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Diffstat (limited to 'doc/articles')
24 files changed, 1975 insertions, 2003 deletions
diff --git a/doc/articles/c_go_cgo.html b/doc/articles/c_go_cgo.html new file mode 100644 index 000000000..ac6bb29a2 --- /dev/null +++ b/doc/articles/c_go_cgo.html @@ -0,0 +1,180 @@ +<!--{ +"Title": "C? Go? Cgo!", +"Template": true +}--> + +<p> +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. +</p> + +<p> +To lead with an example, here's a Go package that provides two functions - +<code>Random</code> and <code>Seed</code> - that wrap C's <code>random</code> +and <code>srandom</code> functions. +</p> + +{{code "/doc/progs/cgo1.go" `/package rand/` `/END/`}} + +<p> +Let's look at what's happening here, starting with the import statement. +</p> + +<p> +The <code>rand</code> package imports <code>"C"</code>, but you'll find there's +no such package in the standard Go library. That's because <code>C</code> is a +"pseudo-package", a special name interpreted by cgo as a reference to C's +name space. +</p> + +<p> +The <code>rand</code> package contains four references to the <code>C</code> +package: the calls to <code>C.random</code> and <code>C.srandom</code>, the +conversion <code>C.uint(i)</code>, and the <code>import</code> statement. +</p> + +<p> +The <code>Random</code> function calls the standard C library's <code>random</code> +function and returns the result. In C, <code>random</code> returns a value of the +C type <code>long</code>, which cgo represents as the type <code>C.long</code>. +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: +</p> + +{{code "/doc/progs/cgo1.go" `/func Random/` `/STOP/`}} + +<p> +Here's an equivalent function that uses a temporary variable to illustrate +the type conversion more explicitly: +</p> + +{{code "/doc/progs/cgo2.go" `/func Random/` `/STOP/`}} + +<p> +The <code>Seed</code> function does the reverse, in a way. It takes a +regular Go <code>int</code>, converts it to the C <code>unsigned int</code> +type, and passes it to the C function <code>srandom</code>. +</p> + +{{code "/doc/progs/cgo1.go" `/func Seed/` `/END/`}} + +<p> +Note that cgo knows the <code>unsigned int</code> type as <code>C.uint</code>; +see the <a href="/cmd/cgo">cgo documentation</a> for a complete list of +these numeric type names. +</p> + +<p> +The one detail of this example we haven't examined yet is the comment +above the <code>import</code> statement. +</p> + +{{code "/doc/progs/cgo1.go" `/\/\*/` `/STOP/`}} + +<p> +Cgo recognizes this comment. Any lines starting +with <code>#cgo</code> +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 <code>#include</code> +statement, but they can be almost any C code. The <code>#cgo</code> +directives are +used to provide flags for the compiler and linker when building the C +parts of the package. +</p> + +<p> +There is a limitation: if your program uses any <code>//export</code> +directives, then the C code in the comment may only include declarations +(<code>extern int f();</code>), not definitions (<code>int f() { +return 1; }</code>). You can use <code>//export</code> directives to +make Go functions accessible to C code. +</p> + +<p> +The <code>#cgo</code> and <code>//export</code> directives are +documented in +the <a href="/cmd/cgo/">cgo documentation</a>. +</p> + +<p> +<b>Strings and things</b> +</p> + +<p> +Unlike Go, C doesn't have an explicit string type. Strings in C are +represented by a zero-terminated array of chars. +</p> + +<p> +Conversion between Go and C strings is done with the +<code>C.CString</code>, <code>C.GoString</code>, and +<code>C.GoStringN</code> functions. These conversions make a copy of the +string data. +</p> + +<p> +This next example implements a <code>Print</code> function that writes a +string to standard output using C's <code>fputs</code> function from the +<code>stdio</code> library: +</p> + +{{code "/doc/progs/cgo3.go" `/package print/` `/END/`}} + +<p> +Memory allocations made by C code are not known to Go's memory manager. +When you create a C string with <code>C.CString</code> (or any C memory +allocation) you must remember to free the memory when you're done with it +by calling <code>C.free</code>. +</p> + +<p> +The call to <code>C.CString</code> returns a pointer to the start of the +char array, so before the function exits we convert it to an +<a href="/pkg/unsafe/#Pointer"><code>unsafe.Pointer</code></a> and release +the memory allocation with <code>C.free</code>. A common idiom in cgo programs +is to <a href="/doc/articles/defer_panic_recover.html"><code>defer</code></a> +the free immediately after allocating (especially when the code that follows +is more complex than a single function call), as in this rewrite of +<code>Print</code>: +</p> + +{{code "/doc/progs/cgo4.go" `/func Print/` `/END/`}} + +<p> +<b>Building cgo packages</b> +</p> + +<p> +To build cgo packages, just use <a href="/cmd/go/#Compile_packages_and_dependencies">" +<code>go build</code>"</a> or +<a href="/cmd/go/#Compile_and_install_packages_and_dependencies">"<code>go install</code> +"</a> as usual. The go tool recognizes the special <code>"C"</code> import and automatically +uses cgo for those files. +</p> + +<p> +<b>More cgo resources</b> +</p> + +<p> +The <a href="/cmd/cgo/">cgo command</a> documentation has more detail about +the C pseudo-package and the build process. The <a href="/misc/cgo/">cgo examples</a> +in the Go tree demonstrate more advanced concepts. +</p> + +<p> +For a simple, idiomatic example of a cgo-based package, see Russ Cox's <a +href="http://code.google.com/p/gosqlite/source/browse/sqlite/sqlite.go">gosqlite</a>. +Also, the Go Project Dashboard lists <a +href="https://godashboard.appspot.com/project?tag=cgo">several other +cgo packages</a>. +</p> + +<p> +Finally, if you're curious as to how all this works internally, take a look +at the introductory comment of the runtime package's <a href="/src/pkg/runtime/cgocall.c">cgocall.c</a>. +</p> 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 @@ +<!--{ +"Title": "Go Concurrency Patterns: Timing out, moving on", +"Template": true +}--> + +<p> +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 <code>ch</code>, 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: +</p> + +{{code "/doc/progs/timeout1.go" `/timeout :=/` `/STOP/`}} + +<p> +We can then use a <code>select</code> statement to receive from either +<code>ch</code> or <code>timeout</code>. If nothing arrives on <code>ch</code> +after one second, the timeout case is selected and the attempt to read from +<cde>ch</cde> is abandoned. +</p> + +{{code "/doc/progs/timeout1.go" `/select {/` `/STOP/`}} + +<p> +The <code>timeout</code> 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 <code>ch</code> receive happens before the +timeout is reached. The <code>timeout</code> channel will eventually be +deallocated by the garbage collector. +</p> + +<p> +(In this example we used <code>time.Sleep</code> to demonstrate the mechanics +of goroutines and channels. In real programs you should use <code> +<a href="/pkg/time/#After">time.After</a></code>, a function that returns +a channel and sends on that channel after the specified duration.) +</p> + +<p> +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. +</p> + +<p> +The function <code>Query</code> takes a slice of database connections and a +<code>query</code> string. It queries each of the databases in parallel and +returns the first response it receives: +</p> + +{{code "/doc/progs/timeout2.go" `/func Query/` `/STOP/`}} + +<p> +In this example, the closure does a non-blocking send, which it achieves by +using the send operation in <code>select</code> statement with a +<code>default</code> 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. +</p> + +<p> +This problem is a textbook of example of what is known as a +<a href="https://en.wikipedia.org/wiki/Race_condition">race condition</a>, but +the fix is trivial. We just make sure to buffer the channel <code>ch</code> (by +adding the buffer length as the second argument to <a href="/pkg/builtin/#make">make</a>), +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. +</p> + +<p> +These two examples demonstrate the simplicity with which Go can express complex +interactions between goroutines. +</p> diff --git a/doc/articles/defer_panic_recover.html b/doc/articles/defer_panic_recover.html index be97045dd..206b836d8 100644 --- a/doc/articles/defer_panic_recover.html +++ b/doc/articles/defer_panic_recover.html @@ -1,10 +1,7 @@ <!--{ - "Title": "Defer, Panic, and Recover" + "Title": "Defer, Panic, and Recover", + "Template": true }--> -<!-- - DO NOT EDIT: created by - tmpltohtml articles/defer_panic_recover.tmpl ---> <p> Go has the usual mechanisms for control flow: if, for, switch, goto. It also @@ -23,23 +20,7 @@ For example, let's look at a function that opens two files and copies the contents of one file to the other: </p> -<pre><!--{{code "progs/defer.go" `/func CopyFile/` `/STOP/`}} --->func CopyFile(dstName, srcName string) (written int64, err error) { - src, err := os.Open(srcName) - if err != nil { - return - } - - dst, err := os.Create(dstName) - if err != nil { - return - } - - written, err = io.Copy(dst, src) - dst.Close() - src.Close() - return -}</pre> +{{code "/doc/progs/defer.go" `/func CopyFile/` `/STOP/`}} <p> This works, but there is a bug. If the call to os.Create fails, the @@ -50,22 +31,7 @@ noticed and resolved. By introducing defer statements we can ensure that the files are always closed: </p> -<pre><!--{{code "progs/defer2.go" `/func CopyFile/` `/STOP/`}} --->func CopyFile(dstName, srcName string) (written int64, err error) { - src, err := os.Open(srcName) - if err != nil { - return - } - defer src.Close() - - dst, err := os.Create(dstName) - if err != nil { - return - } - defer dst.Close() - - return io.Copy(dst, src) -}</pre> +{{code "/doc/progs/defer2.go" `/func CopyFile/` `/STOP/`}} <p> Defer statements allow us to think about closing each file right after opening @@ -88,13 +54,7 @@ In this example, the expression "i" is evaluated when the Println call is deferred. The deferred call will print "0" after the function returns. </p> -<pre><!--{{code "progs/defer.go" `/func a/` `/STOP/`}} --->func a() { - i := 0 - defer fmt.Println(i) - i++ - return -}</pre> +{{code "/doc/progs/defer.go" `/func a/` `/STOP/`}} <p> 2. <i>Deferred function calls are executed in Last In First Out order @@ -105,12 +65,7 @@ deferred. The deferred call will print "0" after the function returns. This function prints "3210": </p> -<pre><!--{{code "progs/defer.go" `/func b/` `/STOP/`}} --->func b() { - for i := 0; i < 4; i++ { - defer fmt.Print(i) - } -}</pre> +{{code "/doc/progs/defer.go" `/func b/` `/STOP/`}} <p> 3. <i>Deferred functions may read and assign to the returning function's named @@ -122,11 +77,7 @@ In this example, a deferred function increments the return value i <i>after</i> the surrounding function returns. Thus, this function returns 2: </p> -<pre><!--{{code "progs/defer.go" `/func c/` `/STOP/`}} --->func c() (i int) { - defer func() { i++ }() - return 1 -}</pre> +{{code "/doc/progs/defer.go" `/func c/` `/STOP/`}} <p> This is convenient for modifying the error return value of a function; we will @@ -156,36 +107,7 @@ to panic and resume normal execution. Here's an example program that demonstrates the mechanics of panic and defer: </p> -<pre><!--{{code "progs/defer2.go" `/package main/` `/STOP/`}} --->package main - -import "fmt" - -func main() { - f() - fmt.Println("Returned normally from f.") -} - -func f() { - defer func() { - if r := recover(); r != nil { - fmt.Println("Recovered in f", r) - } - }() - fmt.Println("Calling g.") - g(0) - fmt.Println("Returned normally from g.") -} - -func g(i int) { - if i > 3 { - fmt.Println("Panicking!") - panic(fmt.Sprintf("%v", i)) - } - defer fmt.Println("Defer in g", i) - fmt.Println("Printing in g", i) - g(i + 1) -}</pre> +{{code "/doc/progs/defer2.go" `/package main/` `/STOP/`}} <p> The function g takes the int i, and panics if i is greater than 3, or else it diff --git a/doc/articles/defer_panic_recover.tmpl b/doc/articles/defer_panic_recover.tmpl deleted file mode 100644 index 5f48c6ef4..000000000 --- a/doc/articles/defer_panic_recover.tmpl +++ /dev/null @@ -1,195 +0,0 @@ -<!