number of return statements in the +function, the files will be closed. +

+ +

+The behavior of defer statements is straightforward and predictable. There are +three simple rules: +

+ +

+1. A deferred function's arguments are evaluated when the defer statement is +evaluated. +

+ +

+In this example, the expression "i" is evaluated when the Println call is +deferred. The deferred call will print "0" after the function returns. +

+ +{{code "/doc/progs/defer.go" `/func a/` `/STOP/`}} + +

+2. Deferred function calls are executed in Last In First Out order +after the surrounding function returns. +

+ +

+This function prints "3210": +

+ +{{code "/doc/progs/defer.go" `/func b/` `/STOP/`}} + +

+3. Deferred functions may read and assign to the returning function's named +return values. +

+ +

+In this example, a deferred function increments the return value i after +the surrounding function returns. Thus, this function returns 2: +

+ +{{code "/doc/progs/defer.go" `/func c/` `/STOP/`}} + +

+This is convenient for modifying the error return value of a function; we will +see an example of this shortly. +

+ +

+Panic is a built-in function that stops the ordinary flow of control and +begins panicking. When the function F calls panic, execution of F stops, +any deferred functions in F are executed normally, and then F returns to its +caller. To the caller, F then behaves like a call to panic. The process +continues up the stack until all functions in the current goroutine have +returned, at which point the program crashes. Panics can be initiated by +invoking panic directly. They can also be caused by runtime errors, such as +out-of-bounds array accesses. +

+ +

+Recover is a built-in function that regains control of a panicking +goroutine. Recover is only useful inside deferred functions. During normal +execution, a call to recover will return nil and have no other effect. If the +current goroutine is panicking, a call to recover will capture the value given +to panic and resume normal execution. +

+ +

+Here's an example program that demonstrates the mechanics of panic and defer: +

+ +{{code "/doc/progs/defer2.go" `/package main/` `/STOP/`}} + +

+The function g takes the int i, and panics if i is greater than 3, or else it +calls itself with the argument i+1. The function f defers a function that calls +recover and prints the recovered value (if it is non-nil). Try to picture what +the output of this program might be before reading on. +

+ +

+The program will output: +

+ +
Calling g.
+Printing in g 0
+Printing in g 1
+Printing in g 2
+Printing in g 3
+Panicking!
+Defer in g 3
+Defer in g 2
+Defer in g 1
+Defer in g 0
+Recovered in f 4
+Returned normally from f.
+ +

+If we remove the deferred function from f the panic is not recovered and +reaches the top of the goroutine's call stack, terminating the program. This +modified program will output: +

+ +
Calling g.
+Printing in g 0
+Printing in g 1
+Printing in g 2
+Printing in g 3
+Panicking!
+Defer in g 3
+Defer in g 2
+Defer in g 1
+Defer in g 0
+panic: 4
+ 
+panic PC=0x2a9cd8
+[stack trace omitted]
+ +

+For a real-world example of panic and recover, see the +json package from the Go standard library. +It decodes JSON-encoded data with a set of recursive functions. +When malformed JSON is encountered, the parser calls panic to unwind the +stack to the top-level function call, which recovers from the panic and returns +an appropriate error value (see the 'error' and 'unmarshal' methods of +the decodeState type in +decode.go). +

+ +

+The convention in the Go libraries is that even when a package uses panic +internally, its external API still presents explicit error return values. +

+ +

+Other uses of defer (beyond the file.Close example given earlier) +include releasing a mutex: +

+ +
mu.Lock()
+defer mu.Unlock()
+ +

+printing a footer: +

+ +
printHeader()
+defer printFooter()
+ +

+and more. +

+ +

+In summary, the defer statement (with or without panic and recover) provides an +unusual and powerful mechanism for control flow. It can be used to model a +number of features implemented by special-purpose structures in other +programming languages. Try it out. +

diff --git a/doc/articles/error_handling.html b/doc/articles/error_handling.html new file mode 100644 index 000000000..8f4fffb48 --- /dev/null +++ b/doc/articles/error_handling.html @@ -0,0 +1,316 @@ + + +

+If you have written any Go code you have probably encountered the built-in +error type. Go code uses error values to +indicate an abnormal state. For example, the os.Open function +returns a non-nil error value when it fails to open a file. +

+ +{{code "/doc/progs/error.go" `/func Open/`}} + +

+The following code uses os.Open to open a file. If an error +occurs it calls log.Fatal to print the error message and stop. +

+ +{{code "/doc/progs/error.go" `/func openFile/` `/STOP/`}} + +

+You can get a lot done in Go knowing just this about the error +type, but in this article we'll take a closer look at error and +discuss some good practices for error handling in Go. +

+ +

+The error type +

+ +

+The error type is an interface type. An error +variable represents any value that can describe itself as a string. Here is the +interface's declaration: +

+ +
type error interface {
+    Error() string
+}
+ +

+The error type, as with all built in types, is +predeclared in the +universe block. +

+ +

+The most commonly-used error implementation is the +errors package's unexported errorString type. +

+ +{{code "/doc/progs/error.go" `/errorString/` `/STOP/`}} + +

+You can construct one of these values with the errors.New +function. It takes a string that it converts to an errors.errorString +and returns as an error value. +

+ +{{code "/doc/progs/error.go" `/New/` `/STOP/`}} + +

+Here's how you might use errors.New: +

+ +{{code "/doc/progs/error.go" `/func Sqrt/` `/STOP/`}} + +

+A caller passing a negative argument to Sqrt receives a non-nil +error value (whose concrete representation is an +errors.errorString value). The caller can access the error string +("math: square root of...") by calling the error's +Error method, or by just printing it: +

+ +{{code "/doc/progs/error.go" `/func printErr/` `/STOP/`}} + +

+The fmt package formats an error value +by calling its Error() string method. +

+ +

+It is the error implementation's responsibility to summarize the context. +The error returned by os.Open formats as "open /etc/passwd: +permission denied," not just "permission denied." The error returned by our +Sqrt is missing information about the invalid argument. +

+ +

+To add that information, a useful function is the fmt package's +Errorf. It formats a string according to Printf's +rules and returns it as an error created by +errors.New. +

+ +{{code "/doc/progs/error.go" `/fmtError/` `/STOP/`}} + +

+In many cases fmt.Errorf is good enough, but since +error is an interface, you can use arbitrary data structures as +error values, to allow callers to inspect the details of the error. +

+ +

+For instance, our hypothetical callers might want to recover the invalid +argument passed to Sqrt. We can enable that by defining a new +error implementation instead of using errors.errorString: +

+ +{{code "/doc/progs/error.go" `/type NegativeSqrtError/` `/STOP/`}} + +

+A sophisticated caller can then use a +type assertion to check for a +NegativeSqrtError and handle it specially, while callers that just +pass the error to fmt.Println or log.Fatal will see +no change in behavior. +

+ +

+As another example, the json package specifies a +SyntaxError type that the json.Decode function +returns when it encounters a syntax error parsing a JSON blob. +

+ +{{code "/doc/progs/error.go" `/type SyntaxError/` `/STOP/`}} + +

+The Offset field isn't even shown in the default formatting of the +error, but callers can use it to add file and line information to their error +messages: +

+ +{{code "/doc/progs/error.go" `/func decodeError/` `/STOP/`}} + +

+(This is a slightly simplified version of some +actual code +from the Camlistore project.) +

+ +

+The error interface requires only a Error method; +specific error implementations might have additional methods. For instance, the +net package returns errors of type +error, following the usual convention, but some of the error +implementations have additional methods defined by the net.Error +interface: +

+ +
package net
+
+type Error interface {
+    error
+    Timeout() bool   // Is the error a timeout?
+    Temporary() bool // Is the error temporary?
+}
+ +

+Client code can test for a net.Error with a type assertion and +then distinguish transient network errors from permanent ones. For instance, a +web crawler might sleep and retry when it encounters a temporary error and give +up otherwise. +

+ +{{code "/doc/progs/error.go" `/func netError/` `/STOP/`}} + +

+Simplifying repetitive error handling +

+ +

+In Go, error handling is important. The language's design and conventions +encourage you to explicitly check for errors where they occur (as distinct from +the convention in other languages of throwing exceptions and sometimes catching +them). In some cases this makes Go code verbose, but fortunately there are some +techniques you can use to minimize repetitive error handling. +

+ +

+Consider an App Engine +application with an HTTP handler that retrieves a record from the datastore and +formats it with a template. +

+ +{{code "/doc/progs/error2.go" `/func init/` `/STOP/`}} + +

+This function handles errors returned by the datastore.Get +function and viewTemplate's Execute method. In both +cases, it presents a simple error message to the user with the HTTP status code +500 ("Internal Server Error"). This looks like a manageable amount of code, but +add some more HTTP handlers and you quickly end up with many copies of +identical error handling code. +

+ +

+To reduce the repetition we can define our own HTTP appHandler +type that includes an error return value: +

+ +{{code "/doc/progs/error3.go" `/type appHandler/`}} + +

+Then we can change our viewRecord function to return errors: +

+ +{{code "/doc/progs/error3.go" `/func viewRecord/` `/STOP/`}} + +

+This is simpler than the original version, but the http package doesn't understand functions that return +error. +To fix this we can implement the http.Handler interface's +ServeHTTP method on appHandler: +

+ +{{code "/doc/progs/error3.go" `/ServeHTTP/` `/STOP/`}} + +

+The ServeHTTP method calls the appHandler function +and displays the returned error (if any) to the user. Notice that the method's +receiver, fn, is a function. (Go can do that!) The method invokes +the function by calling the receiver in the expression fn(w, r). +

+ +

+Now when registering viewRecord with the http package we use the +Handle function (instead of HandleFunc) as +appHandler is an http.Handler (not an +http.HandlerFunc). +

+ +{{code "/doc/progs/error3.go" `/func init/` `/STOP/`}} + +

+With this basic error handling infrastructure in place, we can make it more +user friendly. Rather than just displaying the error string, it would be better +to give the user a simple error message with an appropriate HTTP status code, +while logging the full error to the App Engine developer console for debugging +purposes. +

+ +

+To do this we create an appError struct containing an +error and some other fields: +

+ +{{code "/doc/progs/error4.go" `/type appError/` `/STOP/`}} + +

+Next we modify the appHandler type to return *appError values: +

+ +{{code "/doc/progs/error4.go" `/type appHandler/`}} + +

+(It's usually a mistake to pass back the concrete type of an error rather than +error, +for reasons discussed in the Go FAQ, +but it's the right thing to do here because ServeHTTP is the only +place that sees the value and uses its contents.) +

+ +

+And make appHandler's ServeHTTP method display the +appError's Message to the user with the correct HTTP +status Code and log the full Error to the developer +console: +

+ +{{code "/doc/progs/error4.go" `/ServeHTTP/` `/STOP/`}} + +

+Finally, we update viewRecord to the new function signature and +have it return more context when it encounters an error: +

+ +{{code "/doc/progs/error4.go" `/func viewRecord/` `/STOP/`}} + +

+This version of viewRecord is the same length as the original, but +now each of those lines has specific meaning and we are providing a friendlier +user experience. +

+ +

+It doesn't end there; we can further improve the error handling in our +application. Some ideas: +

+ + + +

+Conclusion +

+ +

+Proper error handling is an essential requirement of good software. By +employing the techniques described in this post you should be able to write +more reliable and succinct Go code. +

diff --git a/doc/articles/go_command.html b/doc/articles/go_command.html new file mode 100644 index 000000000..1e9e70fd8 --- /dev/null +++ b/doc/articles/go_command.html @@ -0,0 +1,265 @@ + + +

The Go distribution includes a command, named +"go", that +automates the downloading, building, installation, and testing of Go packages +and commands. This document talks about why we wrote a new command, what it +is, what it's not, and how to use it.

+ +

Motivation

+ +

You might have seen early Go talks in which Rob Pike jokes that the idea +for Go arose while waiting for a large Google server to compile. That +really was the motivation for Go: to build a language that worked well +for building the large software that Google writes and runs. It was +clear from the start that such a language must provide a way to +express dependencies between code libraries clearly, hence the package +grouping and the explicit import blocks. It was also clear from the +start that you might want arbitrary syntax for describing the code +being imported; this is why import paths are string literals.

+ +

An explicit goal for Go from the beginning was to be able to build Go +code using only the information found in the source itself, not +needing to write a makefile or one of the many modern replacements for +makefiles. If Go needed a configuration file to explain how to build +your program, then Go would have failed.

+ +

At first, there was no Go compiler, and the initial development +focused on building one and then building libraries for it. For +expedience, we postponed the automation of building Go code by using +make and writing makefiles. When compiling a single package involved +multiple invocations of the Go compiler, we even used a program to +write the makefiles for us. You can find it if you dig through the +repository history.

+ +

The purpose of the new go command is our return to this ideal, that Go +programs should compile without configuration or additional effort on +the part of the developer beyond writing the necessary import +statements.

+ +

Configuration versus convention

+ +

The way to achieve the simplicity of a configuration-free system is to +establish conventions. The system works only to the extent that those conventions +are followed. When we first launched Go, many people published packages that +had to be installed in certain places, under certain names, using certain build +tools, in order to be used. That's understandable: that's the way it works in +most other languages. Over the last few years we consistently reminded people +about the goinstall command +(now replaced by go get) +and its conventions: first, that the import path is derived in a known way from +the URL of the source code; second, that that the place to store the sources in +the local file system is derived in a known way from the import path; third, +that each directory in a source tree corresponds to a single package; and +fourth, that the package is built using only information in the source code. +Today, the vast majority of packages follow these conventions. +The Go ecosystem is simpler and more powerful as a result.

+ +

We received many requests to allow a makefile in a package directory to +provide just a little extra configuration beyond what's in the source code. +But that would have introduced new rules. Because we did not accede to such +requests, we were able to write the go command and eliminate our use of make +or any other build system.

+ +

It is important to understand that the go command is not a general +build tool. It cannot be configured and it does not attempt to build +anything but Go packages. These are important simplifying +assumptions: they simplify not only the implementation but also, more +important, the use of the tool itself.

+ +

Go's conventions

+ +

The go command requires that code adheres to a few key, +well-established conventions.

+ +

First, the import path is derived in an known way from the URL of the +source code. For Bitbucket, GitHub, Google Code, and Launchpad, the +root directory of the repository is identified by the repository's +main URL, without the http:// prefix. Subdirectories are named by +adding to that path. For example, the supplemental networking +libraries for Go are obtained by running

+ +
+hg clone http://code.google.com/p/go.net
+
+ +

and thus the import path for the root directory of that repository is +"code.google.com/p/go.net". The websocket package is stored in a +subdirectory, so its import path is +"code.google.com/p/go.net/websocket".

+ +

These paths are on the long side, but in exchange we get an +automatically managed name space for import paths and the ability for +a tool like the go command to look at an unfamiliar import path and +deduce where to obtain the source code.

+ +

Second, the place to store sources in the local file system is derived +in a known way from the import path. Specifically, the first choice +is $GOPATH/src/<import-path>. If $GOPATH is +unset, the go command will fall back to storing source code alongside the +standard Go packages, in $GOROOT/src/pkg/<import-path>. +If $GOPATH is set to a list of paths, the go command tries +<dir>/src/<import-path> for each of the directories in +that list.

+ +

Each of those trees contains, by convention, a top-level directory named +"bin", for holding compiled executables, and a top-level directory +named "pkg", for holding compiled packages that can be imported, +and the "src" directory, for holding package source files. +Imposing this structure lets us keep each of these directory trees +self-contained: the compiled form and the sources are always near each +other.

+ +

These naming conventions also let us work in the reverse direction, +from a directory name to its import path. This mapping is important +for many of the go command's subcommands, as we'll see below.

+ +

Third, each directory in a source tree corresponds to a single +package. By restricting a directory to a single package, we don't have +to create hybrid import paths that specify first the directory and +then the package within that directory. Also, most file management +tools and UIs work on directories as fundamental units. Tying the +fundamental Go unit—the package—to file system structure means +that file system tools become Go package tools. Copying, moving, or +deleting a package corresponds to copying, moving, or deleting a +directory.

+ +

Fourth, each package is built using only the information present in +the source files. This makes it much more likely that the tool will +be able to adapt to changing build environments and conditions. For +example, if we allowed extra configuration such as compiler flags or +command line recipes, then that configuration would need to be updated +each time the build tools changed; it would also be inherently tied +to the use of a specific tool chain.

+ +

Getting started with the go command

+ +

Finally, a quick tour of how to use the go command, to supplement +the information in How to Write Go Code, +which you might want to read first. Assuming you want +to keep your source code separate from the Go distribution source +tree, the first step is to set $GOPATH, the one piece of global +configuration that the go command needs. The $GOPATH can be a +list of directories, but by far the most common usage should be to set it to a +single directory. In particular, you do not need a separate entry in +$GOPATH for each of your projects. One $GOPATH can +support many projects.

+ +

Here’s an example. Let’s say we decide to keep our Go code in the directory +$HOME/mygo. We need to create that directory and set +$GOPATH accordingly.

+ +
+$ mkdir $HOME/mygo
+$ export GOPATH=$HOME/mygo
+$
+
+ +

Into this directory, we now add some source code. Suppose we want to use +the indexing library from the codesearch project along with a left-leaning +red-black tree. We can install both with the "go get" +subcommand:

+ +
+$ go get code.google.com/p/codesearch/index
+$ go get github.com/petar/GoLLRB/llrb
+$
+
+ +

Both of these projects are now downloaded and installed into our +$GOPATH directory. The one tree now contains the two directories +src/code.google.com/p/codesearch/index/ and +src/github.com/petar/GoLLRB/llrb/, along with the compiled +packages (in pkg/) for those libraries and their dependencies.

+ +

Because we used version control systems (Mercurial and Git) to check +out the sources, the source tree also contains the other files in the +corresponding repositories, such as related packages. The "go list" +subcommand lists the import paths corresponding to its arguments, and +the pattern "./..." means start in the current directory +("./") and find all packages below that directory +("..."):

+ +
+$ go list ./...
+code.google.com/p/codesearch/cmd/cgrep
+code.google.com/p/codesearch/cmd/cindex
+code.google.com/p/codesearch/cmd/csearch
+code.google.com/p/codesearch/index
+code.google.com/p/codesearch/regexp
+code.google.com/p/codesearch/sparse
+github.com/petar/GoLLRB/example
+github.com/petar/GoLLRB/llrb
+$
+
+ +

We can also test those packages:

+ +
+$ go test ./...
+?       code.google.com/p/codesearch/cmd/cgrep   [no test files]
+?       code.google.com/p/codesearch/cmd/cindex  [no test files]
+?       code.google.com/p/codesearch/cmd/csearch [no test files]
+ok      code.google.com/p/codesearch/index       0.239s
+ok      code.google.com/p/codesearch/regexp      0.021s
+?       code.google.com/p/codesearch/sparse      [no test files]
+?       github.com/petar/GoLLRB/example          [no test files]
+ok      github.com/petar/GoLLRB/llrb             0.231s
+$
+
+ +

If a go subcommand is invoked with no paths listed, it operates on the +current directory:

+ +
+$ cd $GOPATH/src/code.google.com/p/codesearch/regexp
+$ go list
+code.google.com/p/codesearch/regexp
+$ go test -v
+=== RUN TestNstateEnc
+--- PASS: TestNstateEnc (0.00 seconds)
+=== RUN TestMatch
+--- PASS: TestMatch (0.01 seconds)
+=== RUN TestGrep
+--- PASS: TestGrep (0.00 seconds)
+PASS
+ok      code.google.com/p/codesearch/regexp     0.021s
+$ go install
+$
+
+ +

That "go install" subcommand installs the latest copy of the +package into the pkg directory. Because the go command can analyze the +dependency graph, "go install" also installs any packages that +this package imports but that are out of date, recursively.

+ +

Notice that "go install" was able to determine the name of the +import path for the package in the current directory, because of the convention +for directory naming. It would be a little more convenient if we could pick +the name of the directory where we kept source code, and we probably wouldn't +pick such a long name, but that ability would require additional configuration +and complexity in the tool. Typing an extra directory name or two is a small +price to pay for the increased simplicity and power.

+ +

As the example shows, it’s fine to work with packages from many different +projects at once within a single $GOPATH root directory.

+ +

Limitations

+ +

As mentioned above, the go command is not a general-purpose build +tool. In particular, it does not have any facility for generating Go +source files during a build. Instead, if you want to use a tool like +yacc or the protocol buffer compiler, you will need to write a +makefile (or a configuration file for the build tool of your choice) +to generate the Go files and then check those generated source files +into your repository. This is more work for you, the package author, +but it is significantly less work for your users, who can use +"go get" without needing to obtain and build +any additional tools.

+ +

More information

+ +

For more information, read How to Write Go Code +and see the go command documentation.

diff --git a/doc/articles/gobs_of_data.html b/doc/articles/gobs_of_data.html new file mode 100644 index 000000000..6b836b2c3 --- /dev/null +++ b/doc/articles/gobs_of_data.html @@ -0,0 +1,315 @@ + + +

+To transmit a data structure across a network or to store it in a file, it must +be encoded and then decoded again. There are many encodings available, of +course: JSON, +XML, Google's +protocol buffers, and more. +And now there's another, provided by Go's gob +package. +

+ +

+Why define a new encoding? It's a lot of work and redundant at that. Why not +just use one of the existing formats? Well, for one thing, we do! Go has +packages supporting all the encodings just mentioned (the +protocol buffer package is in +a separate repository but it's one of the most frequently downloaded). And for +many purposes, including communicating with tools and systems written in other +languages, they're the right choice. +

+ +

+But for a Go-specific environment, such as communicating between two servers +written in Go, there's an opportunity to build something much easier to use and +possibly more efficient. +

+ +

+Gobs work with the language in a way that an externally-defined, +language-independent encoding cannot. At the same time, there are lessons to be +learned from the existing systems. +

+ +

+Goals +

+ +

+The gob package was designed with a number of goals in mind. +

+ +

+First, and most obvious, it had to be very easy to use. First, because Go has +reflection, there is no need for a separate interface definition language or +"protocol compiler". The data structure itself is all the package should need +to figure out how to encode and decode it. On the other hand, this approach +means that gobs will never work as well with other languages, but that's OK: +gobs are unashamedly Go-centric. +

+ +

+Efficiency is also important. Textual representations, exemplified by XML and +JSON, are too slow to put at the center of an efficient communications network. +A binary encoding is necessary. +

+ +

+Gob streams must be self-describing. Each gob stream, read from the beginning, +contains sufficient information that the entire stream can be parsed by an +agent that knows nothing a priori about its contents. This property means that +you will always be able to decode a gob stream stored in a file, even long +after you've forgotten what data it represents. +

+ +

+There were also some things to learn from our experiences with Google protocol +buffers. +

+ +

+Protocol buffer misfeatures +

+ +

+Protocol buffers had a major effect on the design of gobs, but have three +features that were deliberately avoided. (Leaving aside the property that +protocol buffers aren't self-describing: if you don't know the data definition +used to encode a protocol buffer, you might not be able to parse it.) +

+ +

+First, protocol buffers only work on the data type we call a struct in Go. You +can't encode an integer or array at the top level, only a struct with fields +inside it. That seems a pointless restriction, at least in Go. If all you want +to send is an array of integers, why should you have to put it into a +struct first? +

+ +

+Next, a protocol buffer definition may specify that fields T.x and +T.y are required to be present whenever a value of type +T is encoded or decoded. Although such required fields may seem +like a good idea, they are costly to implement because the codec must maintain a +separate data structure while encoding and decoding, to be able to report when +required fields are missing. They're also a maintenance problem. Over time, one +may want to modify the data definition to remove a required field, but that may +cause existing clients of the data to crash. It's better not to have them in the +encoding at all. (Protocol buffers also have optional fields. But if we don't +have required fields, all fields are optional and that's that. There will be +more to say about optional fields a little later.) +

+ +

+The third protocol buffer misfeature is default values. If a protocol buffer +omits the value for a "defaulted" field, then the decoded structure behaves as +if the field were set to that value. This idea works nicely when you have +getter and setter methods to control access to the field, but is harder to +handle cleanly when the container is just a plain idiomatic struct. Required +fields are also tricky to implement: where does one define the default values, +what types do they have (is text UTF-8? uninterpreted bytes? how many bits in a +float?) and despite the apparent simplicity, there were a number of +complications in their design and implementation for protocol buffers. We +decided to leave them out of gobs and fall back to Go's trivial but effective +defaulting rule: unless you set something otherwise, it has the "zero value" +for that type - and it doesn't need to be transmitted. +

+ +

+So gobs end up looking like a sort of generalized, simplified protocol buffer. +How do they work? +

+ +

+Values +

+ +

+The encoded gob data isn't about int8s and uint16s. +Instead, somewhat analogous to constants in Go, its integer values are abstract, +sizeless numbers, either signed or unsigned. When you encode an +int8, its value is transmitted as an unsized, variable-length +integer. When you encode an int64, its value is also transmitted as +an unsized, variable-length integer. (Signed and unsigned are treated +distinctly, but the same unsized-ness applies to unsigned values too.) If both +have the value 7, the bits sent on the wire will be identical. When the receiver +decodes that value, it puts it into the receiver's variable, which may be of +arbitrary integer type. Thus an encoder may send a 7 that came from an +int8, but the receiver may store it in an int64. This +is fine: the value is an integer and as a long as it fits, everything works. (If +it doesn't fit, an error results.) This decoupling from the size of the variable +gives some flexibility to the encoding: we can expand the type of the integer +variable as the software evolves, but still be able to decode old data. +

+ +

+This flexibility also applies to pointers. Before transmission, all pointers are +flattened. Values of type int8, *int8, +**int8, ****int8, etc. are all transmitted as an +integer value, which may then be stored in int of any size, or +*int, or ******int, etc. Again, this allows for +flexibility. +

+ +

+Flexibility also happens because, when decoding a struct, only those fields +that are sent by the encoder are stored in the destination. Given the value +

+ +{{code "/doc/progs/gobs1.go" `/type T/` `/STOP/`}} + +

+the encoding of t sends only the 7 and 8. Because it's zero, the +value of Y isn't even sent; there's no need to send a zero value. +

+ +

+The receiver could instead decode the value into this structure: +

+ +{{code "/doc/progs/gobs1.go" `/type U/` `/STOP/`}} + +

+and acquire a value of u with only X set (to the +address of an int8 variable set to 7); the Z field is +ignored - where would you put it? When decoding structs, fields are matched by +name and compatible type, and only fields that exist in both are affected. This +simple approach finesses the "optional field" problem: as the type +T evolves by adding fields, out of date receivers will still +function with the part of the type they recognize. Thus gobs provide the +important result of optional fields - extensibility - without any additional +mechanism or notation. +

+ +

+From integers we can build all the other types: bytes, strings, arrays, slices, +maps, even floats. Floating-point values are represented by their IEEE 754 +floating-point bit pattern, stored as an integer, which works fine as long as +you know their type, which we always do. By the way, that integer is sent in +byte-reversed order because common values of floating-point numbers, such as +small integers, have a lot of zeros at the low end that we can avoid +transmitting. +

+ +

+One nice feature of gobs that Go makes possible is that they allow you to define +your own encoding by having your type satisfy the +GobEncoder and +GobDecoder interfaces, in a manner +analogous to the JSON package's +Marshaler and +Unmarshaler and also to the +Stringer interface from +package fmt. This facility makes it possible to +represent special features, enforce constraints, or hide secrets when you +transmit data. See the documentation for +details. +

+ +

+Types on the wire +

+ +

+The first time you send a given type, the gob package includes in the data +stream a description of that type. In fact, what happens is that the encoder is +used to encode, in the standard gob encoding format, an internal struct that +describes the type and gives it a unique number. (Basic types, plus the layout +of the type description structure, are predefined by the software for +bootstrapping.) After the type is described, it can be referenced by its type +number. +

+ +

+Thus when we send our first type T, the gob encoder sends a +description of T and tags it with a type number, say 127. All +values, including the first, are then prefixed by that number, so a stream of +T values looks like: +

+ +
+("define type id" 127, definition of type T)(127, T value)(127, T value), ...
+
+ +

+These type numbers make it possible to describe recursive types and send values +of those types. Thus gobs can encode types such as trees: +

+ +{{code "/doc/progs/gobs1.go" `/type Node/` `/STOP/`}} + +

+(It's an exercise for the reader to discover how the zero-defaulting rule makes +this work, even though gobs don't represent pointers.) +

+ +

+With the type information, a gob stream is fully self-describing except for the +set of bootstrap types, which is a well-defined starting point. +

+ +

+Compiling a machine +

+ +

+The first time you encode a value of a given type, the gob package builds a +little interpreted machine specific to that data type. It uses reflection on +the type to construct that machine, but once the machine is built it does not +depend on reflection. The machine uses package unsafe and some trickery to +convert the data into the encoded bytes at high speed. It could use reflection +and avoid unsafe, but would be significantly slower. (A similar high-speed +approach is taken by the protocol buffer support for Go, whose design was +influenced by the implementation of gobs.) Subsequent values of the same type +use the already-compiled machine, so they can be encoded right away. +

+ +

+Decoding is similar but harder. When you decode a value, the gob package holds +a byte slice representing a value of a given encoder-defined type to decode, +plus a Go value into which to decode it. The gob package builds a machine for +that pair: the gob type sent on the wire crossed with the Go type provided for +decoding. Once that decoding machine is built, though, it's again a +reflectionless engine that uses unsafe methods to get maximum speed. +

+ +

+Use +

+ +

+There's a lot going on under the hood, but the result is an efficient, +easy-to-use encoding system for transmitting data. Here's a complete example +showing differing encoded and decoded types. Note how easy it is to send and +receive values; all you need to do is present values and variables to the +gob package and it does all the work. +

+ +{{code "/doc/progs/gobs2.go" `/package main/` `$`}} + +

+You can compile and run this example code in the +Go Playground. +

+ +

+The rpc package builds on gobs to turn this +encode/decode automation into transport for method calls across the network. +That's a subject for another article. +

+ +

+Details +

+ +

+The gob package documentation, especially the +file doc.go, expands on many of the +details described here and includes a full worked example showing how the +encoding represents data. If you are interested in the innards of the gob +implementation, that's a good place to start. +

diff --git a/doc/articles/godoc_documenting_go_code.html b/doc/articles/godoc_documenting_go_code.html new file mode 100644 index 000000000..ca66076ad --- /dev/null +++ b/doc/articles/godoc_documenting_go_code.html @@ -0,0 +1,139 @@ + + +

+The Go project takes documentation seriously. Documentation is a huge part of +making software accessible and maintainable. Of course it must be well-written +and accurate, but it also must be easy to write and to maintain. Ideally, it +should be coupled to the code itself so the documentation evolves along with the +code. The easier it is for programmers to produce good documentation, the better +for everyone. +

+ +

+To that end, we have developed the godoc documentation +tool. This article describes godoc's approach to documentation, and explains how +you can use our conventions and tools to write good documentation for your own +projects. +

+ +

+Godoc parses Go source code - including comments - and produces documentation as +HTML or plain text. The end result is documentation tightly coupled with the +code it documents. For example, through godoc's web interface you can navigate +from a function's documentation to its +implementation with one click. +

+ +

+Godoc is conceptually related to Python's +Docstring and Java's +Javadoc, +but its design is simpler. The comments read by godoc are not language +constructs (as with Docstring) nor must they have their own machine-readable +syntax (as with Javadoc). Godoc comments are just good comments, the sort you +would want to read even if godoc didn't exist. +

+ +

+The convention is simple: to document a type, variable, constant, function, or +even a package, write a regular comment directly preceding its declaration, with +no intervening blank line. Godoc will then present that comment as text +alongside the item it documents. For example, this is the documentation for the +fmt package's Fprint +function: +

+ +{{code "/src/pkg/fmt/print.go" `/Fprint formats using the default/` `/func Fprint/`}} + +

+Notice this comment is a complete sentence that begins with the name of the +element it describes. This important convention allows us to generate +documentation in a variety of formats, from plain text to HTML to UNIX man +pages, and makes it read better when tools truncate it for brevity, such as when +they extract the first line or sentence. +