--{ - "Title": "Defer, Panic, and Recover" -}--> -{{donotedit}} -<p> -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. -</p> - -<p> -A <b>defer statement</b> 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. -</p> - -<p> -For example, let's look at a function that opens two files and copies the -contents of one file to the other: -</p> - -{{code "progs/defer.go" `/func CopyFile/` `/STOP/`}} - -<p> -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: -</p> - -{{code "progs/defer2.go" `/func CopyFile/` `/STOP/`}} - -<p> -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 <i>will</i> be closed. -</p> - -<p> -The behavior of defer statements is straightforward and predictable. There are -three simple rules: -</p> - -<p> -1. <i>A deferred function's arguments are evaluated when the defer statement is -evaluated.</i> -</p> - -<p> -In this example, the expression "i" is evaluated when the Println call is -deferred. The deferred call will print "0" after the function returns. -</p> - -{{code "progs/defer.go" `/func a/` `/STOP/`}} - -<p> -2. <i>Deferred function calls are executed in Last In First Out order -</i>after<i> the surrounding function returns.</i> -</p> - -<p> -This function prints "3210": -</p> - -{{code "progs/defer.go" `/func b/` `/STOP/`}} - -<p> -3. <i>Deferred functions may read and assign to the returning function's named -return values.</i> -</p> - -<p> -In this example, a deferred function increments the return value i <i>after</i> -the surrounding function returns. Thus, this function returns 2: -</p> - -{{code "progs/defer.go" `/func c/` `/STOP/`}} - -<p> -This is convenient for modifying the error return value of a function; we will -see an example of this shortly. -</p> - -<p> -<b>Panic</b> is a built-in function that stops the ordinary flow of control and -begins <i>panicking</i>. 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. -</p> - -<p> -<b>Recover</b> 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. -</p> - -<p> -Here's an example program that demonstrates the mechanics of panic and defer: -</p> - -{{code "progs/defer2.go" `/package main/` `/STOP/`}} - -<p> -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. -</p> - -<p> -The program will output: -</p> - -<pre>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.</pre> - -<p> -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: -</p> - -<pre>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]</pre> - -<p> -For a real-world example of <b>panic</b> and <b>recover</b>, see the -<a href="/pkg/encoding/json/">json package</a> 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' functions in -<a href="/src/pkg/encoding/json/decode.go">decode.go</a>). -</p> - -<p> -The convention in the Go libraries is that even when a package uses panic -internally, its external API still presents explicit error return values. -</p> - -<p> -Other uses of <b>defer</b> (beyond the file.Close() example given earlier) -include releasing a mutex: -</p> - -<pre>mu.Lock() -defer mu.Unlock()</pre> - -<p> -printing a footer: -</p> - -<pre>printHeader() -defer printFooter()</pre> - -<p> -and more. -</p> - -<p> -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. -</p> diff --git a/doc/articles/error_handling.html b/doc/articles/error_handling.html index ac33f1dab..8f4fffb48 100644 --- a/doc/articles/error_handling.html +++ b/doc/articles/error_handling.html @@ -1,10 +1,7 @@ <!--{ - "Title": "Error Handling and Go" + "Title": "Error Handling and Go", + "Template": true }--> -<!-- - DO NOT EDIT: created by - tmpltohtml articles/error_handling.tmpl ---> <p> If you have written any Go code you have probably encountered the built-in @@ -13,20 +10,14 @@ indicate an abnormal state. For example, the <code>os.Open</code> function returns a non-nil <code>error</code> value when it fails to open a file. </p> -<pre><!--{{code "progs/error.go" `/func Open/`}} --->func Open(name string) (file *File, err error)</pre> +{{code "/doc/progs/error.go" `/func Open/`}} <p> The following code uses <code>os.Open</code> to open a file. If an error occurs it calls <code>log.Fatal</code> to print the error message and stop. </p> -<pre><!--{{code "progs/error.go" `/func openFile/` `/STOP/`}} ---> f, err := os.Open("filename.ext") - if err != nil { - log.Fatal(err) - } - // do something with the open *File f</pre> +{{code "/doc/progs/error.go" `/func openFile/` `/STOP/`}} <p> You can get a lot done in Go knowing just this about the <code>error</code> @@ -59,15 +50,7 @@ The most commonly-used <code>error</code> implementation is the <a href="/pkg/errors/">errors</a> package's unexported <code>errorString</code> type. </p> -<pre><!--{{code "progs/error.go" `/errorString/` `/STOP/`}} --->// errorString is a trivial implementation of error. -type errorString struct { - s string -} - -func (e *errorString) Error() string { - return e.s -}</pre> +{{code "/doc/progs/error.go" `/errorString/` `/STOP/`}} <p> You can construct one of these values with the <code>errors.New</code> @@ -75,23 +58,13 @@ function. It takes a string that it converts to an <code>errors.errorString</cod and returns as an <code>error</code> value. </p> -<pre><!--{{code "progs/error.go" `/New/` `/STOP/`}} --->// New returns an error that formats as the given text. -func New(text string) error { - return &errorString{text} -}</pre> +{{code "/doc/progs/error.go" `/New/` `/STOP/`}} <p> Here's how you might use <code>errors.New</code>: </p> -<pre><!--{{code "progs/error.go" `/func Sqrt/` `/STOP/`}} --->func Sqrt(f float64) (float64, error) { - if f < 0 { - return 0, errors.New("math: square root of negative number") - } - // implementation -}</pre> +{{code "/doc/progs/error.go" `/func Sqrt/` `/STOP/`}} <p> A caller passing a negative argument to <code>Sqrt</code> receives a non-nil @@ -101,11 +74,7 @@ A caller passing a negative argument to <code>Sqrt</code> receives a non-nil <code>Error</code> method, or by just printing it: </p> -<pre><!--{{code "progs/error.go" `/func printErr/` `/STOP/`}} ---> f, err := Sqrt(-1) - if err != nil { - fmt.Println(err) - }</pre> +{{code "/doc/progs/error.go" `/func printErr/` `/STOP/`}} <p> The <a href="/pkg/fmt/">fmt</a> package formats an <code>error</code> value @@ -126,10 +95,7 @@ rules and returns it as an <code>error</code> created by <code>errors.New</code>. </p> -<pre><!--{{code "progs/error.go" `/fmtError/` `/STOP/`}} ---> if f < 0 { - return 0, fmt.Errorf("math: square root of negative number %g", f) - }</pre> +{{code "/doc/progs/error.go" `/fmtError/` `/STOP/`}} <p> In many cases <code>fmt.Errorf</code> is good enough, but since @@ -143,12 +109,7 @@ argument passed to <code>Sqrt</code>. We can enable that by defining a new error implementation instead of using <code>errors.errorString</code>: </p> -<pre><!--{{code "progs/error.go" `/type NegativeSqrtError/` `/STOP/`}} --->type NegativeSqrtError float64 - -func (f NegativeSqrtError) Error() string { - return fmt.Sprintf("math: square root of negative number %g", float64(f)) -}</pre> +{{code "/doc/progs/error.go" `/type NegativeSqrtError/` `/STOP/`}} <p> A sophisticated caller can then use a @@ -164,13 +125,7 @@ As another example, the <a href="/pkg/encoding/json/">json</a> package specifies returns when it encounters a syntax error parsing a JSON blob. </p> -<pre><!--{{code "progs/error.go" `/type SyntaxError/` `/STOP/`}} --->type SyntaxError struct { - msg string // description of error - Offset int64 // error occurred after reading Offset bytes -} - -func (e *SyntaxError) Error() string { return e.msg }</pre> +{{code "/doc/progs/error.go" `/type SyntaxError/` `/STOP/`}} <p> The <code>Offset</code> field isn't even shown in the default formatting of the @@ -178,14 +133,7 @@ error, but callers can use it to add file and line information to their error messages: </p> -<pre><!--{{code "progs/error.go" `/func decodeError/` `/STOP/`}} ---> if err := dec.Decode(&val); err != nil { - if serr, ok := err.(*json.SyntaxError); ok { - line, col := findLine(f, serr.Offset) - return fmt.Errorf("%s:%d:%d: %v", f.Name(), line, col, err) - } - return err - }</pre> +{{code "/doc/progs/error.go" `/func decodeError/` `/STOP/`}} <p> (This is a slightly simplified version of some @@ -217,14 +165,7 @@ web crawler might sleep and retry when it encounters a temporary error and give up otherwise. </p> -<pre><!--{{code "progs/error.go" `/func netError/` `/STOP/`}} ---> if nerr, ok := err.(net.Error); ok && nerr.Temporary() { - time.Sleep(1e9) - continue - } - if err != nil { - log.Fatal(err) - }</pre> +{{code "/doc/progs/error.go" `/func netError/` `/STOP/`}} <p> <b>Simplifying repetitive error handling</b> @@ -244,23 +185,7 @@ application with an HTTP handler that retrieves a record from the datastore and formats it with a template. </p> -<pre><!--{{code "progs/error2.go" `/func init/` `/STOP/`}} --->func init() { - http.HandleFunc("/view", viewRecord) -} - -func viewRecord(w http.ResponseWriter, r *http.Request) { - c := appengine.NewContext(r) - key := datastore.NewKey(c, "Record", r.FormValue("id"), 0, nil) - record := new(Record) - if err := datastore.Get(c, key, record); err != nil { - http.Error(w, err.Error(), 500) - return - } - if err := viewTemplate.Execute(w, record); err != nil { - http.Error(w, err.Error(), 500) - } -}</pre> +{{code "/doc/progs/error2.go" `/func init/` `/STOP/`}} <p> This function handles errors returned by the <code>datastore.Get</code> @@ -276,23 +201,13 @@ To reduce the repetition we can define our own HTTP <code>appHandler</code> type that includes an <code>error</code> return value: </p> -<pre><!--{{code "progs/error3.go" `/type appHandler/`}} --->type appHandler func(http.ResponseWriter, *http.Request) error</pre> +{{code "/doc/progs/error3.go" `/type appHandler/`}} <p> Then we can change our <code>viewRecord</code> function to return errors: </p> -<pre><!--{{code "progs/error3.go" `/func viewRecord/` `/STOP/`}} --->func viewRecord(w http.ResponseWriter, r *http.Request) error { - c := appengine.NewContext(r) - key := datastore.NewKey(c, "Record", r.FormValue("id"), 0, nil) - record := new(Record) - if err := datastore.Get(c, key, record); err != nil { - return err - } - return viewTemplate.Execute(w, record) -}</pre> +{{code "/doc/progs/error3.go" `/func viewRecord/` `/STOP/`}} <p> This is simpler than the original version, but the <a @@ -302,12 +217,7 @@ To fix this we can implement the <code>http.Handler</code> interface's <code>ServeHTTP</code> method on <code>appHandler</code>: </p> -<pre><!--{{code "progs/error3.go" `/ServeHTTP/` `/STOP/`}} --->func (fn appHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) { - if err := fn(w, r); err != nil { - http.Error(w, err.Error(), 500) - } -}</pre> +{{code "/doc/progs/error3.go" `/ServeHTTP/` `/STOP/`}} <p> The <code>ServeHTTP</code> method calls the <code>appHandler</code> function @@ -323,10 +233,7 @@ Now when registering <code>viewRecord</code> with the http package we use the <code>http.HandlerFunc</code>). </p> -<pre><!--{{code "progs/error3.go" `/func init/` `/STOP/`}} --->func init() { - http.Handle("/view", appHandler(viewRecord)) -}</pre> +{{code "/doc/progs/error3.go" `/func init/` `/STOP/`}} <p> With this basic error handling infrastructure in place, we can make it more @@ -341,24 +248,19 @@ To do this we create an <code>appError</code> struct containing an <code>error</code> and some other fields: </p> -<pre><!--{{code "progs/error4.go" `/type appError/` `/STOP/`}} --->type appError struct { - Error error - Message string - Code int -}</pre> +{{code "/doc/progs/error4.go" `/type appError/` `/STOP/`}} <p> Next we modify the appHandler type to return <code>*appError</code> values: </p> -<pre><!