+ +

+Comments on package declarations should provide general package documentation. +These comments can be short, like the sort +package's brief description: +

+ +{{code "/src/pkg/sort/sort.go" `/Package sort provides/` `/package sort/`}} + +

+They can also be detailed like the gob package's +overview. That package uses another convention for packages +that need large amounts of introductory documentation: the package comment is +placed in its own file, doc.go, which +contains only those comments and a package clause. +

+ +

+When writing package comments of any size, keep in mind that their first +sentence will appear in godoc's package list. +

+ +

+Comments that are not adjacent to a top-level declaration are omitted from +godoc's output, with one notable exception. Top-level comments that begin with +the word "BUG(who)” are recognized as known bugs, and included in +the "Bugs” section of the package documentation. The "who” part should be the +user name of someone who could provide more information. For example, this is a +known issue from the bytes package: +

+ +
+// BUG(r): The rule Title uses for word boundaries does not handle Unicode punctuation properly.
+
+ +

+Godoc treats executable commands somewhat differently. Instead of inspecting the +command source code, it looks for a Go source file belonging to the special +package "documentation”. The comment on the "package documentation” clause is +used as the command's documentation. For example, see the +godoc documentation and its corresponding +doc.go file. +

+ +

+There are a few formatting rules that Godoc uses when converting comments to +HTML: +

+ + + +

+Note that none of these rules requires you to do anything out of the ordinary. +

+ +

+In fact, the best thing about godoc's minimal approach is how easy it is to use. +As a result, a lot of Go code, including all of the standard library, already +follows the conventions. +

+ +

+Your own code can present good documentation just by having comments as +described above. Any Go packages installed inside $GOROOT/src/pkg +and any GOPATH work spaces will already be accessible via godoc's +command-line and HTTP interfaces, and you can specify additional paths for +indexing via the -path flag or just by running "godoc ." +in the source directory. See the godoc documentation +for more details. +

diff --git a/doc/articles/gos_declaration_syntax.html b/doc/articles/gos_declaration_syntax.html new file mode 100644 index 000000000..455cced1d --- /dev/null +++ b/doc/articles/gos_declaration_syntax.html @@ -0,0 +1,348 @@ + + +

+Newcomers to Go wonder why the declaration syntax is different from the +tradition established in the C family. In this post we'll compare the +two approaches and explain why Go's declarations look as they do. +

+ +

+C syntax +

+ +

+First, let's talk about C syntax. C took an unusual and clever approach +to declaration syntax. Instead of describing the types with special +syntax, one writes an expression involving the item being declared, and +states what type that expression will have. Thus +

+ +
+int x;
+
+ +

+declares x to be an int: the expression 'x' will have type int. In +general, to figure out how to write the type of a new variable, write an +expression involving that variable that evaluates to a basic type, then +put the basic type on the left and the expression on the right. +

+ +

+Thus, the declarations +

+ +
+int *p;
+int a[3];
+
+ +

+state that p is a pointer to int because '*p' has type int, and that a +is an array of ints because a[3] (ignoring the particular index value, +which is punned to be the size of the array) has type int. +

+ +

+What about functions? Originally, C's function declarations wrote the +types of the arguments outside the parens, like this: +

+ +
+int main(argc, argv)
+    int argc;
+    char *argv[];
+{ /* ... */ }
+
+ +

+Again, we see that main is a function because the expression main(argc, +argv) returns an int. In modern notation we'd write +

+ +
+int main(int argc, char *argv[]) { /* ... */ }
+
+ +

+but the basic structure is the same. +

+ +

+This is a clever syntactic idea that works well for simple types but can +get confusing fast. The famous example is declaring a function pointer. +Follow the rules and you get this: +

+ +
+int (*fp)(int a, int b);
+
+ +

+Here, fp is a pointer to a function because if you write the expression +(*fp)(a, b) you'll call a function that returns int. What if one of fp's +arguments is itself a function? +

+ +
+int (*fp)(int (*ff)(int x, int y), int b)
+
+ +

+That's starting to get hard to read. +

+ +

+Of course, we can leave out the name of the parameters when we declare a +function, so main can be declared +

+ +
+int main(int, char *[])
+
+ +

+Recall that argv is declared like this, +

+ +
+char *argv[]
+
+ +

+so you drop the name from the middle of its declaration to construct +its type. It's not obvious, though, that you declare something of type +char *[] by putting its name in the middle. +

+ +

+And look what happens to fp's declaration if you don't name the +parameters: +

+ +
+int (*fp)(int (*)(int, int), int)
+
+ +

+Not only is it not obvious where to put the name inside +

+ +
+int (*)(int, int)
+
+ +

+it's not exactly clear that it's a function pointer declaration at all. +And what if the return type is a function pointer? +

+ +
+int (*(*fp)(int (*)(int, int), int))(int, int)
+
+ +

+It's hard even to see that this declaration is about fp. +

+ +

+You can construct more elaborate examples but these should illustrate +some of the difficulties that C's declaration syntax can introduce. +

+ +

+There's one more point that needs to be made, though. Because type and +declaration syntax are the same, it can be difficult to parse +expressions with types in the middle. This is why, for instance, C casts +always parenthesize the type, as in +

+ +
+(int)M_PI
+
+ +

+Go syntax +

+ +

+Languages outside the C family usually use a distinct type syntax in +declarations. Although it's a separate point, the name usually comes +first, often followed by a colon. Thus our examples above become +something like (in a fictional but illustrative language) +

+ +
+x: int
+p: pointer to int
+a: array[3] of int
+
+ +

+These declarations are clear, if verbose - you just read them left to +right. Go takes its cue from here, but in the interests of brevity it +drops the colon and removes some of the keywords: +

+ +
+x int
+p *int
+a [3]int
+
+ +

+There is no direct correspondence between the look of [3]int and how to +use a in an expression. (We'll come back to pointers in the next +section.) You gain clarity at the cost of a separate syntax. +

+ +

+Now consider functions. Let's transcribe the declaration for main, even +though the main function in Go takes no arguments: +

+ +
+func main(argc int, argv *[]byte) int
+
+ +

+Superficially that's not much different from C, but it reads well from +left to right: +

+ +

+function main takes an int and a pointer to a slice of bytes and returns an int. +

+ +

+Drop the parameter names and it's just as clear - they're always first +so there's no confusion. +

+ +
+func main(int, *[]byte) int
+
+ +

+One value of this left-to-right style is how well it works as the types +become more complex. Here's a declaration of a function variable +(analogous to a function pointer in C): +

+ +
+f func(func(int,int) int, int) int
+
+ +

+Or if f returns a function: +

+ +
+f func(func(int,int) int, int) func(int, int) int
+
+ +

+It still reads clearly, from left to right, and it's always obvious +which name is being declared - the name comes first. +

+ +

+The distinction between type and expression syntax makes it easy to +write and invoke closures in Go: +

+ +
+sum := func(a, b int) int { return a+b } (3, 4)
+
+ +

+Pointers +

+ +

+Pointers are the exception that proves the rule. Notice that in arrays +and slices, for instance, Go's type syntax puts the brackets on the left +of the type but the expression syntax puts them on the right of the +expression: +

+ +
+var a []int
+x = a[1]
+
+ +

+For familiarity, Go's pointers use the * notation from C, but we could +not bring ourselves to make a similar reversal for pointer types. Thus +pointers work like this +

+ +
+var p *int
+x = *p
+
+ +

+We couldn't say +

+ +
+var p *int
+x = p*
+
+ +

+because that postfix * would conflate with multiplication. We could have +used the Pascal ^, for example: +

+ +
+var p ^int
+x = p^
+
+ +

+and perhaps we should have (and chosen another operator for xor), +because the prefix asterisk on both types and expressions complicates +things in a number of ways. For instance, although one can write +

+ +
+[]int("hi")
+
+ +

+as a conversion, one must parenthesize the type if it starts with a *: +

+ +
+(*int)(nil)
+
+ +

+Had we been willing to give up * as pointer syntax, those parentheses +would be unnecessary. +

+ +

+So Go's pointer syntax is tied to the familiar C form, but those ties +mean that we cannot break completely from using parentheses to +disambiguate types and expressions in the grammar. +

+ +

+Overall, though, we believe Go's type syntax is easier to understand +than C's, especially when things get complicated. +

+ +

+Notes +

+ +

+Go's declarations read left to right. It's been pointed out that C's +read in a spiral! See +The "Clockwise/Spiral Rule" by David Anderson. +

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+Package image/draw defines +only one operation: drawing a source image onto a destination +image, through an optional mask image. This one operation is +surprisingly versatile and can perform a number of common image +manipulation tasks elegantly and efficiently. +

+ +

+Composition is performed pixel by pixel in the style of the Plan 9 +graphics library and the X Render extension. The model is based on +the classic "Compositing Digital Images" paper by Porter and Duff, +with an additional mask parameter: dst = (src IN mask) OP dst. +For a fully opaque mask, this reduces to the original Porter-Duff +formula: dst = src OP dst. In Go, a nil mask image is equivalent +to an infinitely sized, fully opaque mask image. +

+ +

+The Porter-Duff paper presented +12 different composition operators, +but with an explicit mask, only 2 of these are needed in practice: +source-over-destination and source. In Go, these operators are +represented by the Over and Src constants. The Over operator +performs the natural layering of a source image over a destination +image: the change to the destination image is smaller where the +source (after masking) is more transparent (that is, has lower +alpha). The Src operator merely copies the source (after masking) +with no regard for the destination image's original content. For +fully opaque source and mask images, the two operators produce the +same output, but the Src operator is usually faster. +

+ +

Geometric Alignment

+ +

+Composition requires associating destination pixels with source and +mask pixels. Obviously, this requires destination, source and mask +images, and a composition operator, but it also requires specifying +what rectangle of each image to use. Not every drawing should write +to the entire destination: when updating an animating image, it is +more efficient to only draw the parts of the image that have +changed. Not every drawing should read from the entire source: when +using a sprite that combines many small images into one large one, +only a part of the image is needed. Not every drawing should read +from the entire mask: a mask image that collects a font's glyphs is +similar to a sprite. Thus, drawing also needs to know three +rectangles, one for each image. Since each rectangle has the same +width and height, it suffices to pass a destination rectangle `r` +and two points sp and mp: the source rectangle is equal to r +translated so that r.Min in the destination image aligns with +sp in the source image, and similarly for mp. The effective +rectangle is also clipped to each image's bounds in their +respective co-ordinate space. +

+ +

+ +

+ +

+The DrawMask +function takes seven arguments, but an explicit mask and mask-point +are usually unnecessary, so the +Draw function takes five: +

+ +
+// Draw calls DrawMask with a nil mask.
+func Draw(dst Image, r image.Rectangle, src image.Image, sp image.Point, op Op)
+func DrawMask(dst Image, r image.Rectangle, src image.Image, sp image.Point,
+	mask image.Image, mp image.Point, op Op)
+
+ +

+The destination image must be mutable, so the image/draw package +defines a draw.Image +interface which has a Set method. +

+ +{{code "../src/pkg/image/draw/draw.go" `/type Image/` `/}/`}} + +

Filling a Rectangle

+ +

+To fill a rectangle with a solid color, use an image.Uniform +source. The ColorImage type re-interprets a Color as a +practically infinite-sized Image of that color. For those +familiar with the design of Plan 9's draw library, there is no need +for an explicit "repeat bit" in Go's slice-based image types; the +concept is subsumed by Uniform. +

+ +{{code "/doc/progs/image_draw.go" `/ZERO/` `/STOP/`}} + +

+To initialize a new image to all-blue: +

+ +{{code "/doc/progs/image_draw.go" `/BLUE/` `/STOP/`}} + +

+To reset an image to transparent (or black, if the destination +image's color model cannot represent transparency), use +image.Transparent, which is an image.Uniform: +

+ +{{code "/doc/progs/image_draw.go" `/RESET/` `/STOP/`}} + +

+ +

+ + +

Copying an Image

+ +

+To copy from a rectangle sr in the source image to a rectangle +starting at a point dp in the destination, convert the source +rectangle into the destination image's co-ordinate space: +

+ +{{code "/doc/progs/image_draw.go" `/RECT/` `/STOP/`}} + +

+Alternatively: +

+ +{{code "/doc/progs/image_draw.go" `/RECT2/` `/STOP/`}} + +

+To copy the entire source image, use sr = src.Bounds(). +

+ +

+ +

+ +

Scrolling an Image

+ +

+Scrolling an image is just copying an image to itself, with +different destination and source rectangles. Overlapping +destination and source images are perfectly valid, just as Go's +built-in copy function can handle overlapping destination and +source slices. To scroll an image m by 20 pixels: +

+ +{{code "/doc/progs/image_draw.go" `/SCROLL/` `/STOP/`}} + +

+ +

Converting an Image to RGBA

+ +

+The result of decoding an image format might not be an +image.RGBA: decoding a GIF results in an image.Paletted, +decoding a JPEG results in a ycbcr.YCbCr, and the result of +decoding a PNG depends on the image data. To convert any image to +an image.RGBA: +

+ +{{code "/doc/progs/image_draw.go" `/CONV/` `/STOP/`}} + +

+ +

+ +

Drawing Through a Mask

+ +

+To draw an image through a circular mask with center p and radius +r: +

+ +{{code "/doc/progs/image_draw.go" `/CIRCLESTRUCT/` `/STOP/`}} +{{code "/doc/progs/image_draw.go" `/CIRCLE2/` `/STOP/`}} + +

+ +

+ +

Drawing Font Glyphs

+ +

+To draw a font glyph in blue starting from a point p, draw with +an image.ColorImage source and an image.Alpha mask. For +simplicity, we aren't performing any sub-pixel positioning or +rendering, or correcting for a font's height above a baseline. +

+ +{{code "/doc/progs/image_draw.go" `/GLYPH/` `/STOP/`}} + +

+ +

+ +

Performance

+ +

+The image/draw package implementation demonstrates how to provide +an image manipulation function that is both general purpose, yet +efficient for common cases. The DrawMask function takes arguments +of interface types, but immediately makes type assertions that its +arguments are of specific struct types, corresponding to common +operations like drawing one image.RGBA image onto another, or +drawing an image.Alpha mask (such as a font glyph) onto an +image.RGBA image. If a type assertion succeeds, that type +information is used to run a specialized implementation of the +general algorithm. If the assertions fail, the fallback code path +uses the generic At and Set methods. The fast-paths are purely +a performance optimization; the resultant destination image is the +same either way. In practice, only a small number of special cases +are necessary to support typical applications. +

+ + diff --git a/doc/articles/image_package.html b/doc/articles/image_package.html new file mode 100644 index 000000000..a9d2f3581 --- /dev/null +++ b/doc/articles/image_package.html @@ -0,0 +1,312 @@ + + +

+The image and +image/color packages define a number of types: +color.Color and color.Model describe colors, +image.Point and image.Rectangle describe basic 2-D +geometry, and image.Image brings the two concepts together to +represent a rectangular grid of colors. A +separate article covers image +composition with the image/draw package. +

+ +

+Colors and Color Models +

+ +

+Color is an interface that defines the minimal +method set of any type that can be considered a color: one that can be converted +to red, green, blue and alpha values. The conversion may be lossy, such as +converting from CMYK or YCbCr color spaces. +

+ +{{code "/src/pkg/image/color/color.go" `/type Color interface/` `/^}/`}} + +

+There are three important subtleties about the return values. First, the red, +green and blue are alpha-premultiplied: a fully saturated red that is also 25% +transparent is represented by RGBA returning a 75% r. Second, the channels have +a 16-bit effective range: 100% red is represented by RGBA returning an r of +65535, not 255, so that converting from CMYK or YCbCr is not as lossy. Third, +the type returned is uint32, even though the maximum value is 65535, to +guarantee that multiplying two values together won't overflow. Such +multiplications occur when blending two colors according to an alpha mask from a +third color, in the style of +Porter and Duff's +classic algebra: +

+ +
+dstr, dstg, dstb, dsta := dst.RGBA()
+srcr, srcg, srcb, srca := src.RGBA()
+_, _, _, m := mask.RGBA()
+const M = 1<<16 - 1
+// The resultant red value is a blend of dstr and srcr, and ranges in [0, M].
+// The calculation for green, blue and alpha is similar.
+dstr = (dstr*(M-m) + srcr*m) / M
+
+ +

+The last line of that code snippet would have been more complicated if we worked +with non-alpha-premultiplied colors, which is why Color uses +alpha-premultiplied values. +

+ +

+The image/color package also defines a number of concrete types that implement +the Color interface. For example, +RGBA is a struct that represents +the classic "8 bits per channel" color. +

+ +{{code "/src/pkg/image/color/color.go" `/type RGBA struct/` `/^}/`}} + +

+Note that the R field of an RGBA is an 8-bit +alpha-premultiplied color in the range [0, 255]. RGBA satisfies the +Color interface by multiplying that value by 0x101 to generate a +16-bit alpha-premultiplied color in the range [0, 65535]. Similarly, the +NRGBA struct type represents +an 8-bit non-alpha-premultiplied color, as used by the PNG image format. When +manipulating an NRGBA's fields directly, the values are +non-alpha-premultiplied, but when calling the RGBA method, the +return values are alpha-premultiplied. +

+ +

+A Model is simply +something that can convert Colors to other Colors, possibly lossily. For +example, the GrayModel can convert any Color to a +desaturated Gray. A +Palette can convert any Color to one from a +limited palette. +

+ +{{code "/src/pkg/image/color/color.go" `/type Model interface/` `/^}/`}} + +{{code "/src/pkg/image/color/color.go" `/type Palette \[\]Color/`}} + +

+Points and Rectangles +

+ +

+A Point is an (x, y) co-ordinate +on the integer grid, with axes increasing right and down. It is neither a pixel +nor a grid square. A Point has no intrinsic width, height or +color, but the visualizations below use a small colored square. +

+ +{{code "/src/pkg/image/geom.go" `/type Point struct/` `/^}/`}} + +

+ +

+ +{{code "/doc/progs/image_package1.go" `/p := image.Point/`}} + +

+A Rectangle is an axis-aligned +rectangle on the integer grid, defined by its top-left and bottom-right +Point. A Rectangle also has no intrinsic color, but +the visualizations below outline rectangles with a thin colored line, and call +out their Min and Max Points. +

+ +{{code "/src/pkg/image/geom.go" `/type Rectangle struct/` `/^}/`}} + +

+For convenience, image.Rect(x0, y0, x1, y1) is equivalent to +image.Rectangle{image.Point{x0, y0}, image.Point{x1, y1}}, but is +much easier to type. +

+ +

+A Rectangle is inclusive at the top-left and exclusive at the +bottom-right. For a Point p and a Rectangle r, +p.In(r) if and only if +r.Min.X <= p.X && p.X < r.Max.X, and similarly for Y. This is analagous to how +a slice s[i0:i1] is inclusive at the low end and exclusive at the +high end. (Unlike arrays and slices, a Rectangle often has a +non-zero origin.) +

+ +

+ +

+ +{{code "/doc/progs/image_package2.go" `/r := image.Rect/` `/fmt.Println/`}} + +

+Adding a Point to a Rectangle translates the +Rectangle. Points and Rectangles are not restricted to be in the +bottom-right quadrant. +

+ +

+ +

+ +{{code "/doc/progs/image_package3.go" `/r := image.Rect/` `/fmt.Println/`}} + +

+Intersecting two Rectangles yields another Rectangle, which may be empty. +

+ +

+ +

+ +{{code "/doc/progs/image_package4.go" `/r := image.Rect/` `/fmt.Printf/`}} + +

+Points and Rectangles are passed and returned by value. A function that takes a +Rectangle argument will be as efficient as a function that takes +two Point arguments, or four int arguments. +

+ +

+Images +

+ +

+An Image maps every grid square in a +Rectangle to a Color from a Model. +"The pixel at (x, y)" refers to the color of the grid square defined by the +points (x, y), (x+1, y), (x+1, y+1) and (x, y+1). +

+ +{{code "/src/pkg/image/image.go" `/type Image interface/` `/^}/`}} + +

+A common mistake is assuming that an Image's bounds start at (0, +0). For example, an animated GIF contains a sequence of Images, and each +Image after the first typically only holds pixel data for the area +that changed, and that area doesn't necessarily start at (0, 0). The correct +way to iterate over an Image m's pixels looks like: +

+ +
+b := m.Bounds()
+for y := b.Min.Y; y < b.Max.Y; y++ {
+	for x := b.Min.X; y < b.Max.X; x++ {
+		doStuffWith(m.At(x, y))
+	}
+}
+
+ +

+Image implementations do not have to be based on an in-memory +slice of pixel data. For example, a +Uniform is an +Image of enormous bounds and uniform color, whose in-memory +representation is simply that color. +

+ +{{code "/src/pkg/image/names.go" `/type Uniform struct/` `/^}/`}} + +

+Typically, though, programs will want an image based on a slice. Struct types +like RGBA and +Gray (which other packages refer +to as image.RGBA and image.Gray) hold slices of pixel +data and implement the Image interface. +

+ +{{code "/src/pkg/image/image.go" `/type RGBA struct/` `/^}/`}} + +

+These types also provide a Set(x, y int, c color.Color) method +that allows modifying the image one pixel at a time. +

+ +{{code "/doc/progs/image_package5.go" `/m := image.New/` `/m.Set/`}} + +

+If you're reading or writing a lot of pixel data, it can be more efficient, but +more complicated, to access these struct type's Pix field directly. +

+ +

+The slice-based Image implementations also provide a +SubImage method, which returns an Image backed by the +same array. Modifying the pixels of a sub-image will affect the pixels of the +original image, analagous to how modifying the contents of a sub-slice +s[i0:i1] will affect the contents of the original slice +s. +

+ + + +{{code "/doc/progs/image_package6.go" `/m0 := image.New/` `/fmt.Println\(m0.Stride/`}} + +

+For low-level code that works on an image's Pix field, be aware +that ranging over Pix can affect pixels outside an image's bounds. +In the example above, the pixels covered by m1.Pix are shaded in +blue. Higher-level code, such as the At and Set +methods or the image/draw package, will clip +their operations to the image's bounds. +

+ +

+Image Formats +

+ +

+The standard package library supports a number of common image formats, such as +GIF, JPEG and PNG. If you know the format of a source image file, you can +decode from an io.Reader directly. +

+ +
+import (
+	"image/jpeg"
+	"image/png"
+	"io"
+)
+
+// convertJPEGToPNG converts from JPEG to PNG.
+func convertJPEGToPNG(w io.Writer, r io.Reader) error {
+	img, err := jpeg.Decode(r)
+	if err != nil {
+		return err
+	}
+	return png.Encode(w, img)
+}
+
+ +

+If you have image data of unknown format, the +image.Decode function can detect +the format. The set of recognized formats is constructed at run time and is not +limited to those in the standard package library. An image format package +typically registers its format in an init function, and the main package will +"underscore import" such a package solely for the side effect of format +registration. +

+ +
+import (
+	"image"
+	"image/png"
+	"io"
+
+	_ "code.google.com/p/vp8-go/webp"
+	_ "image/jpeg"
+)
+
+// convertToPNG converts from any recognized format to PNG.
+func convertToPNG(w io.Writer, r io.Reader) error {
+	img, _, err := image.Decode(r)
+	if err != nil {
+		return err
+	}
+	return png.Encode(w, img)
+}
+
diff --git a/doc/articles/index.html b/doc/articles/index.html new file mode 100644 index 000000000..5f70734ec --- /dev/null +++ b/doc/articles/index.html @@ -0,0 +1,7 @@ + + +

+See the Documents page for a complete list of Go articles. +

diff --git a/doc/articles/json_and_go.html b/doc/articles/json_and_go.html new file mode 100644 index 000000000..af7776c0a --- /dev/null +++ b/doc/articles/json_and_go.html @@ -0,0 +1,356 @@ + + +

+JSON (JavaScript Object Notation) is a simple data interchange format. +Syntactically it resembles the objects and lists of JavaScript. It is most +commonly used for communication between web back-ends and JavaScript programs +running in the browser, but it is used in many other places, too. Its home page, +json.org, provides a wonderfully clear and concise +definition of the standard. +

+ +

+With the json package it's a snap to read and +write JSON data from your Go programs. +

+ +

+Encoding +

+ +

+To encode JSON data we use the +Marshal function. +

+ +
+func Marshal(v interface{}) ([]byte, error)
+
+ +

+Given the Go data structure, Message, +

+ +{{code "/doc/progs/json1.go" `/type Message/` `/STOP/`}} + +

+and an instance of Message +

+ +{{code "/doc/progs/json1.go" `/m :=/`}} + +

+we can marshal a JSON-encoded version of m using json.Marshal: +

+ +{{code "/doc/progs/json1.go" `/b, err :=/`}} + +

+If all is well, err will be nil and b +will be a []byte containing this JSON data: +

+ +
+b == []byte(`{"Name":"Alice","Body":"Hello","Time":1294706395881547000}`)
+
+ +

+Only data structures that can be represented as valid JSON will be encoded: +

+ + + +

+The json package only accesses the exported fields of struct types (those that +begin with an uppercase letter). Therefore only the the exported fields of a +struct will be present in the JSON output. +

+ +

+Decoding +

+ +

+To decode JSON data we use the +Unmarshal function. +

+ +
+func Unmarshal(data []byte, v interface{}) error
+
+ +

+We must first create a place where the decoded data will be stored +

+ +{{code "/doc/progs/json1.go" `/var m Message/`}} + +

+and call json.Unmarshal, passing it a []byte of JSON +data and a pointer to m +

+ +{{code "/doc/progs/json1.go" `/err := json.Unmarshal/`}} + +

+If b contains valid JSON that fits in m, after the +call err will be nil and the data from b +will have been stored in the struct m, as if by an assignment +like: +

+ +{{code "/doc/progs/json1.go" `/m = Message/` `/STOP/`}} + +

+How does Unmarshal identify the fields in which to store the +decoded data? For a given JSON key "Foo", Unmarshal +will look through the destination struct's fields to find (in order of +preference): +

+ + + +

+What happens when the structure of the JSON data doesn't exactly match the Go +type? +

+ +{{code "/doc/progs/json1.go" `/"Food":"Pickle"/` `/STOP/`}} + +

+Unmarshal will decode only the fields that it can find in the +destination type. In this case, only the Name field of m will be populated, +and the Food field will be ignored. This behavior is particularly useful when +you wish to pick only a few specific fields out of a large JSON blob. It also +means that any unexported fields in the destination struct will be unaffected +by Unmarshal. +

+ +

+But what if you don't know the structure of your JSON data beforehand? +

+ +

+Generic JSON with interface{} +

+ +

+The interface{} (empty interface) type describes an interface with +zero methods. Every Go type implements at least zero methods and therefore +satisfies the empty interface. +

+ +

+The empty interface serves as a general container type: +

+ +{{code "/doc/progs/json2.go" `/var i interface{}/` `/STOP/`}} + +

+A type assertion accesses the underlying concrete type: +

+ +{{code "/doc/progs/json2.go" `/r := i/` `/STOP/`}} + +

+Or, if the underlying type is unknown, a type switch determines the type: +

+ +{{code "/doc/progs/json2.go" `/switch v/` `/STOP/`}} + + +The json package uses map[string]interface{} and +[]interface{} values to store arbitrary JSON objects and arrays; +it will happily unmarshal any valid JSON blob into a plain +interface{} value. The default concrete Go types are: + + + +

+Decoding arbitrary data +

+ +

+Consider this JSON data, stored in the variable b: +

+ +{{code "/doc/progs/json3.go" `/b :=/`}} + +

+Without knowing this data's structure, we can decode it into an +interface{} value with Unmarshal: +

+ +{{code "/doc/progs/json3.go" `/var f interface/` `/STOP/`}} + +

+At this point the Go value in f would be a map whose keys are +strings and whose values are themselves stored as empty interface values: +

+ +{{code "/doc/progs/json3.go" `/f = map/` `/STOP/`}} + +

+To access this data we can use a type assertion to access f's +underlying map[string]interface{}: +

+ +{{code "/doc/progs/json3.go" `/m := f/`}} + +

+We can then iterate through the map with a range statement and use a type switch +to access its values as their concrete types: +

+ +{{code "/doc/progs/json3.go" `/for k, v/` `/STOP/`}} + +

+In this way you can work with unknown JSON data while still enjoying the +benefits of type safety. +

+ +

+Reference Types +

+ +

+Let's define a Go type to contain the data from the previous example: +

+ +{{code "/doc/progs/json4.go" `/type FamilyMember/` `/STOP/`}} + +{{code "/doc/progs/json4.go" `/var m FamilyMember/` `/STOP/`}} + +

+Unmarshaling that data into a FamilyMember value works as +expected, but if we look closely we can see a remarkable thing has happened. +With the var statement we allocated a FamilyMember struct, and +then provided a pointer to that value to Unmarshal, but at that +time the Parents field was a nil slice value. To +populate the Parents field, Unmarshal allocated a new +slice behind the scenes. This is typical of how Unmarshal works +with the supported reference types (pointers, slices, and maps). +

+ +

+Consider unmarshaling into this data structure: +

+ +
+type Foo struct {
+    Bar *Bar
+}
+
+ +

+If there were a Bar field in the JSON object, +Unmarshal would allocate a new Bar and populate it. +If not, Bar would be left as a nil pointer. +

+ +

+From this a useful pattern arises: if you have an application that receives a +few distinct message types, you might define "receiver" structure like +

+ +
+type IncomingMessage struct {
+    Cmd *Command
+    Msg *Message
+}
+
+ +

+and the sending party can populate the Cmd field and/or the +Msg field of the top-level JSON object, depending on the type of +message they want to communicate. Unmarshal, when decoding the +JSON into an IncomingMessage struct, will only allocate the data +structures present in the JSON data. To know which messages to process, the +programmer need simply test that either Cmd or Msg is +not nil. +

+ +

+Streaming Encoders and Decoders +

+ +

+The json package provides Decoder and Encoder types +to support the common operation of reading and writing streams of JSON data. +The NewDecoder and NewEncoder functions wrap the +io.Reader and +io.Writer interface types. +

+ +
+func NewDecoder(r io.Reader) *Decoder
+func NewEncoder(w io.Writer) *Encoder
+
+ +

+Here's an example program that reads a series of JSON objects from standard +input, removes all but the Name field from each object, and then +writes the objects to standard output: +

+ +{{code "/doc/progs/json5.go" `/package main/` `$`}} + +

+Due to the ubiquity of Readers and Writers, these Encoder and +Decoder types can be used in a broad range of scenarios, such as +reading and writing to HTTP connections, WebSockets, or files. +

+ +

+References +

+ +

+For more information see the json package documentation. For an example usage of +json see the source files of the jsonrpc package. +

diff --git a/doc/articles/json_rpc_tale_of_interfaces.html b/doc/articles/json_rpc_tale_of_interfaces.html new file mode 100644 index 000000000..a545f55f6 --- /dev/null +++ b/doc/articles/json_rpc_tale_of_interfaces.html @@ -0,0 +1,78 @@ + + +

+Here we present an example where Go's +interfaces made it +easy to refactor some existing code to make it more flexible and extensible. +Originally, the standard library's RPC package used +a custom wire format called gob. For a +particular application, we wanted to use JSON +as an alternate wire format. +

+ +

+We first defined a pair of interfaces to describe the functionality of the +existing wire format, one for the client, and one for the server (depicted +below). +

+ +
+type ServerCodec interface {
+	ReadRequestHeader(*Request) error
+	ReadRequestBody(interface{}) error
+	WriteResponse(*Response, interface{}) error
+	Close() error
+}
+
+ +

+On the server side, we then changed two internal function signatures to accept +the ServerCodec interface instead of our existing +gob.Encoder. Here's one of them: +

+ +
+func sendResponse(sending *sync.Mutex, req *Request,
+	reply interface{}, enc *gob.Encoder, errmsg string)
+
+ +

+became +

+ +
+func sendResponse(sending *sync.Mutex, req *Request,
+		reply interface{}, enc ServerCodec, errmsg string)
+
+ +

+We then wrote a trivial gobServerCodec wrapper to reproduce the +original functionality. From there it is simple to build a +jsonServerCodec. +