--{{code "progs/error4.go" `/type appHandler/`}} --->type appHandler func(http.ResponseWriter, *http.Request) *appError</pre> +{{code "/doc/progs/error4.go" `/type appHandler/`}} <p> (It's usually a mistake to pass back the concrete type of an error rather than -<code>error</code>, for reasons to be discussed in another article, but -it's the right thing to do here because <code>ServeHTTP</code> is the only +<code>error</code>, +for reasons discussed in <a href="/doc/go_faq.html#nil_error">the Go FAQ</a>, +but it's the right thing to do here because <code>ServeHTTP</code> is the only place that sees the value and uses its contents.) </p> @@ -369,33 +271,14 @@ status <code>Code</code> and log the full <code>Error</code> to the developer console: </p> -<pre><!--{{code "progs/error4.go" `/ServeHTTP/` `/STOP/`}} --->func (fn appHandler) ServeHTTP(w http.ResponseWriter, r *http.Request) { - if e := fn(w, r); e != nil { // e is *appError, not os.Error. - c := appengine.NewContext(r) - c.Errorf("%v", e.Error) - http.Error(w, e.Message, e.Code) - } -}</pre> +{{code "/doc/progs/error4.go" `/ServeHTTP/` `/STOP/`}} <p> Finally, we update <code>viewRecord</code> to the new function signature and have it return more context when it encounters an error: </p> -<pre><!--{{code "progs/error4.go" `/func viewRecord/` `/STOP/`}} --->func viewRecord(w http.ResponseWriter, r *http.Request) *appError { - c := appengine.NewContext(r) - key := datastore.NewKey(c, "Record", r.FormValue("id"), 0, nil) - record := new(Record) - if err := datastore.Get(c, key, record); err != nil { - return &appError{err, "Record not found", 404} - } - if err := viewTemplate.Execute(w, record); err != nil { - return &appError{err, "Can't display record", 500} - } - return nil -}</pre> +{{code "/doc/progs/error4.go" `/func viewRecord/` `/STOP/`}} <p> This version of <code>viewRecord</code> is the same length as the original, but diff --git a/doc/articles/error_handling.tmpl b/doc/articles/error_handling.tmpl deleted file mode 100644 index 56b7fb309..000000000 --- a/doc/articles/error_handling.tmpl +++ /dev/null @@ -1,314 +0,0 @@ -<!--{ - "Title": "Error Handling and Go" -}--> -{{donotedit}} -<p> -If you have written any Go code you have probably encountered the built-in -<code>error</code> type. Go code uses <code>error</code> values to -indicate an abnormal state. For example, the <code>os.Open</code> function -returns a non-nil <code>error</code> value when it fails to open a file. -</p> - -{{code "progs/error.go" `/func Open/`}} - -<p> -The following code uses <code>os.Open</code> to open a file. If an error -occurs it calls <code>log.Fatal</code> to print the error message and stop. -</p> - -{{code "progs/error.go" `/func openFile/` `/STOP/`}} - -<p> -You can get a lot done in Go knowing just this about the <code>error</code> -type, but in this article we'll take a closer look at <code>error</code> and -discuss some good practices for error handling in Go. -</p> - -<p> -<b>The error type</b> -</p> - -<p> -The <code>error</code> type is an interface type. An <code>error</code> -variable represents any value that can describe itself as a string. Here is the -interface's declaration: -</p> - -<pre>type error interface { - Error() string -}</pre> - -<p> -The <code>error</code> type, as with all built in types, is -<a href="/doc/go_spec.html#Predeclared_identifiers">predeclared</a> in the -<a href="/doc/go_spec.html#Blocks">universe block</a>. -</p> - -<p> -The most commonly-used <code>error</code> implementation is the -<a href="/pkg/errors/">errors</a> package's unexported <code>errorString</code> type. -</p> - -{{code "progs/error.go" `/errorString/` `/STOP/`}} - -<p> -You can construct one of these values with the <code>errors.New</code> -function. It takes a string that it converts to an <code>errors.errorString</code> -and returns as an <code>error</code> value. -</p> - -{{code "progs/error.go" `/New/` `/STOP/`}} - -<p> -Here's how you might use <code>errors.New</code>: -</p> - -{{code "progs/error.go" `/func Sqrt/` `/STOP/`}} - -<p> -A caller passing a negative argument to <code>Sqrt</code> receives a non-nil -<code>error</code> value (whose concrete representation is an -<code>errors.errorString</code> value). The caller can access the error string -("math: square root of...") by calling the <code>error</code>'s -<code>Error</code> method, or by just printing it: -</p> - -{{code "progs/error.go" `/func printErr/` `/STOP/`}} - -<p> -The <a href="/pkg/fmt/">fmt</a> package formats an <code>error</code> value -by calling its <code>Error() string</code> method. -</p> - -<p> -It is the error implementation's responsibility to summarize the context. -The error returned by <code>os.Open</code> formats as "open /etc/passwd: -permission denied," not just "permission denied." The error returned by our -<code>Sqrt</code> is missing information about the invalid argument. -</p> - -<p> -To add that information, a useful function is the <code>fmt</code> package's -<code>Errorf</code>. It formats a string according to <code>Printf</code>'s -rules and returns it as an <code>error</code> created by -<code>errors.New</code>. -</p> - -{{code "progs/error.go" `/fmtError/` `/STOP/`}} - -<p> -In many cases <code>fmt.Errorf</code> is good enough, but since -<code>error</code> is an interface, you can use arbitrary data structures as -error values, to allow callers to inspect the details of the error. -</p> - -<p> -For instance, our hypothetical callers might want to recover the invalid -argument passed to <code>Sqrt</code>. We can enable that by defining a new -error implementation instead of using <code>errors.errorString</code>: -</p> - -{{code "progs/error.go" `/type NegativeSqrtError/` `/STOP/`}} - -<p> -A sophisticated caller can then use a -<a href="/doc/go_spec.html#Type_assertions">type assertion</a> to check for a -<code>NegativeSqrtError</code> and handle it specially, while callers that just -pass the error to <code>fmt.Println</code> or <code>log.Fatal</code> will see -no change in behavior. -</p> - -<p> -As another example, the <a href="/pkg/encoding/json/">json</a> package specifies a -<code>SyntaxError</code> type that the <code>json.Decode</code> function -returns when it encounters a syntax error parsing a JSON blob. -</p> - -{{code "progs/error.go" `/type SyntaxError/` `/STOP/`}} - -<p> -The <code>Offset</code> 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: -</p> - -{{code "progs/error.go" `/func decodeError/` `/STOP/`}} - -<p> -(This is a slightly simplified version of some -<a href="http://camlistore.org/code/?p=camlistore.git;a=blob;f=lib/go/camli/jsonconfig/eval.go#l68">actual code</a> -from the <a href="http://camlistore.org">Camlistore</a> project.) -</p> - -<p> -The <code>error</code> interface requires only a <code>Error</code> method; -specific error implementations might have additional methods. For instance, the -<a href="/pkg/net/">net</a> package returns errors of type -<code>error</code>, following the usual convention, but some of the error -implementations have additional methods defined by the <code>net.Error</code> -interface: -</p> - -<pre>package net - -type Error interface { - error - Timeout() bool // Is the error a timeout? - Temporary() bool // Is the error temporary? -}</pre> - -<p> -Client code can test for a <code>net.Error</code> 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. -</p> - -{{code "progs/error.go" `/func netError/` `/STOP/`}} - -<p> -<b>Simplifying repetitive error handling</b> -</p> - -<p> -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. -</p> - -<p> -Consider an <a href="http://code.google.com/appengine/docs/go/">App Engine</a> -application with an HTTP handler that retrieves a record from the datastore and -formats it with a template. -</p> - -{{code "progs/error2.go" `/func init/` `/STOP/`}} - -<p> -This function handles errors returned by the <code>datastore.Get</code> -function and <code>viewTemplate</code>'s <code>Execute</code> 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. -</p> - -<p> -To reduce the repetition we can define our own HTTP <code>appHandler</code> -type that includes an <code>error</code> return value: -</p> - -{{code "progs/error3.go" `/type appHandler/`}} - -<p> -Then we can change our <code>viewRecord</code> function to return errors: -</p> - -{{code "progs/error3.go" `/func viewRecord/` `/STOP/`}} - -<p> -This is simpler than the original version, but the <a -href="/pkg/net/http/">http</a> package doesn't understand functions that return -<code>error</code>. -To fix this we can implement the <code>http.Handler</code> interface's -<code>ServeHTTP</code> method on <code>appHandler</code>: -</p> - -{{code "progs/error3.go" `/ServeHTTP/` `/STOP/`}} - -<p> -The <code>ServeHTTP</code> method calls the <code>appHandler</code> function -and displays the returned error (if any) to the user. Notice that the method's -receiver, <code>fn</code>, is a function. (Go can do that!) The method invokes -the function by calling the receiver in the expression <code>fn(w, r)</code>. -</p> - -<p> -Now when registering <code>viewRecord</code> with the http package we use the -<code>Handle</code> function (instead of <code>HandleFunc</code>) as -<code>appHandler</code> is an <code>http.Handler</code> (not an -<code>http.HandlerFunc</code>). -</p> - -{{code "progs/error3.go" `/func init/` `/STOP/`}} - -<p> -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. -</p> - -<p> -To do this we create an <code>appError</code> struct containing an -<code>error</code> and some other fields: -</p> - -{{code "progs/error4.go" `/type appError/` `/STOP/`}} - -<p> -Next we modify the appHandler type to return <code>*appError</code> values: -</p> - -{{code "progs/error4.go" `/type appHandler/`}} - -<p> -(It's usually a mistake to pass back the concrete type of an error rather than -<code>error</code>, for reasons to be discussed in another article, but -it's the right thing to do here because <code>ServeHTTP</code> is the only -place that sees the value and uses its contents.) -</p> - -<p> -And make <code>appHandler</code>'s <code>ServeHTTP</code> method display the -<code>appError</code>'s <code>Message</code> to the user with the correct HTTP -status <code>Code</code> and log the full <code>Error</code> to the developer -console: -</p> - -{{code "progs/error4.go" `/ServeHTTP/` `/STOP/`}} - -<p> -Finally, we update <code>viewRecord</code> to the new function signature and -have it return more context when it encounters an error: -</p> - -{{code "progs/error4.go" `/func viewRecord/` `/STOP/`}} - -<p> -This version of <code>viewRecord</code> is the same length as the original, but -now each of those lines has specific meaning and we are providing a friendlier -user experience. -</p> - -<p> -It doesn't end there; we can further improve the error handling in our -application. Some ideas: -</p> - -<ul> -<li>give the error handler a pretty HTML template, -<li>make debugging easier by writing the stack trace to the HTTP response when -the user is an administrator, -<li>write a constructor function for <code>appError</code> that stores the -stack trace for easier debugging, -<li>recover from panics inside the <code>appHandler</code>, logging the error -to the console as "Critical," while telling the user "a serious error -has occurred." This is a nice touch to avoid exposing the user to inscrutable -error messages caused by programming errors. -See the <a href="defer_panic_recover.html">Defer, Panic, and Recover</a> -article for more details. -</ul> - -<p> -<b>Conclusion</b> -</p> - -<p> -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. -</p> 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 @@ +<!--{ + "title": "About the go command" +}--> + +<p>The Go distribution includes a command, named +"<code><a href="/cmd/go/">go</a></code>", 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.</p> + +<h2>Motivation</h2> + +<p>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.</p> + +<p>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.</p> + +<p>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.</p> + +<p>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.</p> + +<h2>Configuration versus convention</h2> + +<p>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 <code>goinstall</code> command +(now replaced by <a href="/cmd/go/#Download_and_install_packages_and_dependencies"><code>go get</code></a>) +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.</p> + +<p>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.