+ +

+After some similar changes to the client side, this was the full extent of the +work we needed to do on the RPC package. This whole exercise took about 20 +minutes! After tidying up and testing the new code, the +final changeset +was submitted. +

+ +

+In an inheritance-oriented language like Java or C++, the obvious path would be +to generalize the RPC class, and create JsonRPC and GobRPC subclasses. However, +this approach becomes tricky if you want to make a further generalization +orthogonal to that hierarchy. (For example, if you were to implement an +alternate RPC standard). In our Go package, we took a route that is both +conceptually simpler and requires less code be written or changed. +

+ +

+A vital quality for any codebase is maintainability. As needs change, it is +essential to adapt your code easily and cleanly, lest it become unwieldy to work +with. We believe Go's lightweight, composition-oriented type system provides a +means of structuring code that scales. +

diff --git a/doc/articles/laws_of_reflection.html b/doc/articles/laws_of_reflection.html new file mode 100644 index 000000000..826a054f2 --- /dev/null +++ b/doc/articles/laws_of_reflection.html @@ -0,0 +1,649 @@ + + +

+Reflection in computing is the +ability of a program to examine its own structure, particularly +through types; it's a form of metaprogramming. It's also a great +source of confusion. +

+ +

+In this article we attempt to clarify things by explaining how +reflection works in Go. Each language's reflection model is +different (and many languages don't support it at all), but +this article is about Go, so for the rest of this article the word +"reflection" should be taken to mean "reflection in Go". +

+ +

Types and interfaces

+ +

+Because reflection builds on the type system, let's start with a +refresher about types in Go. +

+ +

+Go is statically typed. Every variable has a static type, that is, +exactly one type known and fixed at compile time: int, +float32, *MyType, []byte, +and so on. If we declare +

+ +{{code "/doc/progs/interface.go" `/type MyInt/` `/STOP/`}} + +

+then i has type int and j +has type MyInt. The variables i and +j have distinct static types and, although they have +the same underlying type, they cannot be assigned to one another +without a conversion. +

+ +

+One important category of type is interface types, which represent +fixed sets of methods. An interface variable can store any concrete +(non-interface) value as long as that value implements the +interface's methods. A well-known pair of examples is +io.Reader and io.Writer, the types +Reader and Writer from the +io package: +

+ +{{code "/doc/progs/interface.go" `/// Reader/` `/STOP/`}} + +

+Any type that implements a Read (or +Write) method with this signature is said to implement +io.Reader (or io.Writer). For the +purposes of this discussion, that means that a variable of type +io.Reader can hold any value whose type has a +Read method: +

+ +{{code "/doc/progs/interface.go" `/func readers/` `/STOP/`}} + +

+It's important to be clear that whatever concrete value +r may hold, r's type is always +io.Reader: Go is statically typed and the static type +of r is io.Reader.

+ +

+An extremely important example of an interface type is the empty +interface: +

+ +
+interface{}
+
+ +

+It represents the empty set of methods and is satisfied by any +value at all, since any value has zero or more methods. +

+ +

+Some people say that Go's interfaces are dynamically typed, but +that is misleading. They are statically typed: a variable of +interface type always has the same static type, and even though at +run time the value stored in the interface variable may change +type, that value will always satisfy the interface. +

+ +

+We need to be precise about all this because reflection and +interfaces are closely related. +

+ +

The representation of an interface

+ +

+Russ Cox has written a +detailed blog post +about the representation of interface values in Go. It's not necessary to +repeat the full story here, but a simplified summary is in order. +

+ +

+A variable of interface type stores a pair: the concrete value +assigned to the variable, and that value's type descriptor. +To be more precise, the value is the underlying concrete data item +that implements the interface and the type describes the full type +of that item. For instance, after +

+ +{{code "/doc/progs/interface.go" `/func typeAssertions/` `/STOP/`}} + +

+r contains, schematically, the (value, type) pair, +(tty, *os.File). Notice that the type +*os.File implements methods other than +Read; even though the interface value provides access +only to the Read method, the value inside carries all +the type information about that value. That's why we can do things +like this: +

+ +{{code "/doc/progs/interface.go" `/var w io.Writer/` `/STOP/`}} + +

+The expression in this assignment is a type assertion; what it +asserts is that the item inside r also implements +io.Writer, and so we can assign it to w. +After the assignment, w will contain the pair +(tty, *os.File). That's the same pair as +was held in r. The static type of the interface +determines what methods may be invoked with an interface variable, +even though the concrete value inside may have a larger set of +methods. +

+ +

+Continuing, we can do this: +

+ +{{code "/doc/progs/interface.go" `/var empty interface{}/` `/STOP/`}} + +

+and our empty interface value e will again contain +that same pair, (tty, *os.File). That's +handy: an empty interface can hold any value and contains all the +information we could ever need about that value. +

+ +

+(We don't need a type assertion here because it's known statically +that w satisfies the empty interface. In the example +where we moved a value from a Reader to a +Writer, we needed to be explicit and use a type +assertion because Writer's methods are not a +subset of Reader's.) +

+ +

+One important detail is that the pair inside an interface always +has the form (value, concrete type) and cannot have the form +(value, interface type). Interfaces do not hold interface +values. +

+ +

+Now we're ready to reflect. +

+ +

The first law of reflection

+ +

1. Reflection goes from interface value to reflection object.

+ +

+At the basic level, reflection is just a mechanism to examine the +type and value pair stored inside an interface variable. To get +started, there are two types we need to know about in +package reflect: +Type and +Value. Those two types +give access to the contents of an interface variable, and two +simple functions, called reflect.TypeOf and +reflect.ValueOf, retrieve reflect.Type +and reflect.Value pieces out of an interface value. +(Also, from the reflect.Value it's easy to get +to the reflect.Type, but let's keep the +Value and Type concepts separate for +now.) +

+ +

+Let's start with TypeOf: +

+ +{{code "/doc/progs/interface2.go" `/package main/` `/STOP main/`}} + +

+This program prints +

+ +
+type: float64
+
+ +

+You might be wondering where the interface is here, since the program looks +like it's passing the float64 variable x, not an +interface value, to reflect.TypeOf. But it's there; as +godoc reports, the signature of +reflect.TypeOf includes an empty interface: +

+ +
+// TypeOf returns the reflection Type of the value in the interface{}.
+func TypeOf(i interface{}) Type
+
+ +

+When we call reflect.TypeOf(x), x is +first stored in an empty interface, which is then passed as the +argument; reflect.TypeOf unpacks that empty interface +to recover the type information. +

+ +

+The reflect.ValueOf function, of course, recovers the +value (from here on we'll elide the boilerplate and focus just on +the executable code): +

+ +{{code "/doc/progs/interface2.go" `/START f9/` `/STOP/`}} + +

+prints +

+ +
+value: <float64 Value>
+
+ +

+Both reflect.Type and reflect.Value have +lots of methods to let us examine and manipulate them. One +important example is that Value has a +Type method that returns the Type of a +reflect.Value. Another is that both Type +and Value have a Kind method that returns +a constant indicating what sort of item is stored: +Uint, Float64, Slice, and so +on. Also methods on Value with names like +Int and Float let us grab values (as +int64 and float64) stored inside: +

+ +{{code "/doc/progs/interface2.go" `/START f1/` `/STOP/`}} + +

+prints +

+ +
+type: float64
+kind is float64: true
+value: 3.4
+
+ +

+There are also methods like SetInt and +SetFloat but to use them we need to understand +settability, the subject of the third law of reflection, discussed +below. +

+ +

+The reflection library has a couple of properties worth singling +out. First, to keep the API simple, the "getter" and "setter" +methods of Value operate on the largest type that can +hold the value: int64 for all the signed integers, for +instance. That is, the Int method of +Value returns an int64 and the +SetInt value takes an int64; it may be +necessary to convert to the actual type involved: +

+ +{{code "/doc/progs/interface2.go" `/START f2/` `/STOP/`}} + +

+The second property is that the Kind of a reflection +object describes the underlying type, not the static type. If a +reflection object contains a value of a user-defined integer type, +as in +

+ +{{code "/doc/progs/interface2.go" `/START f3/` `/STOP/`}} + +

+the Kind of v is still +reflect.Int, even though the static type of +x is MyInt, not int. In +other words, the Kind cannot discriminate an int from +a MyInt even though the Type can. +

+ +

The second law of reflection

+ +

2. Reflection goes from reflection object to interface +value.

+ +

+Like physical reflection, reflection in Go generates its own +inverse. +

+ +

+Given a reflect.Value we can recover an interface +value using the Interface method; in effect the method +packs the type and value information back into an interface +representation and returns the result: +

+ +
+// Interface returns v's value as an interface{}.
+func (v Value) Interface() interface{}
+
+ +

+As a consequence we can say +

+ +{{code "/doc/progs/interface2.go" `/START f3b/` `/STOP/`}} + +

+to print the float64 value represented by the +reflection object v. +

+ +

+We can do even better, though. The arguments to +fmt.Println, fmt.Printf and so on are all +passed as empty interface values, which are then unpacked by the +fmt package internally just as we have been doing in +the previous examples. Therefore all it takes to print the contents +of a reflect.Value correctly is to pass the result of +the Interface method to the formatted print +routine: +

+ +{{code "/doc/progs/interface2.go" `/START f3c/` `/STOP/`}} + +

+(Why not fmt.Println(v)? Because v is a +reflect.Value; we want the concrete value it holds.) +Since our value is a float64, we can even use a +floating-point format if we want: +

+ +{{code "/doc/progs/interface2.go" `/START f3d/` `/STOP/`}} + +

+and get in this case +

+ +
+3.4e+00
+
+ +

+Again, there's no need to type-assert the result of +v.Interface() to float64; the empty +interface value has the concrete value's type information inside +and Printf will recover it. +

+ +

+In short, the Interface method is the inverse of the +ValueOf function, except that its result is always of +static type interface{}. +

+ +

+Reiterating: Reflection goes from interface values to reflection +objects and back again. +

+ +

The third law of reflection

+ +

3. To modify a reflection object, the value must be settable.

+ +

+The third law is the most subtle and confusing, but it's easy +enough to understand if we start from first principles. +

+ +

+Here is some code that does not work, but is worth studying. +

+ +{{code "/doc/progs/interface2.go" `/START f4/` `/STOP/`}} + +

+If you run this code, it will panic with the cryptic message +

+ +
+panic: reflect.Value.SetFloat using unaddressable value
+
+ +

+The problem is not that the value 7.1 is not +addressable; it's that v is not settable. Settability +is a property of a reflection Value, and not all +reflection Values have it. +

+ +

+The CanSet method of Value reports the +settability of a Value; in our case, +

+ +{{code "/doc/progs/interface2.go" `/START f5/` `/STOP/`}} + +

+prints +

+ +
+settability of v: false
+
+ +

+It is an error to call a Set method on an non-settable +Value. But what is settability? +

+ +

+Settability is a bit like addressability, but stricter. It's the +property that a reflection object can modify the actual storage +that was used to create the reflection object. Settability is +determined by whether the reflection object holds the original +item. When we say +

+ +{{code "/doc/progs/interface2.go" `/START f6/` `/STOP/`}} + +

+we pass a copy of x to +reflect.ValueOf, so the interface value created as the +argument to reflect.ValueOf is a copy of +x, not x itself. Thus, if the +statement +

+ +{{code "/doc/progs/interface2.go" `/START f6b/` `/STOP/`}} + +

+were allowed to succeed, it would not update x, even +though v looks like it was created from +x. Instead, it would update the copy of x +stored inside the reflection value and x itself would +be unaffected. That would be confusing and useless, so it is +illegal, and settability is the property used to avoid this +issue. +

+ +

+If this seems bizarre, it's not. It's actually a familiar situation +in unusual garb. Think of passing x to a +function: +

+ +
+f(x)
+
+ +

+We would not expect f to be able to modify +x because we passed a copy of x's value, +not x itself. If we want f to modify +x directly we must pass our function the address of +x (that is, a pointer to x):

+ +

+f(&x) +

+ +

+This is straightforward and familiar, and reflection works the same +way. If we want to modify x by reflection, we must +give the reflection library a pointer to the value we want to +modify. +

+ +

+Let's do that. First we initialize x as usual +and then create a reflection value that points to it, called +p. +

+ +{{code "/doc/progs/interface2.go" `/START f7/` `/STOP/`}} + +

+The output so far is +

+ +
+type of p: *float64
+settability of p: false
+
+ +

+The reflection object p isn't settable, but it's not +p we want to set, it's (in effect) *p. To +get to what p points to, we call the Elem +method of Value, which indirects through the pointer, +and save the result in a reflection Value called +v: +

+ +{{code "/doc/progs/interface2.go" `/START f7b/` `/STOP/`}} + +

+Now v is a settable reflection object, as the output +demonstrates, +

+ +
+settability of v: true
+
+ +

+and since it represents x, we are finally able to use +v.SetFloat to modify the value of +x: +

+ +{{code "/doc/progs/interface2.go" `/START f7c/` `/STOP/`}} + +

+The output, as expected, is +

+ +
+7.1
+7.1
+
+ +

+Reflection can be hard to understand but it's doing exactly what +the language does, albeit through reflection Types and +Values that can disguise what's going on. Just keep in +mind that reflection Values need the address of something in order +to modify what they represent. +

+ +

Structs

+ +

+In our previous example v wasn't a pointer itself, it +was just derived from one. A common way for this situation to arise +is when using reflection to modify the fields of a structure. As +long as we have the address of the structure, we can modify its +fields. +

+ +

+Here's a simple example that analyzes a struct value, t. We create +the reflection object with the address of the struct because we'll want to +modify it later. Then we set typeOfT to its type and iterate over +the fields using straightforward method calls +(see package reflect for details). +Note that we extract the names of the fields from the struct type, but the +fields themselves are regular reflect.Value objects. +

+ +{{code "/doc/progs/interface2.go" `/START f8/` `/STOP/`}} + +

+The output of this program is +

+ +
+0: A int = 23
+1: B string = skidoo
+
+ +

+There's one more point about settability introduced in +passing here: the field names of T are upper case +(exported) because only exported fields of a struct are +settable. +

+ +

+Because s contains a settable reflection object, we +can modify the fields of the structure. +

+ +{{code "/doc/progs/interface2.go" `/START f8b/` `/STOP/`}} + +

+And here's the result: +

+ +
+t is now {77 Sunset Strip}
+
+ +

+If we modified the program so that s was created from +t, not &t, the calls to +SetInt and SetString would fail as the +fields of t would not be settable. +

+ +

Conclusion

+ +

+Here again are the laws of reflection: +

+ +
    +
  1. Reflection goes from interface value to reflection +object.
    2. +
  2. Reflection goes from reflection object to interface +value.
    3. +
  3. To modify a reflection object, the value must be settable.
    4. +
+ +

+Once you understand these laws reflection in Go becomes much easier +to use, although it remains subtle. It's a powerful tool that +should be used with care and avoided unless strictly +necessary. +

+ +

+There's plenty more to reflection that we haven't covered — +sending and receiving on channels, allocating memory, using slices +and maps, calling methods and functions — but this post is +long enough. We'll cover some of those topics in a later +article. +

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+Go's slice type provides a convenient and efficient means of working with +sequences of typed data. Slices are analogous to arrays in other languages, but +have some unusual properties. This article will look at what slices are and how +they are used. +

+ +

+Arrays +

+ +

+The slice type is an abstraction built on top of Go's array type, and so to +understand slices we must first understand arrays. +

+ +

+An array type definition specifies a length and an element type. For example, +the type [4]int represents an array of four integers. An array's +size is fixed; its length is part of its type ([4]int and +[5]int are distinct, incompatible types). Arrays can be indexed in +the usual way, so the expression s[n] accesses the nth +element: +

+ +
+var a [4]int
+a[0] = 1
+i := a[0]
+// i == 1
+
+ +

+Arrays do not need to be initialized explicitly; the zero value of an array is +a ready-to-use array whose elements are themselves zeroed: +

+ +
+// a[2] == 0, the zero value of the int type
+
+ +

+The in-memory representation of [4]int is just four integer values laid out sequentially: +

+ +

+ +

+ +

+Go's arrays are values. An array variable denotes the entire array; it is not a +pointer to the first array element (as would be the case in C). This means +that when you assign or pass around an array value you will make a copy of its +contents. (To avoid the copy you could pass a pointer to the array, but +then that's a pointer to an array, not an array.) One way to think about arrays +is as a sort of struct but with indexed rather than named fields: a fixed-size +composite value. +

+ +

+An array literal can be specified like so: +

+ +
+b := [2]string{"Penn", "Teller"}
+
+ +

+Or, you can have the compiler count the array elements for you: +

+ +
+b := [...]string{"Penn", "Teller"}
+
+ +

+In both cases, the type of b is [2]string. +

+ +

+Slices +

+ +

+Arrays have their place, but they're a bit inflexible, so you don't see them +too often in Go code. Slices, though, are everywhere. They build on arrays to +provide great power and convenience. +

+ +

+The type specification for a slice is []T, where T is +the type of the elements of the slice. Unlike an array type, a slice type has +no specified length. +

+ +

+A slice literal is declared just like an array literal, except you leave out +the element count: +

+ +
+letters := []string{"a", "b", "c", "d"}
+
+ +

+A slice can be created with the built-in function called make, +which has the signature, +

+ +
+func make([]T, len, cap) []T
+
+ +

+where T stands for the element type of the slice to be created. The +make function takes a type, a length, and an optional capacity. +When called, make allocates an array and returns a slice that +refers to that array. +

+ +
+var s []byte
+s = make([]byte, 5, 5)
+// s == []byte{0, 0, 0, 0, 0}
+
+ +

+When the capacity argument is omitted, it defaults to the specified length. +Here's a more succinct version of the same code: +

+ +
+s := make([]byte, 5)
+
+ +

+The length and capacity of a slice can be inspected using the built-in +len and cap functions. +

+ +
+len(s) == 5
+cap(s) == 5
+
+ +

+The next two sections discuss the relationship between length and capacity. +

+ +

+The zero value of a slice is nil. The len and +cap functions will both return 0 for a nil slice. +

+ +

+A slice can also be formed by "slicing" an existing slice or array. Slicing is +done by specifying a half-open range with two indices separated by a colon. For +example, the expression b[1:4] creates a slice including elements +1 through 3 of b (the indices of the resulting slice will be 0 +through 2). +

+ +
+b := []byte{'g', 'o', 'l', 'a', 'n', 'g'}
+// b[1:4] == []byte{'o', 'l', 'a'}, sharing the same storage as b
+
+ +

+The start and end indices of a slice expression are optional; they default to zero and the slice's length respectively: +

+ +
+// b[:2] == []byte{'g', 'o'}
+// b[2:] == []byte{'l', 'a', 'n', 'g'}
+// b[:] == b
+
+ +

+This is also the syntax to create a slice given an array: +

+ +
+x := [3]string{"Лайка", "Белка", "Стрелка"}
+s := x[:] // a slice referencing the storage of x
+
+ +

+Slice internals +

+ +

+A slice is a descriptor of an array segment. It consists of a pointer to the +array, the length of the segment, and its capacity (the maximum length of the +segment). +

+ +

+ +

+ +

+Our variable s, created earlier by make([]byte, 5), +is structured like this: +

+ +

+ +

+ +

+The length is the number of elements referred to by the slice. The capacity is +the number of elements in the underlying array (beginning at the element +referred to by the slice pointer). The distinction between length and capacity +will be made clear as we walk through the next few examples. +

+ +

+As we slice s, observe the changes in the slice data structure and +their relation to the underlying array: +

+ +
+s = s[2:4]
+
+ +

+ +

+ +

+Slicing does not copy the slice's data. It creates a new slice value that +points to the original array. This makes slice operations as efficient as +manipulating array indices. Therefore, modifying the elements (not the +slice itself) of a re-slice modifies the elements of the original slice: +

+ +
+d := []byte{'r', 'o', 'a', 'd'}
+e := d[2:] 
+// e == []byte{'a', 'd'}
+e[1] == 'm'
+// e == []byte{'a', 'm'}
+// d == []byte{'r', 'o', 'a', 'm'}
+
+ +

+Earlier we sliced s to a length shorter than its capacity. We can +grow s to its capacity by slicing it again: +

+ +
+s = s[:cap(s)]
+
+ +

+ +

+ +

+A slice cannot be grown beyond its capacity. Attempting to do so will cause a +runtime panic, just as when indexing outside the bounds of a slice or array. +Similarly, slices cannot be re-sliced below zero to access earlier elements in +the array. +

+ +

+Growing slices (the copy and append functions) +

+ +

+To increase the capacity of a slice one must create a new, larger slice and +copy the contents of the original slice into it. This technique is how dynamic +array implementations from other languages work behind the scenes. The next +example doubles the capacity of s by making a new slice, +t, copying the contents of s into t, and +then assigning the slice value t to s: +

+ +
+t := make([]byte, len(s), (cap(s)+1)*2) // +1 in case cap(s) == 0
+for i := range s {
+        t[i] = s[i]
+}
+s = t
+
+ +

+The looping piece of this common operation is made easier by the built-in copy +function. As the name suggests, copy copies data from a source slice to a +destination slice. It returns the number of elements copied. +

+ +
+func copy(dst, src []T) int
+
+ +

+The copy function supports copying between slices of different +lengths (it will copy only up to the smaller number of elements). In addition, +copy can handle source and destination slices that share the same +underlying array, handling overlapping slices correctly. +

+ +

+Using copy, we can simplify the code snippet above: +

+ +
+t := make([]byte, len(s), (cap(s)+1)*2)
+copy(t, s)
+s = t
+
+ +

+A common operation is to append data to the end of a slice. This function +appends byte elements to a slice of bytes, growing the slice if necessary, and +returns the updated slice value: +

+ +{{code "/doc/progs/slices.go" `/AppendByte/` `/STOP/`}} + +

+One could use AppendByte like this: +

+ +
+p := []byte{2, 3, 5}
+p = AppendByte(p, 7, 11, 13)
+// p == []byte{2, 3, 5, 7, 11, 13}
+
+ +

+Functions like AppendByte are useful because they offer complete +control over the way the slice is grown. Depending on the characteristics of +the program, it may be desirable to allocate in smaller or larger chunks, or to +put a ceiling on the size of a reallocation. +

+ +

+But most programs don't need complete control, so Go provides a built-in +append function that's good for most purposes; it has the +signature +

+ +
+func append(s []T, x ...T) []T 
+
+ +

+The append function appends the elements x to the end +of the slice s, and grows the slice if a greater capacity is +needed. +

+ +
+a := make([]int, 1)
+// a == []int{0}
+a = append(a, 1, 2, 3)
+// a == []int{0, 1, 2, 3}
+
+ +

+To append one slice to another, use ... to expand the second +argument to a list of arguments. +

+ +
+a := []string{"John", "Paul"}
+b := []string{"George", "Ringo", "Pete"}
+a = append(a, b...) // equivalent to "append(a, b[0], b[1], b[2])"
+// a == []string{"John", "Paul", "George", "Ringo", "Pete"}
+
+ +

+Since the zero value of a slice (nil) acts like a zero-length +slice, you can declare a slice variable and then append to it in a loop: +

+ +{{code "/doc/progs/slices.go" `/Filter/` `/STOP/`}} + +

+A possible "gotcha" +

+ +

+As mentioned earlier, re-slicing a slice doesn't make a copy of the underlying +array. The full array will be kept in memory until it is no longer referenced. +Occasionally this can cause the program to hold all the data in memory when +only a small piece of it is needed. +

+ +

+For example, this FindDigits function loads a file into memory and +searches it for the first group of consecutive numeric digits, returning them +as a new slice. +

+ +{{code "/doc/progs/slices.go" `/digit/` `/STOP/`}} + +

+This code behaves as advertised, but the returned []byte points +into an array containing the entire file. Since the slice references the +original array, as long as the slice is kept around the garbage collector can't +release the array; the few useful bytes of the file keep the entire contents in +memory. +

+ +

+To fix this problem one can copy the interesting data to a new slice before +returning it: +

+ +{{code "/doc/progs/slices.go" `/CopyDigits/` `/STOP/`}} + +

+A more concise version of this function could be constructed by using +append. This is left as an exercise for the reader. +

+ +

+Further Reading +

+ +

+Effective Go contains an +in-depth treatment of slices +and arrays, +and the Go language specification +defines slices and their +associated +helper +functions. +

diff --git a/doc/articles/wiki/Makefile b/doc/articles/wiki/Makefile new file mode 100644 index 000000000..0cb907185 --- /dev/null +++ b/doc/articles/wiki/Makefile @@ -0,0 +1,20 @@ +# Copyright 2010 The Go Authors. All rights reserved. +# Use of this source code is governed by a BSD-style +# license that can be found in the LICENSE file. + +all: index.html + +CLEANFILES:=srcextract.bin htmlify.bin get.bin + +index.html: wiki.html srcextract.bin htmlify.bin + PATH=.:$$PATH awk '/^!/{system(substr($$0,2)); next} {print}' < wiki.html | tr -d '\r' > index.html + +test: get.bin + bash ./test.sh + rm -f get.6 get.bin + +%.bin: %.go + go build -o $@ $^ + +clean: + rm -f $(CLEANFILES) diff --git a/doc/articles/wiki/edit.html b/doc/articles/wiki/edit.html new file mode 100644 index 000000000..c14953b17 --- /dev/null +++ b/doc/articles/wiki/edit.html @@ -0,0 +1,6 @@ +