</p> + +<p>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.</p> + +<h2>Go's conventions</h2> + +<p>The <code>go</code> command requires that code adheres to a few key, +well-established conventions.</p> + +<p>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 <code>http://</code> prefix. Subdirectories are named by +adding to that path. For example, the supplemental networking +libraries for Go are obtained by running</p> + +<pre> +hg clone http://code.google.com/p/go.net +</pre> + +<p>and thus the import path for the root directory of that repository is +"<code>code.google.com/p/go.net</code>". The websocket package is stored in a +subdirectory, so its import path is +"<code>code.google.com/p/go.net/websocket</code>".</p> + +<p>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.</p> + +<p>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 <code>$GOPATH/src/<import-path></code>. If <code>$GOPATH</code> is +unset, the go command will fall back to storing source code alongside the +standard Go packages, in <code>$GOROOT/src/pkg/<import-path></code>. +If <code>$GOPATH</code> is set to a list of paths, the go command tries +<code><dir>/src/<import-path></code> for each of the directories in +that list.</p> + +<p>Each of those trees contains, by convention, a top-level directory named +"<code>bin</code>", for holding compiled executables, and a top-level directory +named "<code>pkg</code>", for holding compiled packages that can be imported, +and the "<code>src</code>" 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.</p> + +<p>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.</p> + +<p>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.</p> + +<p>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.</p> + +<h2>Getting started with the go command</h2> + +<p>Finally, a quick tour of how to use the go command, to supplement +the information in <a href="/doc/code.html">How to Write Go Code</a>, +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 <code>$GOPATH</code>, the one piece of global +configuration that the go command needs. The <code>$GOPATH</code> 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 +<code>$GOPATH</code> for each of your projects. One <code>$GOPATH</code> can +support many projects.</p> + +<p>Here’s an example. Let’s say we decide to keep our Go code in the directory +<code>$HOME/mygo</code>. We need to create that directory and set +<code>$GOPATH</code> accordingly.</p> + +<pre> +$ mkdir $HOME/mygo +$ export GOPATH=$HOME/mygo +$ +</pre> + +<p>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 "<code>go get</code>" +subcommand:</p> + +<pre> +$ go get code.google.com/p/codesearch/index +$ go get github.com/petar/GoLLRB/llrb +$ +</pre> + +<p>Both of these projects are now downloaded and installed into our +<code>$GOPATH</code> directory. The one tree now contains the two directories +<code>src/code.google.com/p/codesearch/index/</code> and +<code>src/github.com/petar/GoLLRB/llrb/</code>, along with the compiled +packages (in <code>pkg/</code>) for those libraries and their dependencies.</p> + +<p>Because we used version control systems (Mercurial and Git) to check +out the sources, the source tree also contains the other files in the +corresponding repositories, such as related packages. The "<code>go list</code>" +subcommand lists the import paths corresponding to its arguments, and +the pattern "<code>./...</code>" means start in the current directory +("<code>./</code>") and find all packages below that directory +("<code>...</code>"):</p> + +<pre> +$ 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 +$ +</pre> + +<p>We can also test those packages:</p> + +<pre> +$ 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 +$ +</pre> + +<p>If a go subcommand is invoked with no paths listed, it operates on the +current directory:</p> + +<pre> +$ 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 +$ +</pre> + +<p>That "<code>go install</code>" subcommand installs the latest copy of the +package into the pkg directory. Because the go command can analyze the +dependency graph, "<code>go install</code>" also installs any packages that +this package imports but that are out of date, recursively.</p> + +<p>Notice that "<code>go install</code>" 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.</p> + +<p>As the example shows, it’s fine to work with packages from many different +projects at once within a single <code>$GOPATH</code> root directory.</p> + +<h2>Limitations</h2> + +<p>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 +"<code>go get</code>" without needing to obtain and build +any additional tools.</p> + +<h2>More information</h2> + +<p>For more information, read <a href="/doc/code.html">How to Write Go Code</a> +and see the <a href="/cmd/go/">go command documentation</a>.</p> 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 @@ +<!--{ +"Title": "Gobs of data", +"Template": true +}--> + +<p> +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: <a href="http://www.json.org/">JSON</a>, +<a href="http://www.w3.org/XML/">XML</a>, Google's +<a href="http://code.google.com/p/protobuf">protocol buffers</a>, and more. +And now there's another, provided by Go's <a href="/pkg/encoding/gob/">gob</a> +package. +</p> + +<p> +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 +<a href="/pkg/">packages</a> supporting all the encodings just mentioned (the +<a href="http://code.google.com/p/goprotobuf">protocol buffer package</a> 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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +<b>Goals</b> +</p> + +<p> +The gob package was designed with a number of goals in mind. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +There were also some things to learn from our experiences with Google protocol +buffers. +</p> + +<p> +<b>Protocol buffer misfeatures</b> +</p> + +<p> +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.) +</p> + +<p> +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? +</p> + +<p> +Next, a protocol buffer definition may specify that fields <code>T.x</code> and +<code>T.y</code> are required to be present whenever a value of type +<code>T</code> 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.) +</p> + +<p> +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. +</p> + +<p> +So gobs end up looking like a sort of generalized, simplified protocol buffer. +How do they work? +</p> + +<p> +<b>Values</b> +</p> + +<p> +The encoded gob data isn't about <code>int8</code>s and <code>uint16</code>s. +Instead, somewhat analogous to constants in Go, its integer values are abstract, +sizeless numbers, either signed or unsigned. When you encode an +<code>int8</code>, its value is transmitted as an unsized, variable-length +integer. When you encode an <code>int64</code>, 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 +<code>int8</code>, but the receiver may store it in an <code>int64</code>. 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. +</p> + +<p> +This flexibility also applies to pointers. Before transmission, all pointers are +flattened. Values of type <code>int8</code>, <code>*int8</code>, +<code>**int8</code>, <code>****int8</code>, etc. are all transmitted as an +integer value, which may then be stored in <code>int</code> of any size, or +<code>*int</code>, or <code>******int</code>, etc. Again, this allows for +flexibility. +</p> + +<p> +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 +</p> + +{{code "/doc/progs/gobs1.go" `/type T/` `/STOP/`}} + +<p> +the encoding of <code>t</code> sends only the 7 and 8. Because it's zero, the +value of <code>Y</code> isn't even sent; there's no need to send a zero value. +</p> + +<p> +The receiver could instead decode the value into this structure: +</p> + +{{code "/doc/progs/gobs1.go" `/type U/` `/STOP/`}} + +<p> +and acquire a value of <code>u</code> with only <code>X</code> set (to the +address of an <code>int8</code> variable set to 7); the <code>Z</code> 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 +<code>T</code> 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. +</p> + +<p> +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. +</p> + +<p> +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 +<a href="/pkg/encoding/gob/#GobEncoder">GobEncoder</a> and +<a href="/pkg/encoding/gob/#GobDecoder">GobDecoder</a> interfaces, in a manner +analogous to the <a href="/pkg/encoding/json/">JSON</a> package's +<a href="/pkg/encoding/json/#Marshaler">Marshaler</a> and +<a href="/pkg/encoding/json/#Unmarshaler">Unmarshaler</a> and also to the +<a href="/pkg/fmt/#Stringer">Stringer</a> interface from +<a href="/pkg/fmt/">package fmt</a>. This facility makes it possible to +represent special features, enforce constraints, or hide secrets when you +transmit data. See the <a href="/pkg/encoding/gob/">documentation</a> for +details. +</p> + +<p> +<b>Types on the wire</b> +</p> + +<p> +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. +</p> + +<p> +Thus when we send our first type <code>T</code>, the gob encoder sends a +description of <code>T</code> and tags it with a type number, say 127. All +values, including the first, are then prefixed by that number, so a stream of +<code>T</code> values looks like: +</p> + +<pre> +("define type id" 127, definition of type T)(127, T value)(127, T value), ... +</pre> + +<p> +These type numbers make it possible to describe recursive types and send values +of those types. Thus gobs can encode types such as trees: +</p> + +{{code "/doc/progs/gobs1.go" `/type Node/` `/STOP/`}} + +<p> +(It's an exercise for the reader to discover how the zero-defaulting rule makes +this work, even though gobs don't represent pointers.) +</p> + +<p> +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. +</p> + +<p> +<b>Compiling a machine</b> +</p> + +<p> +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. +</p> + +<p> +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. +</p> + +<p> +<b>Use</b> +</p> + +<p> +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 +<a href="/pkg/encoding/gob/">gob package</a> and it does all the work. +</p> + +{{code "/doc/progs/gobs2.go" `/package main/` `$`}} + +<p> +You can compile and run this example code in the +<a href="http://play.golang.org/p/_-OJV-rwMq">Go Playground</a>. +</p> + +<p> +The <a href="/pkg/net/rpc/">rpc package</a> builds on gobs to turn this +encode/decode automation into transport for method calls across the network. +That's a subject for another article. +</p> + +<p> +<b>Details</b> +</p> + +<p> +The <a href="/pkg/encoding/gob/">gob package documentation</a>, especially the +file <a href="/src/pkg/encoding/gob/doc.go">doc.go</a>, 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. +</p> 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 @@ +<!--{ +"Title": "Godoc: documenting Go code", +"Template": true +}--> + +<p> +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. +</p> + +<p> +To that end, we have developed the <a href="/cmd/godoc/">godoc</a> 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. +</p> + +<p> +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 <a href="/pkg/strings/#HasPrefix">documentation</a> to its +<a href="/src/pkg/strings/strings.go?