Editing {{.Title |html}}

+ +
+
+
+
diff --git a/doc/articles/wiki/final-noclosure.go b/doc/articles/wiki/final-noclosure.go new file mode 100644 index 000000000..a23cf7a27 --- /dev/null +++ b/doc/articles/wiki/final-noclosure.go @@ -0,0 +1,104 @@ +// Copyright 2010 The Go Authors. All rights reserved. +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. + +package main + +import ( + "errors" + "html/template" + "io/ioutil" + "net/http" + "regexp" +) + +type Page struct { + Title string + Body []byte +} + +func (p *Page) save() error { + filename := p.Title + ".txt" + return ioutil.WriteFile(filename, p.Body, 0600) +} + +func loadPage(title string) (*Page, error) { + filename := title + ".txt" + body, err := ioutil.ReadFile(filename) + if err != nil { + return nil, err + } + return &Page{Title: title, Body: body}, nil +} + +func viewHandler(w http.ResponseWriter, r *http.Request) { + title, err := getTitle(w, r) + if err != nil { + return + } + p, err := loadPage(title) + if err != nil { + http.Redirect(w, r, "/edit/"+title, http.StatusFound) + return + } + renderTemplate(w, "view", p) +} + +func editHandler(w http.ResponseWriter, r *http.Request) { + title, err := getTitle(w, r) + if err != nil { + return + } + p, err := loadPage(title) + if err != nil { + p = &Page{Title: title} + } + renderTemplate(w, "edit", p) +} + +func saveHandler(w http.ResponseWriter, r *http.Request) { + title, err := getTitle(w, r) + if err != nil { + return + } + body := r.FormValue("body") + p := &Page{Title: title, Body: []byte(body)} + err = p.save() + if err != nil { + http.Error(w, err.Error(), http.StatusInternalServerError) + return + } + http.Redirect(w, r, "/view/"+title, http.StatusFound) +} + +func renderTemplate(w http.ResponseWriter, tmpl string, p *Page) { + t, err := template.ParseFiles(tmpl + ".html") + if err != nil { + http.Error(w, err.Error(), http.StatusInternalServerError) + return + } + err = t.Execute(w, p) + if err != nil { + http.Error(w, err.Error(), http.StatusInternalServerError) + } +} + +const lenPath = len("/view/") + +var titleValidator = regexp.MustCompile("^[a-zA-Z0-9]+$") + +func getTitle(w http.ResponseWriter, r *http.Request) (title string, err error) { + title = r.URL.Path[lenPath:] + if !titleValidator.MatchString(title) { + http.NotFound(w, r) + err = errors.New("Invalid Page Title") + } + return +} + +func main() { + http.HandleFunc("/view/", viewHandler) + http.HandleFunc("/edit/", editHandler) + http.HandleFunc("/save/", saveHandler) + http.ListenAndServe(":8080", nil) +} diff --git a/doc/articles/wiki/final-noerror.go b/doc/articles/wiki/final-noerror.go new file mode 100644 index 000000000..e11d268e2 --- /dev/null +++ b/doc/articles/wiki/final-noerror.go @@ -0,0 +1,55 @@ +// Copyright 2010 The Go Authors. All rights reserved. +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. + +package main + +import ( + "html/template" + "io/ioutil" + "net/http" +) + +type Page struct { + Title string + Body []byte +} + +func (p *Page) save() error { + filename := p.Title + ".txt" + return ioutil.WriteFile(filename, p.Body, 0600) +} + +func loadPage(title string) (*Page, error) { + filename := title + ".txt" + body, err := ioutil.ReadFile(filename) + if err != nil { + return nil, err + } + return &Page{Title: title, Body: body}, nil +} + +const lenPath = len("/view/") + +func editHandler(w http.ResponseWriter, r *http.Request) { + title := r.URL.Path[lenPath:] + p, err := loadPage(title) + if err != nil { + p = &Page{Title: title} + } + t, _ := template.ParseFiles("edit.html") + t.Execute(w, p) +} + +func viewHandler(w http.ResponseWriter, r *http.Request) { + title := r.URL.Path[lenPath:] + p, _ := loadPage(title) + t, _ := template.ParseFiles("view.html") + t.Execute(w, p) +} + +func main() { + http.HandleFunc("/view/", viewHandler) + http.HandleFunc("/edit/", editHandler) + http.ListenAndServe(":8080", nil) +} diff --git a/doc/articles/wiki/final-parsetemplate.go b/doc/articles/wiki/final-parsetemplate.go new file mode 100644 index 000000000..6234c08f2 --- /dev/null +++ b/doc/articles/wiki/final-parsetemplate.go @@ -0,0 +1,93 @@ +// Copyright 2010 The Go Authors. All rights reserved. +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. + +package main + +import ( + "html/template" + "io/ioutil" + "net/http" + "regexp" +) + +type Page struct { + Title string + Body []byte +} + +func (p *Page) save() error { + filename := p.Title + ".txt" + return ioutil.WriteFile(filename, p.Body, 0600) +} + +func loadPage(title string) (*Page, error) { + filename := title + ".txt" + body, err := ioutil.ReadFile(filename) + if err != nil { + return nil, err + } + return &Page{Title: title, Body: body}, nil +} + +func viewHandler(w http.ResponseWriter, r *http.Request, title string) { + p, err := loadPage(title) + if err != nil { + http.Redirect(w, r, "/edit/"+title, http.StatusFound) + return + } + renderTemplate(w, "view", p) +} + +func editHandler(w http.ResponseWriter, r *http.Request, title string) { + p, err := loadPage(title) + if err != nil { + p = &Page{Title: title} + } + renderTemplate(w, "edit", p) +} + +func saveHandler(w http.ResponseWriter, r *http.Request, title string) { + body := r.FormValue("body") + p := &Page{Title: title, Body: []byte(body)} + err := p.save() + if err != nil { + http.Error(w, err.Error(), http.StatusInternalServerError) + return + } + http.Redirect(w, r, "/view/"+title, http.StatusFound) +} + +func renderTemplate(w http.ResponseWriter, tmpl string, p *Page) { + t, err := template.ParseFiles(tmpl + ".html") + if err != nil { + http.Error(w, err.Error(), http.StatusInternalServerError) + return + } + err = t.Execute(w, p) + if err != nil { + http.Error(w, err.Error(), http.StatusInternalServerError) + } +} + +const lenPath = len("/view/") + +var titleValidator = regexp.MustCompile("^[a-zA-Z0-9]+$") + +func makeHandler(fn func(http.ResponseWriter, *http.Request, string)) http.HandlerFunc { + return func(w http.ResponseWriter, r *http.Request) { + title := r.URL.Path[lenPath:] + if !titleValidator.MatchString(title) { + http.NotFound(w, r) + return + } + fn(w, r, title) + } +} + +func main() { + http.HandleFunc("/view/", makeHandler(viewHandler)) + http.HandleFunc("/edit/", makeHandler(editHandler)) + http.HandleFunc("/save/", makeHandler(saveHandler)) + http.ListenAndServe(":8080", nil) +} diff --git a/doc/articles/wiki/final-template.go b/doc/articles/wiki/final-template.go new file mode 100644 index 000000000..f295b9d60 --- /dev/null +++ b/doc/articles/wiki/final-template.go @@ -0,0 +1,67 @@ +// Copyright 2010 The Go Authors. All rights reserved. +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. + +package main + +import ( + "html/template" + "io/ioutil" + "net/http" +) + +type Page struct { + Title string + Body []byte +} + +func (p *Page) save() error { + filename := p.Title + ".txt" + return ioutil.WriteFile(filename, p.Body, 0600) +} + +func loadPage(title string) (*Page, error) { + filename := title + ".txt" + body, err := ioutil.ReadFile(filename) + if err != nil { + return nil, err + } + return &Page{Title: title, Body: body}, nil +} + +const lenPath = len("/view/") + +func editHandler(w http.ResponseWriter, r *http.Request) { + title := r.URL.Path[lenPath:] + p, err := loadPage(title) + if err != nil { + p = &Page{Title: title} + } + renderTemplate(w, "edit", p) +} + +func viewHandler(w http.ResponseWriter, r *http.Request) { + title := r.URL.Path[lenPath:] + p, _ := loadPage(title) + renderTemplate(w, "view", p) +} + +func saveHandler(w http.ResponseWriter, r *http.Request) { + title := r.URL.Path[lenPath:] + body := r.FormValue("body") + p := &Page{Title: title, Body: []byte(body)} + p.save() + http.Redirect(w, r, "/view/"+title, http.StatusFound) +} + +func renderTemplate(w http.ResponseWriter, tmpl string, p *Page) { + t, _ := template.ParseFiles(tmpl + ".html") + t.Execute(w, p) +} + +func main() { + http.HandleFunc("/view/", viewHandler) + http.HandleFunc("/edit/", editHandler) + http.HandleFunc("/save/", saveHandler) + http.ListenAndServe(":8080", nil) +} diff --git a/doc/articles/wiki/final.go b/doc/articles/wiki/final.go new file mode 100644 index 000000000..e93cdee47 --- /dev/null +++ b/doc/articles/wiki/final.go @@ -0,0 +1,90 @@ +// Copyright 2010 The Go Authors. All rights reserved. +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. + +package main + +import ( + "html/template" + "io/ioutil" + "net/http" + "regexp" +) + +type Page struct { + Title string + Body []byte +} + +func (p *Page) save() error { + filename := p.Title + ".txt" + return ioutil.WriteFile(filename, p.Body, 0600) +} + +func loadPage(title string) (*Page, error) { + filename := title + ".txt" + body, err := ioutil.ReadFile(filename) + if err != nil { + return nil, err + } + return &Page{Title: title, Body: body}, nil +} + +func viewHandler(w http.ResponseWriter, r *http.Request, title string) { + p, err := loadPage(title) + if err != nil { + http.Redirect(w, r, "/edit/"+title, http.StatusFound) + return + } + renderTemplate(w, "view", p) +} + +func editHandler(w http.ResponseWriter, r *http.Request, title string) { + p, err := loadPage(title) + if err != nil { + p = &Page{Title: title} + } + renderTemplate(w, "edit", p) +} + +func saveHandler(w http.ResponseWriter, r *http.Request, title string) { + body := r.FormValue("body") + p := &Page{Title: title, Body: []byte(body)} + err := p.save() + if err != nil { + http.Error(w, err.Error(), http.StatusInternalServerError) + return + } + http.Redirect(w, r, "/view/"+title, http.StatusFound) +} + +var templates = template.Must(template.ParseFiles("edit.html", "view.html")) + +func renderTemplate(w http.ResponseWriter, tmpl string, p *Page) { + err := templates.ExecuteTemplate(w, tmpl+".html", p) + if err != nil { + http.Error(w, err.Error(), http.StatusInternalServerError) + } +} + +const lenPath = len("/view/") + +var titleValidator = regexp.MustCompile("^[a-zA-Z0-9]+$") + +func makeHandler(fn func(http.ResponseWriter, *http.Request, string)) http.HandlerFunc { + return func(w http.ResponseWriter, r *http.Request) { + title := r.URL.Path[lenPath:] + if !titleValidator.MatchString(title) { + http.NotFound(w, r) + return + } + fn(w, r, title) + } +} + +func main() { + http.HandleFunc("/view/", makeHandler(viewHandler)) + http.HandleFunc("/edit/", makeHandler(editHandler)) + http.HandleFunc("/save/", makeHandler(saveHandler)) + http.ListenAndServe(":8080", nil) +} diff --git a/doc/articles/wiki/get.go b/doc/articles/wiki/get.go new file mode 100644 index 000000000..c6e9bf28b --- /dev/null +++ b/doc/articles/wiki/get.go @@ -0,0 +1,54 @@ +// Copyright 2011 The Go Authors. All rights reserved. +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. + +package main + +import ( + "flag" + "fmt" + "io" + "log" + "net" + "net/http" + "os" + "strings" +) + +var ( + post = flag.String("post", "", "urlencoded form data to POST") + addr = flag.Bool("addr", false, "find open address and print to stdout") +) + +func main() { + flag.Parse() + if *addr { + l, err := net.Listen("tcp", "127.0.0.1:0") + if err != nil { + log.Fatal(err) + } + defer l.Close() + fmt.Print(l.Addr()) + return + } + url := flag.Arg(0) + if url == "" { + log.Fatal("no url supplied") + } + var r *http.Response + var err error + if *post != "" { + b := strings.NewReader(*post) + r, err = http.Post(url, "application/x-www-form-urlencoded", b) + } else { + r, err = http.Get(url) + } + if err != nil { + log.Fatal(err) + } + defer r.Body.Close() + _, err = io.Copy(os.Stdout, r.Body) + if err != nil { + log.Fatal(err) + } +} diff --git a/doc/articles/wiki/htmlify.go b/doc/articles/wiki/htmlify.go new file mode 100644 index 000000000..2a845a174 --- /dev/null +++ b/doc/articles/wiki/htmlify.go @@ -0,0 +1,16 @@ +// Copyright 2010 The Go Authors. All rights reserved. +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. + +package main + +import ( + "io/ioutil" + "os" + "text/template" +) + +func main() { + b, _ := ioutil.ReadAll(os.Stdin) + template.HTMLEscape(os.Stdout, b) +} diff --git a/doc/articles/wiki/http-sample.go b/doc/articles/wiki/http-sample.go new file mode 100644 index 000000000..ac8cc4f2d --- /dev/null +++ b/doc/articles/wiki/http-sample.go @@ -0,0 +1,15 @@ +package main + +import ( + "fmt" + "net/http" +) + +func handler(w http.ResponseWriter, r *http.Request) { + fmt.Fprintf(w, "Hi there, I love %s!", r.URL.Path[1:]) +} + +func main() { + http.HandleFunc("/", handler) + http.ListenAndServe(":8080", nil) +} diff --git a/doc/articles/wiki/index.html b/doc/articles/wiki/index.html new file mode 100644 index 000000000..52bf7e798 --- /dev/null +++ b/doc/articles/wiki/index.html @@ -0,0 +1,721 @@ + + +

Introduction

+ +

+Covered in this tutorial: +

+ + +

+Assumed knowledge: +

+ + +

Getting Started

+ +

+At present, you need to have a FreeBSD, Linux, OS X, or Windows machine to run Go. +We will use $ to represent the command prompt. +

+ +

+Install Go (see the Installation Instructions). +

+ +

+Make a new directory for this tutorial inside your GOPATH and cd to it: +

+ +
+$ mkdir gowiki
+$ cd gowiki
+
+ +

+Create a file named wiki.go, open it in your favorite editor, and +add the following lines: +

+ +
+package main
+
+import (
+	"fmt"
+	"io/ioutil"
+)
+
+ +

+We import the fmt and ioutil packages from the Go +standard library. Later, as we implement additional functionality, we will +add more packages to this import declaration. +

+ +

Data Structures

+ +

+Let's start by defining the data structures. A wiki consists of a series of +interconnected pages, each of which has a title and a body (the page content). +Here, we define Page as a struct with two fields representing +the title and body. +

+ +{{code "doc/articles/wiki/part1.go" `/^type Page/` `/}/`}} + +

+The type []byte means "a byte slice". +(See Slices: usage and +internals for more on slices.) +The Body element is a []byte rather than +string because that is the type expected by the io +libraries we will use, as you'll see below. +

+ +

+The Page struct describes how page data will be stored in memory. +But what about persistent storage? We can address that by creating a +save method on Page: +

+ +{{code "doc/articles/wiki/part1.go" `/^func.*Page.*save/` `/}/`}} + +

+This method's signature reads: "This is a method named save that +takes as its receiver p, a pointer to Page . It takes +no parameters, and returns a value of type error." +

+ +

+This method will save the Page's Body to a text +file. For simplicity, we will use the Title as the file name. +

+ +

+The save method returns an error value because +that is the return type of WriteFile (a standard library function +that writes a byte slice to a file). The save method returns the +error value, to let the application handle it should anything go wrong while +writing the file. If all goes well, Page.save() will return +nil (the zero-value for pointers, interfaces, and some other +types). +

+ +

+The octal integer constant 0600, passed as the third parameter to +WriteFile, indicates that the file should be created with +read-write permissions for the current user only. (See the Unix man page +open(2) for details.) +

+ +

+We will want to load pages, too: +

+ +{{code "doc/articles/wiki/part1-noerror.go" `/^func loadPage/` `/^}/`}} + +

+The function loadPage constructs the file name from +Title, reads the file's contents into a new +Page, and returns a pointer to that new page. +

+ +

+Functions can return multiple values. The standard library function +io.ReadFile returns []byte and error. +In loadPage, error isn't being handled yet; the "blank identifier" +represented by the underscore (_) symbol is used to throw away the +error return value (in essence, assigning the value to nothing). +

+ +

+But what happens if ReadFile encounters an error? For example, +the file might not exist. We should not ignore such errors. Let's modify the +function to return *Page and error. +

+ +{{code "doc/articles/wiki/part1.go" `/^func loadPage/` `/^}/`}} + +

+Callers of this function can now check the second parameter; if it is +nil then it has successfully loaded a Page. If not, it will be an +error that can be handled by the caller (see the +language specification for details). +

+ +

+At this point we have a simple data structure and the ability to save to and +load from a file. Let's write a main function to test what we've +written: +

+ +{{code "doc/articles/wiki/part1.go" `/^func main/` `/^}/`}} + +

+After compiling and executing this code, a file named TestPage.txt +would be created, containing the contents of p1. The file would +then be read into the struct p2, and its Body element +printed to the screen. +

+ +

+You can compile and run the program like this: +

+ +
+$ go build wiki.go
+$ ./wiki
+This is a sample page.
+
+ +

+(If you're using Windows you must type "wiki" without the +"./" to run the program.) +

+ +

+Click here to view the code we've written so far. +

+ +

Introducing the net/http package (an interlude)

+ +

+Here's a full working example of a simple web server: +

+ +{{code "doc/articles/wiki/http-sample.go"}} + +

+The main function begins with a call to +http.HandleFunc, which tells the http package to +handle all requests to the web root ("/") with +handler. +

+ +

+It then calls http.ListenAndServe, specifying that it should +listen on port 8080 on any interface (":8080"). (Don't +worry about its second parameter, nil, for now.) +This function will block until the program is terminated. +

+ +

+The function handler is of the type http.HandlerFunc. +It takes an http.ResponseWriter and an http.Request as +its arguments. +

+ +

+An http.ResponseWriter value assembles the HTTP server's response; by writing +to it, we send data to the HTTP client. +

+ +

+An http.Request is a data structure that represents the client +HTTP request. The string r.URL.Path is the path component +of the request URL. The trailing [1:] means +"create a sub-slice of Path from the 1st character to the end." +This drops the leading "/" from the path name. +

+ +

+If you run this program and access the URL: +

+
http://localhost:8080/monkeys
+

+the program would present a page containing: +

+
Hi there, I love monkeys!
+ +

Using net/http to serve wiki pages

+ +

+To use the net/http package, it must be imported: +

+ +
+import (
+	"fmt"
+	"net/http"
+	"io/ioutil"
+)
+
+ +

+Let's create a handler to view a wiki page: +

+ +{{code "doc/articles/wiki/part2.go" `/^const lenPath/`}} + +{{code "doc/articles/wiki/part2.go" `/^func viewHandler/` `/^}/`}} + +

+First, this function extracts the page title from r.URL.Path, +the path component of the request URL. The global constant +lenPath is the length of the leading "/view/" +component of the request path. +The Path is re-sliced with [lenPath:] to drop the +first 6 characters of the string. This is because the path will invariably +begin with "/view/", which is not part of the page title. +

+ +

+The function then loads the page data, formats the page with a string of simple +HTML, and writes it to w, the http.ResponseWriter. +

+ +

+Again, note the use of _ to ignore the error +return value from loadPage. This is done here for simplicity +and generally considered bad practice. We will attend to this later. +

+ +

+To use this handler, we create a main function that +initializes http using the viewHandler to handle +any requests under the path /view/. +

+ +{{code "doc/articles/wiki/part2.go" `/^func main/` `/^}/`}} + +

+Click here to view the code we've written so far. +

+ +

+Let's create some page data (as test.txt), compile our code, and +try serving a wiki page. +

+ +

+Open test.txt file in your editor, and save the string "Hello world" (without quotes) +in it. +

+ +
+$ go build wiki.go
+$ ./wiki
+
+ +

+With this web server running, a visit to http://localhost:8080/view/test +should show a page titled "test" containing the words "Hello world". +

+ +

Editing Pages

+ +

+A wiki is not a wiki without the ability to edit pages. Let's create two new +handlers: one named editHandler to display an 'edit page' form, +and the other named saveHandler to save the data entered via the +form. +

+ +

+First, we add them to main(): +

+ +{{code "doc/articles/wiki/final-noclosure.go" `/^func main/` `/^}/`}} + +

+The function editHandler loads the page +(or, if it doesn't exist, create an empty Page struct), +and displays an HTML form. +

+ +{{code "doc/articles/wiki/notemplate.go" `/^func editHandler/` `/^}/`}} + +

+This function will work fine, but all that hard-coded HTML is ugly. +Of course, there is a better way. +

+ +

The html/template package

+ +

+The html/template package is part of the Go standard library. +We can use html/template to keep the HTML in a separate file, +allowing us to change the layout of our edit page without modifying the +underlying Go code. +

+ +

+First, we must add html/template to the list of imports: +

+ +
+import (
+	"http"
+	"io/ioutil"
+	"os"
+	"html/template"
+)
+
+ +

+Let's create a template file containing the HTML form. +Open a new file named edit.html, and add the following lines: +

+ +{{code "doc/articles/wiki/edit.html"}} + +

+Modify editHandler to use the template, instead of the hard-coded +HTML: +

+ +{{code "doc/articles/wiki/final-noerror.go" `/^func editHandler/` `/^}/`}} + +

+The function template.ParseFiles will read the contents of +edit.html and return a *template.Template. +

+ +

+The method t.Execute executes the template, writing the +generated HTML to the http.ResponseWriter. +The .Title and .Body dotted identifiers refer to +p.Title and p.Body. +

+ +

+Template directives are enclosed in double curly braces. +The printf "%s" .Body instruction is a function call +that outputs .Body as a string instead of a stream of bytes, +the same as a call to fmt.Printf. +The |html part of each directive pipes the value through the +html formatter before outputting it, which escapes HTML +characters (such as replacing > with &gt;), +preventing user data from corrupting the form HTML. +

+ +

+Now that we've removed the fmt.Fprintf statement, we can remove +"fmt" from the import list. +

+ +

+While we're working with templates, let's create a template for our +viewHandler called view.html: +

+ +{{code "doc/articles/wiki/view.html"}} + +

+Modify viewHandler accordingly: +

+ +{{code "doc/articles/wiki/final-noerror.go" `/^func viewHandler/` `/^}/`}} + +

+Notice that we've used almost exactly the same templating code in both +handlers. Let's remove this duplication by moving the templating code +to its own function: +