#L312">implementation</a> with one click. +</p> + +<p> +Godoc is conceptually related to Python's +<a href="http://www.python.org/dev/peps/pep-0257/">Docstring</a> and Java's +<a href="http://www.oracle.com/technetwork/java/javase/documentation/index-jsp-135444.html">Javadoc</a>, +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. +</p> + +<p> +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 +<code>fmt</code> package's <a href="/pkg/fmt/#Fprint"><code>Fprint</code></a> +function: +</p> + +{{code "/src/pkg/fmt/print.go" `/Fprint formats using the default/` `/func Fprint/`}} + +<p> +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. +</p> + +<p> +Comments on package declarations should provide general package documentation. +These comments can be short, like the <a href="/pkg/sort/"><code>sort</code></a> +package's brief description: +</p> + +{{code "/src/pkg/sort/sort.go" `/Package sort provides/` `/package sort/`}} + +<p> +They can also be detailed like the <a href="/pkg/encoding/gob/">gob package</a>'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, <a href="/src/pkg/encoding/gob/doc.go">doc.go</a>, which +contains only those comments and a package clause. +</p> + +<p> +When writing package comments of any size, keep in mind that their first +sentence will appear in godoc's <a href="/pkg/">package list</a>. +</p> + +<p> +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 <code>"BUG(who)”</code> 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 <a href="/pkg/bytes/#bugs">bytes package</a>: +</p> + +<pre> +// BUG(r): The rule Title uses for word boundaries does not handle Unicode punctuation properly. +</pre> + +<p> +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 +<a href="/cmd/godoc/">godoc documentation</a> and its corresponding +<a href="/src/cmd/godoc/doc.go">doc.go</a> file. +</p> + +<p> +There are a few formatting rules that Godoc uses when converting comments to +HTML: +</p> + +<ul> +<li> +Subsequent lines of text are considered part of the same paragraph; you must +leave a blank line to separate paragraphs. +</li> +<li> +Pre-formatted text must be indented relative to the surrounding comment text +(see gob's <a href="/src/pkg/encoding/gob/doc.go">doc.go</a> for an example). +</li> +<li> +URLs will be converted to HTML links; no special markup is necessary. +</li> +</ul> + +<p> +Note that none of these rules requires you to do anything out of the ordinary. +</p> + +<p> +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. +</p> + +<p> +Your own code can present good documentation just by having comments as +described above. Any Go packages installed inside <code>$GOROOT/src/pkg</code> +and any <code>GOPATH</code> work spaces will already be accessible via godoc's +command-line and HTTP interfaces, and you can specify additional paths for +indexing via the <code>-path</code> flag or just by running <code>"godoc ."</code> +in the source directory. See the <a href="/cmd/godoc/">godoc documentation</a> +for more details. +</p> 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 @@ +<!--{ +"Title": "Go's Declaration Syntax" +}--> + +<p> +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. +</p> + +<p> +<b>C syntax</b> +</p> + +<p> +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 +</p> + +<pre> +int x; +</pre> + +<p> +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. +</p> + +<p> +Thus, the declarations +</p> + +<pre> +int *p; +int a[3]; +</pre> + +<p> +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. +</p> + +<p> +What about functions? Originally, C's function declarations wrote the +types of the arguments outside the parens, like this: +</p> + +<pre> +int main(argc, argv) + int argc; + char *argv[]; +{ /* ... */ } +</pre> + +<p> +Again, we see that main is a function because the expression main(argc, +argv) returns an int. In modern notation we'd write +</p> + +<pre> +int main(int argc, char *argv[]) { /* ... */ } +</pre> + +<p> +but the basic structure is the same. +</p> + +<p> +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: +</p> + +<pre> +int (*fp)(int a, int b); +</pre> + +<p> +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? +</p> + +<pre> +int (*fp)(int (*ff)(int x, int y), int b) +</pre> + +<p> +That's starting to get hard to read. +</p> + +<p> +Of course, we can leave out the name of the parameters when we declare a +function, so main can be declared +</p> + +<pre> +int main(int, char *[]) +</pre> + +<p> +Recall that argv is declared like this, +</p> + +<pre> +char *argv[] +</pre> + +<p> +so you drop the name from the <em>middle</em> 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. +</p> + +<p> +And look what happens to fp's declaration if you don't name the +parameters: +</p> + +<pre> +int (*fp)(int (*)(int, int), int) +</pre> + +<p> +Not only is it not obvious where to put the name inside +</p> + +<pre> +int (*)(int, int) +</pre> + +<p> +it's not exactly clear that it's a function pointer declaration at all. +And what if the return type is a function pointer? +</p> + +<pre> +int (*(*fp)(int (*)(int, int), int))(int, int) +</pre> + +<p> +It's hard even to see that this declaration is about fp. +</p> + +<p> +You can construct more elaborate examples but these should illustrate +some of the difficulties that C's declaration syntax can introduce. +</p> + +<p> +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 +</p> + +<pre> +(int)M_PI +</pre> + +<p> +<b>Go syntax</b> +</p> + +<p> +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) +</p> + +<pre> +x: int +p: pointer to int +a: array[3] of int +</pre> + +<p> +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: +</p> + +<pre> +x int +p *int +a [3]int +</pre> + +<p> +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. +</p> + +<p> +Now consider functions. Let's transcribe the declaration for main, even +though the main function in Go takes no arguments: +</p> + +<pre> +func main(argc int, argv *[]byte) int +</pre> + +<p> +Superficially that's not much different from C, but it reads well from +left to right: +</p> + +<p> +<em>function main takes an int and a pointer to a slice of bytes and returns an int.</em> +</p> + +<p> +Drop the parameter names and it's just as clear - they're always first +so there's no confusion. +</p> + +<pre> +func main(int, *[]byte) int +</pre> + +<p> +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): +</p> + +<pre> +f func(func(int,int) int, int) int +</pre> + +<p> +Or if f returns a function: +</p> + +<pre> +f func(func(int,int) int, int) func(int, int) int +</pre> + +<p> +It still reads clearly, from left to right, and it's always obvious +which name is being declared - the name comes first. +</p> + +<p> +The distinction between type and expression syntax makes it easy to +write and invoke closures in Go: +</p> + +<pre> +sum := func(a, b int) int { return a+b } (3, 4) +</pre> + +<p> +<b>Pointers</b> +</p> + +<p> +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: +</p> + +<pre> +var a []int +x = a[1] +</pre> + +<p> +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 +</p> + +<pre> +var p *int +x = *p +</pre> + +<p> +We couldn't say +</p> + +<pre> +var p *int +x = p* +</pre> + +<p> +because that postfix * would conflate with multiplication. We could have +used the Pascal ^, for example: +</p> + +<pre> +var p ^int +x = p^ +</pre> + +<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 +</p> + +<pre> +[]int("hi") +</pre> + +<p> +as a conversion, one must parenthesize the type if it starts with a *: +</p> + +<pre> +(*int)(nil) +</pre> + +<p> +Had we been willing to give up * as pointer syntax, those parentheses +would be unnecessary. +</p> + +<p> +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. +</p> + +<p> +Overall, though, we believe Go's type syntax is easier to understand +than C's, especially when things get complicated. +</p> + +<p> +<b>Notes</b> +</p> + +<p> +Go's declarations read left to right. It's been pointed out that C's +read in a spiral! See <a href="http://c-faq.com/decl/spiral.anderson.html"> +The "Clockwise/Spiral Rule"</a> by David Anderson. +</p> diff --git a/doc/articles/image-20.png b/doc/articles/image-20.png Binary files differnew file mode 100644 index 000000000..063e43064 --- /dev/null +++ b/doc/articles/image-20.png diff --git a/doc/articles/image-2a.png b/doc/articles/image-2a.png Binary files differnew file mode 100644 index 000000000..3f1c0afff --- /dev/null +++ b/doc/articles/image-2a.png diff --git a/doc/articles/image-2b.png b/doc/articles/image-2b.png Binary files differnew file mode 100644 index 000000000..32b247011 --- /dev/null +++ b/doc/articles/image-2b.png diff --git a/doc/articles/image-2c.png b/doc/articles/image-2c.png Binary files differnew file mode 100644 index 000000000..f9abce5b5 --- /dev/null +++ b/doc/articles/image-2c.png diff --git a/doc/articles/image-2d.png b/doc/articles/image-2d.png Binary files differnew file mode 100644 index 000000000..ed0a9f92c --- /dev/null +++ b/doc/articles/image-2d.png diff --git a/doc/articles/image-2e.png b/doc/articles/image-2e.png Binary files differnew file mode 100644 index 000000000..483b208e3 --- /dev/null +++ b/doc/articles/image-2e.png diff --git a/doc/articles/image-2f.png b/doc/articles/image-2f.png Binary files differnew file mode 100644 index 000000000..3dce02d5f --- /dev/null +++ b/doc/articles/image-2f.png diff --git a/doc/articles/image_draw.html b/doc/articles/image_draw.html new file mode 100644 index 000000000..848b65982 --- /dev/null +++ b/doc/articles/image_draw.html @@ -0,0 +1,222 @@ +<!--{ + "Title": "The Go image/draw package", + "Template": true +}--> + +<p> +<a href="http://golang.org/pkg/image/draw/">Package image/draw</a> 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. +</p> + +<p> +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: <code>dst = (src IN mask) OP dst</code>. +For a fully opaque mask, this reduces to the original Porter-Duff +formula: <code>dst = src OP dst</code>. In Go, a nil mask image is equivalent +to an infinitely sized, fully opaque mask image. +</p> + +<p> +The Porter-Duff paper presented +<a href="http://www.w3.org/TR/SVGCompositing/examples/compop-porterduff-examples.png">12 different composition operators</a>, +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 <code>Over</code> and <code>Src</code> constants. The <code>Over</code> 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 <code>Src</code> 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 <code>Src</code> operator is usually faster. +</p> + +<p><b>Geometric Alignment</b></p> + +<p> +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 <code>sp</code> and <code>mp</code>: the source rectangle is equal to <code>r</code> +translated so that <code>r.Min</code> in the destination image aligns with +<code>sp</code> in the source image, and similarly for <code>mp</code>. The effective +rectangle is also clipped to each image's bounds in their +respective co-ordinate space. +</p> + +<p> +<img src="image-20.png"> +</p> + +<p> +The <a href="http://golang.org/pkg/image/draw/#DrawMask"><code>DrawMask</code></a> +function takes seven arguments, but an explicit mask and mask-point +are usually unnecessary, so the +<a href="http://golang.org/pkg/image/draw/#Draw"><code>Draw</code></a> function takes five: +</p> + +<pre> +// 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) +</pre> + +<p> +The destination image must be mutable, so the image/draw package +defines a <a href="http://golang.org/pkg/image/draw/#Image"><code>draw.Image</code></a> +interface which has a <code>Set</code> method. +</p> + +{{code "../src/pkg/image/draw/draw.go" `/type Image/` `/}/`}} + +<p><b>Filling a Rectangle</b></p> + +<p> +To fill a rectangle with a solid color, use an <code>image.Uniform</code> +source. The <code>ColorImage</code> type re-interprets a <code>Color</code> as a +practically infinite-sized <code>Image</code> 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 <code>Uniform</code>. +</p> + +{{code "/doc/progs/image_draw.go" `/ZERO/` `/STOP/`}} + +<p> +To initialize a new image to all-blue: +</p> + +{{code "/doc/progs/image_draw.go" `/BLUE/` `/STOP/`}} + +<p> +To reset an image to transparent (or black, if the destination +image's color model cannot represent transparency), use +<code>image.Transparent</code>, which is an <code>image.Uniform</code>: +</p> + +{{code "/doc/progs/image_draw.go" `/RESET/` `/STOP/`}} + +<p> +<img src="image-2a.png"> +</p> + + +<p><b>Copying an Image</b></p> + +<p> +To copy from a rectangle <code>sr</code> in the source image to a rectangle +starting at a point <code>dp</code> in the destination, convert the source +rectangle into the destination image's co-ordinate space: +</p> + +{{code "/doc/progs/image_draw.go" `/RECT/` `/STOP/`}} + +<p> +Alternatively: +</p> + +{{code "/doc/progs/image_draw.go" `/RECT2/` `/STOP/`}} + +<p> +To copy the entire source image, use <code>sr = src.Bounds()</code>. +</p> + +<p> +<img src="image-2b.png"> +</p> + +<p><b>Scrolling an Image</b></p> + +<p> +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: +</p> + +{{code "/doc/progs/image_draw.go" `/SCROLL/` `/STOP/`}} + +<p><img src="image-2c.png"></p> + +<p><b>Converting an Image to RGBA</b></p> + +<p> +The result of decoding an image format might not be an +<code>image.RGBA</code>: decoding a GIF results in an <code>image.Paletted</code>, +decoding a JPEG results in a <code>ycbcr.YCbCr</code>, and the result of +decoding a PNG depends on the image data. To convert any image to +an <code>image.RGBA</code>: +</p> + +{{code "/doc/progs/image_draw.go" `/CONV/` `/STOP/`}} + +<p> +<img src="image-2d.png"> +</p> + +<p><b>Drawing Through a Mask</b></p> + +<p> +To draw an image through a circular mask with center <code>p</code> and radius +<code>r</code>: +</p> + +{{code "/doc/progs/image_draw.go" `/CIRCLE/` `/STOP/`}} +{{code "/doc/progs/image_draw.go" `/CIRCLE2/` `/STOP/`}} + +<p> +<img src="image-2e.png"> +</p> + +<p><b>Drawing Font Glyphs</b></p> + +<p> +To draw a font glyph in blue starting from a point <code>p</code>, draw with +an <code>image.ColorImage</code> source and an <code>image.Alpha mask</code>. For +simplicity, we aren't performing any sub-pixel positioning or +rendering, or correcting for a font's height above a baseline. +</p> + +{{code "/doc/progs/image_draw.go" `/GLYPH/` `/STOP/`}} + +<p> +<img src="image-2f.png"> +</p> + +<p><b>Performance</b></p> + +<p> +The image/draw package implementation demonstrates how to provide +an image manipulation function that is both general purpose, yet +efficient for common cases. The <code>DrawMask</code> 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 <code>image.RGBA</code> image onto another, or +drawing an <code>image.Alpha</code> mask (such as a font glyph) onto an +<code>image.RGBA</code> 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 <code>At</code> and <code>Set</code> 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. +</p> + + 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 @@ +<!--{ +"Title": "JSON and Go", +"Template": true +}--> + +<p> +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, +<a href="http://json.org">json.org</a>, provides a wonderfully clear and concise +definition of the standard. +</p> + +<p> +With the <a href="/pkg/encoding/json/">json package</a> it's a snap to read and +write JSON data from your Go programs. +</p> + +<p> +<b>Encoding</b> +</p> + +<p> +To encode JSON data we use the +<a href="/pkg/encoding/json/#Marshal"><code>Marshal</code></a> function. +</p> + +<pre> +func Marshal(v interface{}) ([]byte, error) +</pre> + +<p> +Given the Go data structure, <code>Message</code>, +</p> + +{{code "/doc/progs/json1.go" `/type Message/` `/STOP/`}} + +<p> +and an instance of <code>Message</code> +</p> + +{{code "/doc/progs/json1.go" `/m :=/`}} + +<p> +we can marshal a JSON-encoded version of m using <code>json.Marshal</code>: +</p> + +{{code "/doc/progs/json1.go" `/b, err :=/`}} + +<p> +If all is well, <code>err</code> will be <code>nil</code> and <code>b</code> +will be a <code>[]byte</code> containing this JSON data: +</p> + +<pre> +b == []byte(`{"Name":"Alice","Body":"Hello","Time":1294706395881547000}`) +</pre> + +<p> +Only data structures that can be represented as valid JSON will be encoded: +</p> + +<ul> +<li> +JSON objects only support strings as keys; to encode a Go map type it must be +of the form <code>map[string]T</code> (where <code>T</code> is any Go type +supported by the json package). +</li> +<li> +Channel, complex, and function types cannot be encoded. +</li> +<li> +Cyclic data structures are not supported; they will cause <code>Marshal</code> +to go into an infinite loop. +</li> +<li> +Pointers will be encoded as the values they point to (or 'null' if the pointer +is <code>nil</code>). +</li> +</ul> + +<p> +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. +</p> + +<p> +<b>Decoding</b> +</p> + +<p> +To decode JSON data we use the +<a href="/pkg/encoding/json/#Unmarshal"><code>Unmarshal</code></a> function. +</p> + +<pre> +func Unmarshal(data []byte, v interface{}) error +</pre> + +<p> +We must first create a place where the decoded data will be stored +</p> + +{{code "/doc/progs/json1.go" `/var m Message/`}} + +<p> +and call <code>json.Unmarshal</code>, passing it a <code>[]byte</code> of JSON +data and a pointer to <code>m</code> +</p> + +{{code "/doc/progs/json1.go" `/err := json.Unmarshal/`}} + +<p> +If <code>b</code> contains valid JSON that fits in <code>m</code>, after the +call <code>err</code> will be <code>nil</code> and the data from <code>b</code> +will have been stored in the struct <code>m</code>, as if by an assignment +like: +</p> + +{{code "/doc/progs/json1.go" `/m = Message/` `/STOP/`}} + +<p> +How does <code>Unmarshal</code> identify the fields in which to store the +decoded data? For a given JSON key <code>"Foo"</code>, <code>Unmarshal</code> +will look through the destination struct's fields to find (in order of +preference): +</p> + +<ul> +<li> +An exported field with a tag of <code>"Foo"</code> (see the +<a href="/ref/spec#Struct_types">Go spec</a> for more on struct tags), +</li> +<li> +An exported field named <code>"Foo"</code>, or +</li> +<li> +An exported field named <code>"FOO"</code> or <code>"FoO"</code> or some other +case-insensitive match of <code>"Foo"</code>. +</li> +</ul> + +<p> +What happens when the structure of the JSON data doesn't exactly match the Go +type? +</p> + +{{code "/doc/progs/json1.go" `/"Food":"Pickle"/` `/STOP/`}} + +<p> +<code>Unmarshal</code> 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 <code>Unmarshal</code>. +</p> + +<p> +But what if you don't know the structure of your JSON data beforehand? +</p> + +<p> +<b>Generic JSON with interface{}</b> +</p> + +<p> +The <code>interface{}</code> (empty interface) type describes an interface with +zero methods. Every Go type implements at least zero methods and therefore +satisfies the empty interface. +</p> + +<p> +The empty interface serves as a general container type: +</p> + +{{code "/doc/progs/json2.go" `/var i interface{}/` `/STOP/`}} + +<p> +A type assertion accesses the underlying concrete type: +</p> + +{{code "/doc/progs/json2.go" `/r := i/` `/STOP/`}} + +<p> +Or, if the underlying type is unknown, a type switch determines the type: +</p> + +{{code "/doc/progs/json2.go" `/switch v/` `/STOP/`}} + + +The json package uses <code>map[string]interface{}</code> and +<code>[]interface{}</code> values to store arbitrary JSON objects and arrays; +it will happily unmarshal any valid JSON blob into a plain +<code>interface{}</code> value. The default concrete Go types are: + +<ul> +<li> +<code>bool</code> for JSON booleans, +</li> +<li> +<code>float64</code> for JSON numbers, +</li> +<li> +<code>string</code> for JSON strings, and +</li> +<li> +<code>nil</code> for JSON null. +</li> +</ul> + +<p> +<b>Decoding arbitrary data</b> +</p> + +<p> +Consider this JSON data, stored in the variable <code>b</code>: +</p> + +{{code "/doc/progs/json3.go" `/b :=/`}} + +<p> +Without knowing this data's structure, we can decode it into an +<code>interface{}</code> value with <code>Unmarshal</code>: +</p> + +{{code "/doc/progs/json3.go" `/var f interface/` `/STOP/`}} + +<p> +At this point the Go value in <code>f</code> would be a map whose keys are +strings and whose values are themselves stored as empty interface values: +</p> + +{{code "/doc/progs/json3.go" `/f = map/` `/STOP/`}} + +<p> +To access this data we can use a type assertion to access <code>f</code>'s +underlying <code>map[string]interface{}</code>: +</p> + +{{code "/doc/progs/json3.go" `/m := f/`}} + +<p> +We can then iterate through the map with a range statement and use a type switch +to access its values as their concrete types: +</p> + +{{code "/doc/progs/json3.go" `/for k, v/` `/STOP/`}} + +<p> +In this way you can work with unknown JSON data while still enjoying the +benefits of type safety. +</p> + +<p> +<b>Reference Types</b> +</p> + +<p> +Let's define a Go type to contain the data from the previous example: +</p> + +{{code "/doc/progs/json4.go" `/type FamilyMember/` `/STOP/`}} + +{{code "/doc/progs/json4.go" `/var m FamilyMember/` `/STOP/`}} + +<p> +Unmarshaling that data into a <code>FamilyMember</code> value works as +expected, but if we look closely we can see a remarkable thing has happened. +With the var statement we allocated a <code>FamilyMember</code> struct, and +then provided a pointer to that value to <code>Unmarshal</code>, but at that +time the <code>Parents</code> field was a <code>nil</code> slice value. To +populate the <code>Parents</code> field, <code>Unmarshal</code> allocated a new +slice behind the scenes. This is typical of how <code>Unmarshal</code> works +with the supported reference types (pointers, slices, and maps). +</p> + +<p> +Consider unmarshaling into this data structure: +</p> + +<pre> +type Foo struct { + Bar *Bar +} +</pre> + +<p> +If there were a <code>Bar</code> field in the JSON object, +<code>Unmarshal</code> would allocate a new <code>Bar</code> and populate it. +If not, <code>Bar</code> would be left as a <code>nil</code> pointer. +</p> + +<p> +From this a useful pattern arises: if you have an application that receives a +few distinct message types, you might define "receiver" structure like +</p> + +<pre> +type IncomingMessage struct { + Cmd *Command + Msg *Message +} +</pre> + +<p> +and the sending party can populate the <code>Cmd</code> field and/or the +<code>Msg</code> field of the top-level JSON object, depending on the type of +message they want to communicate. <code>Unmarshal</code>, when decoding the +JSON into an <code>IncomingMessage</code> 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 <code>Cmd</code> or <code>Msg</code> is +not <code>nil</code>. +</p> + +<p> +<b>Streaming Encoders and Decoders</b> +</p> + +<p> +The json package provides <code>Decoder</code> and <code>Encoder</code> types +to support the common operation of reading and writing streams of JSON data. +The <code>NewDecoder</code> and <code>NewEncoder</code> functions wrap the +<a href="/pkg/io/#Reader"><code>io.Reader</code></a> and +<a href="/pkg/io/#Writer"><code>io.Writer</code></a> interface types. +</p> + +<pre> +func NewDecoder(r io.Reader) *Decoder +func NewEncoder(w io.Writer) *Encoder +</pre> + +<p> +Here's an example program that reads a series of JSON objects from standard +input, removes all but the <code>Name</code> field from each object, and then +writes the objects to standard output: +</p> + +{{code "/doc/progs/json5.go" `/package main/` `$`}} + +<p> +Due to the ubiquity of Readers and Writers, these <code>Encoder</code> and +<code>Decoder</code> types can be used in a broad range of scenarios, such as +reading and writing to HTTP connections, WebSockets, or files. +</p> + +<p> +<b>References</b> +</p> + +<p> +For more information see the <a href="/pkg/encoding/json/">json package documentation</a>. For an example usage of +json see the source files of the <a href="/pkg/net/rpc/jsonrpc/">jsonrpc package</a>. +</p> diff --git a/doc/articles/laws_of_reflection.html b/doc/articles/laws_of_reflection.html index 4df70e0d2..a6175f73c 100644 --- a/doc/articles/laws_of_reflection.html +++ b/doc/articles/laws_of_reflection.html @@ -1,11 +1,7 @@ <!--{ - "Title": "The Laws of Reflection" + "Title": "The Laws of Reflection", + "Template": true }--> -<!-- - DO NOT EDIT: created by - tmpltohtml articles/laws_of_reflection.tmpl ---> - <p> Reflection in computing is the @@ -36,11 +32,7 @@ exactly one type known and fixed at compile time: <code>int</code>, and so on. If we declare </p> -<pre><!--{{code "progs/interface.go" `/type MyInt/` `/STOP/`}} --->type MyInt int - -var i int -var j MyInt</pre> +{{code "/doc/progs/interface.go" `/type MyInt/` `/STOP/`}} <p> then <code>i</code> has type <code>int</code> and <code>j</code> @@ -60,16 +52,7 @@ interface's methods. A well-known pair of examples is "http://golang.org/pkg/io/">io package</a>: </p> -<pre><!--{{code "progs/interface.go" `/// Reader/` `/STOP/`}} --->// Reader is the interface that wraps the basic Read method. -type Reader interface { - Read(p []byte) (n int, err error) -} - -// Writer is the interface that wraps the basic Write method. -type Writer interface { - Write(p []byte) (n int, err error) -}</pre> +{{code "/doc/progs/interface.go" `/// Reader/` `/STOP/`}} <p> Any type that implements a <code>Read</code> (or @@ -80,12 +63,7 @@ purposes of this discussion, that means that a variable of type <code>Read</code> method: </p> -<pre><!