+ +{{code "doc/articles/wiki/final-template.go" `/^func viewHandler/` `/^}/`}} +{{code "doc/articles/wiki/final-template.go" `/^func editHandler/` `/^}/`}} +{{code "doc/articles/wiki/final-template.go" `/^func renderTemplate/` `/^}/`}} + +

+The handlers are now shorter and simpler. +

+ +

Handling non-existent pages

+ +

+What if you visit +/view/APageThatDoesntExist? The program will crash. This is +because it ignores the error return value from loadPage. Instead, +if the requested Page doesn't exist, it should redirect the client to the edit +Page so the content may be created: +

+ +{{code "doc/articles/wiki/final-noclosure.go" `/^func viewHandler/` `/^}/`}} + +

+The http.Redirect function adds an HTTP status code of +http.StatusFound (302) and a Location +header to the HTTP response. +

+ +

Saving Pages

+ +

+The function saveHandler will handle the form submission. +

+ +{{code "doc/articles/wiki/final-template.go" `/^func saveHandler/` `/^}/`}} + +

+The page title (provided in the URL) and the form's only field, +Body, are stored in a new Page. +The save() method is then called to write the data to a file, +and the client is redirected to the /view/ page. +

+ +

+The value returned by FormValue is of type string. +We must convert that value to []byte before it will fit into +the Page struct. We use []byte(body) to perform +the conversion. +

+ +

Error handling

+ +

+There are several places in our program where errors are being ignored. This +is bad practice, not least because when an error does occur the program will +crash. A better solution is to handle the errors and return an error message +to the user. That way if something does go wrong, the server will continue to +function and the user will be notified. +

+ +

+First, let's handle the errors in renderTemplate: +

+ +{{code "doc/articles/wiki/final-parsetemplate.go" `/^func renderTemplate/` `/^}/`}} + +

+The http.Error function sends a specified HTTP response code +(in this case "Internal Server Error") and error message. +Already the decision to put this in a separate function is paying off. +

+ +

+Now let's fix up saveHandler: +

+ +{{code "doc/articles/wiki/final-noclosure.go" `/^func saveHandler/` `/^}/`}} + +

+Any errors that occur during p.save() will be reported +to the user. +

+ +

Template caching

+ +

+There is an inefficiency in this code: renderTemplate calls +ParseFiles every time a page is rendered. +A better approach would be to call ParseFiles once at program +initialization, parsing all templates into a single *Template. +Then we can use the +ExecuteTemplate +method to render a specific template. +

+ +

+First we create a global variable named templates, and initialize +it with ParseFiles. +

+ +{{code "doc/articles/wiki/final.go" `/var templates/`}} + +

+The function template.Must is a convenience wrapper that panics +when passed a non-nil error value, and otherwise returns the +*Template unaltered. A panic is appropriate here; if the templates +can't be loaded the only sensible thing to do is exit the program. +

+ +

+A for loop is used with a range statement to iterate +over an array constant containing the names of the templates we want parsed. +If we were to add more templates to our program, we would add their names to +that array. +

+ +

+We then modify the renderTemplate function to call the +templates.ExecuteTemplate method with the name of the appropriate +template: +

+ +{{code "doc/articles/wiki/final.go" `/func renderTemplate/` `/^}/`}} + +

+Note that the template name is the template file name, so we must +append ".html" to the tmpl argument. +

+ +

Validation

+ +

+As you may have observed, this program has a serious security flaw: a user +can supply an arbitrary path to be read/written on the server. To mitigate +this, we can write a function to validate the title with a regular expression. +

+ +

+First, add "regexp" to the import list. +Then we can create a global variable to store our validation regexp: +

+ +{{code "doc/articles/wiki/final-noclosure.go" `/^var titleValidator/`}} + +

+The function regexp.MustCompile will parse and compile the +regular expression, and return a regexp.Regexp. +MustCompile is distinct from Compile in that it will +panic if the expression compilation fails, while Compile returns +an error as a second parameter. +

+ +

+Now, let's write a function that extracts the title string from the request +URL, and tests it against our TitleValidator expression: +

+ +{{code "doc/articles/wiki/final-noclosure.go" `/func getTitle/` `/^}/`}} + +

+If the title is valid, it will be returned along with a nil +error value. If the title is invalid, the function will write a +"404 Not Found" error to the HTTP connection, and return an error to the +handler. +

+ +

+Let's put a call to getTitle in each of the handlers: +

+ +{{code "doc/articles/wiki/final-noclosure.go" `/^func viewHandler/` `/^}/`}} +{{code "doc/articles/wiki/final-noclosure.go" `/^func editHandler/` `/^}/`}} +{{code "doc/articles/wiki/final-noclosure.go" `/^func saveHandler/` `/^}/`}} + +

Introducing Function Literals and Closures

+ +

+Catching the error condition in each handler introduces a lot of repeated code. +What if we could wrap each of the handlers in a function that does this +validation and error checking? Go's +function +literals provide a powerful means of abstracting functionality +that can help us here. +

+ +

+First, we re-write the function definition of each of the handlers to accept +a title string: +

+ +
+func viewHandler(w http.ResponseWriter, r *http.Request, title string)
+func editHandler(w http.ResponseWriter, r *http.Request, title string)
+func saveHandler(w http.ResponseWriter, r *http.Request, title string)
+
+ +

+Now let's define a wrapper function that takes a function of the above +type, and returns a function of type http.HandlerFunc +(suitable to be passed to the function http.HandleFunc): +

+ +
+func makeHandler(fn func (http.ResponseWriter, *http.Request, string)) http.HandlerFunc {
+	return func(w http.ResponseWriter, r *http.Request) {
+		// Here we will extract the page title from the Request,
+		// and call the provided handler 'fn'
+	}
+}
+
+ +

+The returned function is called a closure because it encloses values defined +outside of it. In this case, the variable fn (the single argument +to makeHandler) is enclosed by the closure. The variable +fn will be one of our save, edit, or view handlers. +

+ +

+Now we can take the code from getTitle and use it here +(with some minor modifications): +

+ +{{code "doc/articles/wiki/final.go" `/func makeHandler/` `/^}/`}} + +

+The closure returned by makeHandler is a function that takes +an http.ResponseWriter and http.Request (in other +words, an http.HandlerFunc). +The closure extracts the title from the request path, and +validates it with the TitleValidator regexp. If the +title is invalid, an error will be written to the +ResponseWriter using the http.NotFound function. +If the title is valid, the enclosed handler function +fn will be called with the ResponseWriter, +Request, and title as arguments. +

+ +

+Now we can wrap the handler functions with makeHandler in +main, before they are registered with the http +package: +

+ +{{code "doc/articles/wiki/final.go" `/func main/` `/^}/`}} + +

+Finally we remove the calls to getTitle from the handler functions, +making them much simpler: +

+ +{{code "doc/articles/wiki/final.go" `/^func viewHandler/` `/^}/`}} +{{code "doc/articles/wiki/final.go" `/^func editHandler/` `/^}/`}} +{{code "doc/articles/wiki/final.go" `/^func saveHandler/` `/^}/`}} + +

Try it out!

+ +

+Click here to view the final code listing. +

+ +

+Recompile the code, and run the app: +

+ +
+$ go build wiki.go
+$ ./wiki
+
+ +

+Visiting http://localhost:8080/view/ANewPage +should present you with the page edit form. You should then be able to +enter some text, click 'Save', and be redirected to the newly created page. +

+ +

Other tasks

+ +

+Here are some simple tasks you might want to tackle on your own: +

+ + diff --git a/doc/articles/wiki/notemplate.go b/doc/articles/wiki/notemplate.go new file mode 100644 index 000000000..33006ac95 --- /dev/null +++ b/doc/articles/wiki/notemplate.go @@ -0,0 +1,58 @@ +// Copyright 2010 The Go Authors. All rights reserved. +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. + +package main + +import ( + "fmt" + "io/ioutil" + "net/http" +) + +type Page struct { + Title string + Body []byte +} + +func (p *Page) save() error { + filename := p.Title + ".txt" + return ioutil.WriteFile(filename, p.Body, 0600) +} + +func loadPage(title string) (*Page, error) { + filename := title + ".txt" + body, err := ioutil.ReadFile(filename) + if err != nil { + return nil, err + } + return &Page{Title: title, Body: body}, nil +} + +const lenPath = len("/view/") + +func viewHandler(w http.ResponseWriter, r *http.Request) { + title := r.URL.Path[lenPath:] + p, _ := loadPage(title) + fmt.Fprintf(w, "

%s

%s
", p.Title, p.Body) +} + +func editHandler(w http.ResponseWriter, r *http.Request) { + title := r.URL.Path[lenPath:] + p, err := loadPage(title) + if err != nil { + p = &Page{Title: title} + } + fmt.Fprintf(w, "

Editing %s

"+ + "
"+ + "
"+ + ""+ + "
", + p.Title, p.Title, p.Body) +} + +func main() { + http.HandleFunc("/view/", viewHandler) + http.HandleFunc("/edit/", editHandler) + http.ListenAndServe(":8080", nil) +} diff --git a/doc/articles/wiki/part1-noerror.go b/doc/articles/wiki/part1-noerror.go new file mode 100644 index 000000000..7577b7b46 --- /dev/null +++ b/doc/articles/wiki/part1-noerror.go @@ -0,0 +1,33 @@ +// Copyright 2010 The Go Authors. All rights reserved. +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. + +package main + +import ( + "fmt" + "io/ioutil" +) + +type Page struct { + Title string + Body []byte +} + +func (p *Page) save() error { + filename := p.Title + ".txt" + return ioutil.WriteFile(filename, p.Body, 0600) +} + +func loadPage(title string) *Page { + filename := title + ".txt" + body, _ := ioutil.ReadFile(filename) + return &Page{Title: title, Body: body} +} + +func main() { + p1 := &Page{Title: "TestPage", Body: []byte("This is a sample page.")} + p1.save() + p2 := loadPage("TestPage") + fmt.Println(string(p2.Body)) +} diff --git a/doc/articles/wiki/part1.go b/doc/articles/wiki/part1.go new file mode 100644 index 000000000..d7bf1be97 --- /dev/null +++ b/doc/articles/wiki/part1.go @@ -0,0 +1,36 @@ +// Copyright 2010 The Go Authors. All rights reserved. +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. + +package main + +import ( + "fmt" + "io/ioutil" +) + +type Page struct { + Title string + Body []byte +} + +func (p *Page) save() error { + filename := p.Title + ".txt" + return ioutil.WriteFile(filename, p.Body, 0600) +} + +func loadPage(title string) (*Page, error) { + filename := title + ".txt" + body, err := ioutil.ReadFile(filename) + if err != nil { + return nil, err + } + return &Page{Title: title, Body: body}, nil +} + +func main() { + p1 := &Page{Title: "TestPage", Body: []byte("This is a sample Page.")} + p1.save() + p2, _ := loadPage("TestPage") + fmt.Println(string(p2.Body)) +} diff --git a/doc/articles/wiki/part2.go b/doc/articles/wiki/part2.go new file mode 100644 index 000000000..dd4365c82 --- /dev/null +++ b/doc/articles/wiki/part2.go @@ -0,0 +1,43 @@ +// Copyright 2010 The Go Authors. All rights reserved. +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. + +package main + +import ( + "fmt" + "io/ioutil" + "net/http" +) + +type Page struct { + Title string + Body []byte +} + +func (p *Page) save() error { + filename := p.Title + ".txt" + return ioutil.WriteFile(filename, p.Body, 0600) +} + +func loadPage(title string) (*Page, error) { + filename := title + ".txt" + body, err := ioutil.ReadFile(filename) + if err != nil { + return nil, err + } + return &Page{Title: title, Body: body}, nil +} + +const lenPath = len("/view/") + +func viewHandler(w http.ResponseWriter, r *http.Request) { + title := r.URL.Path[lenPath:] + p, _ := loadPage(title) + fmt.Fprintf(w, "

%s

%s
", p.Title, p.Body) +} + +func main() { + http.HandleFunc("/view/", viewHandler) + http.ListenAndServe(":8080", nil) +} diff --git a/doc/articles/wiki/srcextract.go b/doc/articles/wiki/srcextract.go new file mode 100644 index 000000000..813e25283 --- /dev/null +++ b/doc/articles/wiki/srcextract.go @@ -0,0 +1,76 @@ +// Copyright 2010 The Go Authors. All rights reserved. +// Use of this source code is governed by a BSD-style +// license that can be found in the LICENSE file. + +package main + +import ( + "bytes" + "flag" + "go/ast" + "go/parser" + "go/printer" + "go/token" + "log" + "os" + "text/template" +) + +var ( + srcFn = flag.String("src", "", "source filename") + getName = flag.String("name", "", "func/type name to output") + html = flag.Bool("html", true, "output HTML") + showPkg = flag.Bool("pkg", false, "show package in output") +) + +func main() { + // handle input + flag.Parse() + if *srcFn == "" || *getName == "" { + flag.Usage() + os.Exit(2) + } + // load file + fs := token.NewFileSet() + file, err := parser.ParseFile(fs, *srcFn, nil, 0) + if err != nil { + log.Fatal(err) + } + // create filter + filter := func(name string) bool { + return name == *getName + } + // filter + if !ast.FilterFile(file, filter) { + os.Exit(1) + } + // print the AST + var b bytes.Buffer + printer.Fprint(&b, fs, file) + // drop package declaration + if !*showPkg { + for { + c, err := b.ReadByte() + if c == '\n' || err != nil { + break + } + } + } + // drop leading newlines + for { + b, err := b.ReadByte() + if err != nil { + break + } + if b != '\n' { + os.Stdout.Write([]byte{b}) + break + } + } + // output + if *html { + template.HTMLEscape(os.Stdout, b.Bytes()) + } else { + b.WriteTo(os.Stdout) + } +} diff --git a/doc/articles/wiki/test.bash b/doc/articles/wiki/test.bash new file mode 100755 index 000000000..5c2cb60dc --- /dev/null +++ b/doc/articles/wiki/test.bash @@ -0,0 +1,30 @@ +#!/usr/bin/env bash +# Copyright 2010 The Go Authors. All rights reserved. +# Use of this source code is governed by a BSD-style +# license that can be found in the LICENSE file. + +set -e +wiki_pid= +cleanup() { + kill $wiki_pid + rm -f test_*.out Test.txt final-test.bin final-test.go +} +trap cleanup 0 INT + +go build -o get.bin get.go +addr=$(./get.bin -addr) +sed s/:8080/$addr/ < final.go > final-test.go +go build -o final-test.bin final-test.go +(./final-test.bin) & +wiki_pid=$! + +sleep 1 + +./get.bin http://$addr/edit/Test > test_edit.out +diff -u test_edit.out test_edit.good +./get.bin -post=body=some%20content http://$addr/save/Test +diff -u Test.txt test_Test.txt.good +./get.bin http://$addr/view/Test > test_view.out +diff -u test_view.out test_view.good + +echo PASS diff --git a/doc/articles/wiki/test_Test.txt.good b/doc/articles/wiki/test_Test.txt.good new file mode 100644 index 000000000..f0eec86f6 --- /dev/null +++ b/doc/articles/wiki/test_Test.txt.good @@ -0,0 +1 @@ +some content \ No newline at end of file diff --git a/doc/articles/wiki/test_edit.good b/doc/articles/wiki/test_edit.good new file mode 100644 index 000000000..36c6dbb73 --- /dev/null +++ b/doc/articles/wiki/test_edit.good @@ -0,0 +1,6 @@ +

Editing Test

+ +
+
+
+
diff --git a/doc/articles/wiki/test_view.good b/doc/articles/wiki/test_view.good new file mode 100644 index 000000000..07e8edb22 --- /dev/null +++ b/doc/articles/wiki/test_view.good @@ -0,0 +1,5 @@ +

Test

+ +

[edit]

+ +
some content
diff --git a/doc/articles/wiki/view.html b/doc/articles/wiki/view.html new file mode 100644 index 000000000..023391577 --- /dev/null +++ b/doc/articles/wiki/view.html @@ -0,0 +1,5 @@ +

{{.Title |html}}

+ +

[edit]

+ +
{{printf "%s" .Body |html}}
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