--{{code "progs/interface.go" `/func readers/` `/STOP/`}} ---> var r io.Reader - r = os.Stdin - r = bufio.NewReader(r) - r = new(bytes.Buffer) - // and so on</pre> +{{code "/doc/progs/interface.go" `/func readers/` `/STOP/`}} <p> It's important to be clear that whatever concrete value @@ -138,13 +116,7 @@ that implements the interface and the type describes the full type of that item. For instance, after </p> -<pre><!--{{code "progs/interface.go" `/func typeAssertions/` `/STOP/`}} ---> var r io.Reader - tty, err := os.OpenFile("/dev/tty", os.O_RDWR, 0) - if err != nil { - return nil, err - } - r = tty</pre> +{{code "/doc/progs/interface.go" `/func typeAssertions/` `/STOP/`}} <p> <code>r</code> contains, schematically, the (value, type) pair, @@ -156,9 +128,7 @@ the type information about that value. That's why we can do things like this: </p> -<pre><!--{{code "progs/interface.go" `/var w io.Writer/` `/STOP/`}} ---> var w io.Writer - w = r.(io.Writer)</pre> +{{code "/doc/progs/interface.go" `/var w io.Writer/` `/STOP/`}} <p> The expression in this assignment is a type assertion; what it @@ -176,9 +146,7 @@ methods. Continuing, we can do this: </p> -<pre><!--{{code "progs/interface.go" `/var empty interface{}/` `/STOP/`}} ---> var empty interface{} - empty = w</pre> +{{code "/doc/progs/interface.go" `/var empty interface{}/` `/STOP/`}} <p> and our empty interface value <code>e</code> will again contain @@ -216,7 +184,7 @@ 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 <a href="http://golang.org/pkg/reflect">package reflect</a>: -<a href="http://golang.org/pkg/reflect/#Type">Type</a>and +<a href="http://golang.org/pkg/reflect/#Type">Type</a> and <a href="http://golang.org/pkg/reflect/#Value">Value</a>. Those two types give access to the contents of an interface variable, and two simple functions, called <code>reflect.TypeOf</code> and @@ -232,18 +200,7 @@ now.) Let's start with <code>TypeOf</code>: </p> -<pre><!--{{code "progs/interface2.go" `/package main/` `/STOP main/`}} --->package main - -import ( - "fmt" - "reflect" -) - -func main() { - var x float64 = 3.4 - fmt.Println("type:", reflect.TypeOf(x)) -}</pre> +{{code "/doc/progs/interface2.go" `/package main/` `/STOP main/`}} <p> This program prints @@ -281,9 +238,7 @@ value (from here on we'll elide the boilerplate and focus just on the executable code): </p> -<pre><!--{{code "progs/interface2.go" `/var x/` `/STOP/`}} ---> var x float64 = 3.4 - fmt.Println("type:", reflect.TypeOf(x))</pre> +{{code "/doc/progs/interface2.go" `/START f9/` `/STOP/`}} <p> prints @@ -307,12 +262,7 @@ on. Also methods on <code>Value</code> with names like <code>int64</code> and <code>float64</code>) stored inside: </p> -<pre><!--{{code "progs/interface2.go" `/START f1/` `/STOP/`}} ---> var x float64 = 3.4 - v := reflect.ValueOf(x) - fmt.Println("type:", v.Type()) - fmt.Println("kind is float64:", v.Kind() == reflect.Float64) - fmt.Println("value:", v.Float())</pre> +{{code "/doc/progs/interface2.go" `/START f1/` `/STOP/`}} <p> prints @@ -342,12 +292,7 @@ instance. That is, the <code>Int</code> method of necessary to convert to the actual type involved: </p> -<pre><!--{{code "progs/interface2.go" `/START f2/` `/STOP/`}} ---> var x uint8 = 'x' - v := reflect.ValueOf(x) - fmt.Println("type:", v.Type()) // uint8. - fmt.Println("kind is uint8: ", v.Kind() == reflect.Uint8) // true. - x = uint8(v.Uint()) // v.Uint returns a uint64.</pre> +{{code "/doc/progs/interface2.go" `/START f2/` `/STOP/`}} <p> The second property is that the <code>Kind</code> of a reflection @@ -356,10 +301,7 @@ reflection object contains a value of a user-defined integer type, as in </p> -<pre><!--{{code "progs/interface2.go" `/START f3/` `/START/`}} ---> type MyInt int - var x MyInt = 7 - v := reflect.ValueOf(x)</pre> +{{code "/doc/progs/interface2.go" `/START f3/` `/STOP/`}} <p> the <code>Kind</code> of <code>v</code> is still @@ -395,9 +337,7 @@ func (v Value) Interface() interface{} As a consequence we can say </p> -<pre><!--{{code "progs/interface2.go" `/START f3b/` `/START/`}} ---> y := v.Interface().(float64) // y will have type float64. - fmt.Println(y)</pre> +{{code "/doc/progs/interface2.go" `/START f3b/` `/STOP/`}} <p> to print the <code>float64</code> value represented by the @@ -415,8 +355,7 @@ the <code>Interface</code> method to the formatted print routine: </p> -<pre><!--{{code "progs/interface2.go" `/START f3c/` `/START/`}} ---> fmt.Println(v.Interface())</pre> +{{code "/doc/progs/interface2.go" `/START f3c/` `/STOP/`}} <p> (Why not <code>fmt.Println(v)</code>? Because <code>v</code> is a @@ -425,8 +364,7 @@ Since our value is a <code>float64</code>, we can even use a floating-point format if we want: </p> -<pre><!--{{code "progs/interface2.go" `/START f3d/` `/STOP/`}} ---> fmt.Printf("value is %7.1e\n", v.Interface())</pre> +{{code "/doc/progs/interface2.go" `/START f3d/` `/STOP/`}} <p> and get in this case @@ -467,10 +405,7 @@ enough to understand if we start from first principles. Here is some code that does not work, but is worth studying. </p> -<pre><!--{{code "progs/interface2.go" `/START f4/` `/STOP/`}} ---> var x float64 = 3.4 - v := reflect.ValueOf(x) - v.SetFloat(7.1) // Error: will panic.</pre> +{{code "/doc/progs/interface2.go" `/START f4/` `/STOP/`}} <p> If you run this code, it will panic with the cryptic message @@ -492,10 +427,7 @@ The <code>CanSet</code> method of <code>Value</code> reports the settability of a <code>Value</code>; in our case, </p> -<pre><!--{{code "progs/interface2.go" `/START f5/` `/STOP/`}} ---> var x float64 = 3.4 - v := reflect.ValueOf(x) - fmt.Println("settability of v:", v.CanSet())</pre> +{{code "/doc/progs/interface2.go" `/START f5/` `/STOP/`}} <p> prints @@ -518,9 +450,7 @@ determined by whether the reflection object holds the original item. When we say </p> -<pre><!--{{code "progs/interface2.go" `/START f6/` `/START/`}} ---> var x float64 = 3.4 - v := reflect.ValueOf(x)</pre> +{{code "/doc/progs/interface2.go" `/START f6/` `/STOP/`}} <p> we pass a <em>copy</em> of <code>x</code> to @@ -530,8 +460,7 @@ argument to <code>reflect.ValueOf</code> is a <em>copy</em> of statement </p> -<pre><!--{{code "progs/interface2.go" `/START f6b/` `/STOP/`}} ---> v.SetFloat(7.1)</pre> +{{code "/doc/progs/interface2.go" `/START f6b/` `/STOP/`}} <p> were allowed to succeed, it would not update <code>x</code>, even @@ -577,11 +506,7 @@ and then create a reflection value that points to it, called <code>p</code>. </p> -<pre><!--{{code "progs/interface2.go" `/START f7/` `/START/`}} ---> var x float64 = 3.4 - p := reflect.ValueOf(&x) // Note: take the address of x. - fmt.Println("type of p:", p.Type()) - fmt.Println("settability of p:", p.CanSet())</pre> +{{code "/doc/progs/interface2.go" `/START f7/` `/STOP/`}} <p> The output so far is @@ -601,9 +526,7 @@ and save the result in a reflection <code>Value</code> called <code>v</code>: </p> -<pre><!--{{code "progs/interface2.go" `/START f7b/` `/START/`}} ---> v := p.Elem() - fmt.Println("settability of v:", v.CanSet())</pre> +{{code "/doc/progs/interface2.go" `/START f7b/` `/STOP/`}} <p> Now <code>v</code> is a settable reflection object, as the output @@ -620,10 +543,7 @@ and since it represents <code>x</code>, we are finally able to use <code>x</code>: </p> -<pre><!--{{code "progs/interface2.go" `/START f7c/` `/STOP/`}} ---> v.SetFloat(7.1) - fmt.Println(v.Interface()) - fmt.Println(x)</pre> +{{code "/doc/progs/interface2.go" `/START f7c/` `/STOP/`}} <p> The output, as expected, is @@ -664,22 +584,7 @@ but the fields themselves are regular <code>reflect.Value</code> objects. </p> -<pre><!--{{code "progs/interface2.go" `/START f8/` `/STOP/`}} ---> type T struct { - A int - B string - } - t := T{23, "skidoo"} - s := reflect.ValueOf(&t).Elem() - typeOfT := s.Type() - for i := 0; i < s.NumField(); i++ { - f := s.Field(i) - fmt.Printf("%d: %s %s = %v\n", i, - typeOfT.Field(i).Name, f.Type(), f.Interface()) - } - s.Field(0).SetInt(77) - s.Field(1).SetString("Sunset Strip") - fmt.Println("t is now", t)</pre> +{{code "/doc/progs/interface2.go" `/START f8/` `/STOP/`}} <p> The output of this program is @@ -702,10 +607,7 @@ Because <code>s</code> contains a settable reflection object, we can modify the fields of the structure. </p> -<pre><!--{{code "progs/interface2.go" `/START f8b/` `/STOP/`}} ---> s.Field(0).SetInt(77) - s.Field(1).SetString("Sunset Strip") - fmt.Println("t is now", t)</pre> +{{code "/doc/progs/interface2.go" `/START f8b/` `/STOP/`}} <p> And here's the result: @@ -749,4 +651,4 @@ 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. -</p>
\ No newline at end of file +</p> diff --git a/doc/articles/laws_of_reflection.tmpl b/doc/articles/laws_of_reflection.tmpl deleted file mode 100644 index 7db5d6d3b..000000000 --- a/doc/articles/laws_of_reflection.tmpl +++ /dev/null @@ -1,654 +0,0 @@ -<!--{ - "Title": "The Laws of Reflection" -}--> -{{donotedit}} - -<p> -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. -</p> - -<p> -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". -</p> - -<p><b>Types and interfaces</b></p> - -<p> -Because reflection builds on the type system, let's start with a -refresher about types in Go. -</p> - -<p> -Go is statically typed. Every variable has a static type, that is, -exactly one type known and fixed at compile time: <code>int</code>, -<code>float32</code>, <code>*MyType</code>, <code>[]byte</code>, -and so on. If we declare -</p> - -{{code "progs/interface.go" `/type MyInt/` `/STOP/`}} - -<p> -then <code>i</code> has type <code>int</code> and <code>j</code> -has type <code>MyInt</code>. The variables <code>i</code> and -<code>j</code> have distinct static types and, although they have -the same underlying type, they cannot be assigned to one another -without a conversion. -</p> - -<p> -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 -<code>io.Reader</code> and <code>io.Writer</code>, the types -<code>Reader</code> and <code>Writer</code> from the <a href= -"http://golang.org/pkg/io/">io package</a>: -</p> - -{{code "progs/interface.go" `/// Reader/` `/STOP/`}} - -<p> -Any type that implements a <code>Read</code> (or -<code>Write</code>) method with this signature is said to implement -<code>io.Reader</code> (or <code>io.Writer</code>). For the -purposes of this discussion, that means that a variable of type -<code>io.Reader</code> can hold any value whose type has a -<code>Read</code> method: -</p> - -{{code "progs/interface.go" `/func readers/` `/STOP/`}} - -<p> -It's important to be clear that whatever concrete value -<code>r</code> may hold, <code>r</code>'s type is always -<code>io.Reader</code>: Go is statically typed and the static type -of <code>r</code> is <code>io.Reader</code>.</p> - -<p> -An extremely important example of an interface type is the empty -interface: -</p> - -<pre> -interface{} -</pre> - -<p> -It represents the empty set of methods and is satisfied by any -value at all, since any value has zero or more methods. -</p> - -<p> -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. -</p> - -<p> -We need to be precise about all this because reflection and -interfaces are closely related. -</p> - -<p><b>The representation of an interface</b></p> - -<p> -Russ Cox has written a <a href= -"http://research.swtch.com/2009/12/go-data-structures-interfaces.html"> -detailed blog post</a> 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. -</p> - -<p> -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 -</p> - -{{code "progs/interface.go" `/func typeAssertions/` `/STOP/`}} - -<p> -<code>r</code> contains, schematically, the (value, type) pair, -(<code>tty</code>, <code>*os.File</code>). Notice that the type -<code>*os.File</code> implements methods other than -<code>Read</code>; even though the interface value provides access -only to the <code>Read</code> method, the value inside carries all -the type information about that value. That's why we can do things -like this: -</p> - -{{code "progs/interface.go" `/var w io.Writer/` `/STOP/`}} - -<p> -The expression in this assignment is a type assertion; what it -asserts is that the item inside <code>r</code> also implements -<code>io.Writer</code>, and so we can assign it to <code>w</code>. -After the assignment, <code>w</code> will contain the pair -(<code>tty</code>, <code>*os.File</code>). That's the same pair as -was held in <code>r</code>. 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. -</p> - -<p> -Continuing, we can do this: -</p> - -{{code "progs/interface.go" `/var empty interface{}/` `/STOP/`}} - -<p> -and our empty interface value <code>e</code> will again contain -that same pair, (<code>tty</code>, <code>*os.File</code>). That's -handy: an empty interface can hold any value and contains all the -information we could ever need about that value. -</p> - -<p> -(We don't need a type assertion here because it's known statically -that <code>w</code> satisfies the empty interface. In the example -where we moved a value from a <code>Reader</code> to a -<code>Writer</code>, we needed to be explicit and use a type -assertion because <code>Writer</code>'s methods are not a -subset of <code>Reader</code>'s.) -</p> - -<p> -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. -</p> - -<p> -Now we're ready to reflect. -</p> - -<p><b>The first law of reflection</b></p> - -<p><b>1. Reflection goes from interface value to reflection object.</b></p> - -<p> -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 -<a href="http://golang.org/pkg/reflect">package reflect</a>: -<a href="http://golang.org/pkg/reflect/#Type">Type</a>and -<a href="http://golang.org/pkg/reflect/#Value">Value</a>. Those two types -give access to the contents of an interface variable, and two -simple functions, called <code>reflect.TypeOf</code> and -<code>reflect.ValueOf</code>, retrieve <code>reflect.Type</code> -and <code>reflect.Value</code> pieces out of an interface value. -(Also, from the <code>reflect.Value</code> it's easy to get -to the <code>reflect.Type</code>, but let's keep the -<code>Value</code> and <code>Type</code> concepts separate for -now.) -</p> - -<p> -Let's start with <code>TypeOf</code>: -</p> - -{{code "progs/interface2.go" `/package main/` `/STOP main/`}} - -<p> -This program prints -</p> - -<pre> -type: float64 -</pre> - -<p> -You might be wondering where the interface is here, since the -program looks like it's passing the <code>float64</code> -variable <code>x</code>, not an interface value, to -<code>reflect.TypeOf</code>. But it's there; as <a href= -"http://golang.org/pkg/reflect/#Type.TypeOf">godoc reports</a>, the -signature of <code>reflect.TypeOf</code> includes an empty -interface: -</p> - -<pre> -// TypeOf returns the reflection Type of the value in the interface{}. -func TypeOf(i interface{}) Type -</pre> - -<p> -When we call <code>reflect.TypeOf(x)</code>, <code>x</code> is -first stored in an empty interface, which is then passed as the -argument; <code>reflect.TypeOf</code> unpacks that empty interface -to recover the type information. -</p> - -<p> -The <code>reflect.ValueOf</code> function, of course, recovers the -value (from here on we'll elide the boilerplate and focus just on -the executable code): -</p> - -{{code "progs/interface2.go" `/var x/` `/STOP/`}} - -<p> -prints -</p> - -<pre> -value: <float64 Value> -</pre> - -<p> -Both <code>reflect.Type</code> and <code>reflect.Value</code> have -lots of methods to let us examine and manipulate them. One -important example is that <code>Value</code> has a -<code>Type</code> method that returns the <code>Type</code> of a -<code>reflect.Value</code>. Another is that both <code>Type</code> -and <code>Value</code> have a <code>Kind</code> method that returns -a constant indicating what sort of item is stored: -<code>Uint</code>, <code>Float64</code>, <code>Slice</code>, and so -on. Also methods on <code>Value</code> with names like -<code>Int</code> and <code>Float</code> let us grab values (as -<code>int64</code> and <code>float64</code>) stored inside: -</p> - -{{code "progs/interface2.go" `/START f1/` `/STOP/`}} - -<p> -prints -</p> - -<pre> -type: float64 -kind is float64: true -value: 3.4 -</pre> - -<p> -There are also methods like <code>SetInt</code> and -<code>SetFloat</code> but to use them we need to understand -settability, the subject of the third law of reflection, discussed -below. -</p> - -<p> -The reflection library has a couple of properties worth singling -out. First, to keep the API simple, the "getter" and "setter" -methods of <code>Value</code> operate on the largest type that can -hold the value: <code>int64</code> for all the signed integers, for -instance. That is, the <code>Int</code> method of -<code>Value</code> returns an <code>int64</code> and the -<code>SetInt</code> value takes an <code>int64</code>; it may be -necessary to convert to the actual type involved: -</p> - -{{code "progs/interface2.go" `/START f2/` `/STOP/`}} - -<p> -The second property is that the <code>Kind</code> 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 -</p> - -{{code "progs/interface2.go" `/START f3/` `/START/`}} - -<p> -the <code>Kind</code> of <code>v</code> is still -<code>reflect.Int</code>, even though the static type of -<code>x</code> is <code>MyInt</code>, not <code>int</code>. In -other words, the <code>Kind</code> cannot discriminate an int from -a <code>MyInt</code> even though the <code>Type</code> can. -</p> - -<p><b>The second law of reflection</b></p> - -<p><b>2. Reflection goes from reflection object to interface -value.</b></p> - -<p> -Like physical reflection, reflection in Go generates its own -inverse. -</p> - -<p> -Given a <code>reflect.Value</code> we can recover an interface -value using the <code>Interface</code> method; in effect the method -packs the type and value information back into an interface -representation and returns the result: -</p> - -<pre> -// Interface returns v's value as an interface{}. -func (v Value) Interface() interface{} -</pre> - -<p> -As a consequence we can say -</p> - -{{code "progs/interface2.go" `/START f3b/` `/START/`}} - -<p> -to print the <code>float64</code> value represented by the -reflection object <code>v</code>. -</p> - -<p> -We can do even better, though. The arguments to -<code>fmt.Println</code>, <code>fmt.Printf</code> and so on are all -passed as empty interface values, which are then unpacked by the -<code>fmt</code> package internally just as we have been doing in -the previous examples. Therefore all it takes to print the contents -of a <code>reflect.Value</code> correctly is to pass the result of -the <code>Interface</code> method to the formatted print -routine: -</p> - -{{code "progs/interface2.go" `/START f3c/` `/START/`}} - -<p> -(Why not <code>fmt.Println(v)</code>? Because <code>v</code> is a -<code>reflect.Value</code>; we want the concrete value it holds.) -Since our value is a <code>float64</code>, we can even use a -floating-point format if we want: -</p> - -{{code "progs/interface2.go" `/START f3d/` `/STOP/`}} - -<p> -and get in this case -</p> - -<pre> -3.4e+00 -</pre> - -<p> -Again, there's no need to type-assert the result of -<code>v.Interface()</code> to <code>float64</code>; the empty -interface value has the concrete value's type information inside -and <code>Printf</code> will recover it. -</p> - -<p> -In short, the <code>Interface</code> method is the inverse of the -<code>ValueOf</code> function, except that its result is always of -static type <code>interface{}</code>. -</p> - -<p> -Reiterating: Reflection goes from interface values to reflection -objects and back again. -</p> - -<p><b>The third law of reflection</b></p> - -<p><b>3. To modify a reflection object, the value must be settable.</b></p> - -<p> -The third law is the most subtle and confusing, but it's easy -enough to understand if we start from first principles. -</p> - -<p> -Here is some code that does not work, but is worth studying. -</p> - -{{code "progs/interface2.go" `/START f4/` `/STOP/`}} - -<p> -If you run this code, it will panic with the cryptic message -</p> - -<pre> -panic: reflect.Value.SetFloat using unaddressable value -</pre> - -<p> -The problem is not that the value <code>7.1</code> is not -addressable; it's that <code>v</code> is not settable. Settability -is a property of a reflection <code>Value</code>, and not all -reflection <code>Values</code> have it. -</p> - -<p> -The <code>CanSet</code> method of <code>Value</code> reports the -settability of a <code>Value</code>; in our case, -</p> - -{{code "progs/interface2.go" `/START f5/` `/STOP/`}} - -<p> -prints -</p> - -<pre> -settability of v: false -</pre> - -<p> -It is an error to call a <code>Set</code> method on an non-settable -<code>Value</code>. But what is settability? -</p> - -<p> -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 -</p> - -{{code "progs/interface2.go" `/START f6/` `/START/`}} - -<p> -we pass a <em>copy</em> of <code>x</code> to -<code>reflect.ValueOf</code>, so the interface value created as the -argument to <code>reflect.ValueOf</code> is a <em>copy</em> of -<code>x</code>, not <code>x</code> itself. Thus, if the -statement -</p> - -{{code "progs/interface2.go" `/START f6b/` `/STOP/`}} - -<p> -were allowed to succeed, it would not update <code>x</code>, even -though <code>v</code> looks like it was created from -<code>x</code>. Instead, it would update the copy of <code>x</code> -stored inside the reflection value and <code>x</code> itself would -be unaffected. That would be confusing and useless, so it is -illegal, and settability is the property used to avoid this -issue. -</p> - -<p> -If this seems bizarre, it's not. It's actually a familiar situation -in unusual garb. Think of passing <code>x</code> to a -function: -</p> - -<pre> -f(x) -</pre> - -<p> -We would not expect <code>f</code> to be able to modify -<code>x</code> because we passed a copy of <code>x</code>'s value, -not <code>x</code> itself. If we want <code>f</code> to modify -<code>x</code> directly we must pass our function the address of -<code>x</code> (that is, a pointer to <code>x</code>):</p> - -<p> -<code>f(&x)</code> -</p> - -<p> -This is straightforward and familiar, and reflection works the same -way. If we want to modify <code>x</code> by reflection, we must -give the reflection library a pointer to the value we want to -modify. -</p> - -<p> -Let's do that. First we initialize <code>x</code> as usual -and then create a reflection value that points to it, called -<code>p</code>. -</p> - -{{code "progs/interface2.go" `/START f7/` `/START/`}} - -<p> -The output so far is -</p> - -<pre> -type of p: *float64 -settability of p: false -</pre> - -<p> -The reflection object <code>p</code> isn't settable, but it's not -<code>p</code> we want to set, it's (in effect) <code>*p</code>. To -get to what <code>p</code> points to, we call the <code>Elem</code> -method of <code>Value</code>, which indirects through the pointer, -and save the result in a reflection <code>Value</code> called -<code>v</code>: -</p> - -{{code "progs/interface2.go" `/START f7b/` `/START/`}} - -<p> -Now <code>v</code> is a settable reflection object, as the output -demonstrates, -</p> - -<pre> -settability of v: true -</pre> - -<p> -and since it represents <code>x</code>, we are finally able to use -<code>v.SetFloat</code> to modify the value of -<code>x</code>: -</p> - -{{code "progs/interface2.go" `/START f7c/` `/STOP/`}} - -<p> -The output, as expected, is -</p> - -<pre> -7.1 -7.1 -</pre> - -<p> -Reflection can be hard to understand but it's doing exactly what -the language does, albeit through reflection <code>Types</code> and -<code>Values</code> 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. -</p> - -<p><b>Structs</b></p> - -<p> -In our previous example <code>v</code> 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. -</p> - -<p> -Here's a simple example that analyzes a struct value, -<code>t</code>. We create the reflection object with the address of -the struct because we'll want to modify it later. Then we set -<code>typeOfT</code> to its type and iterate over the fields using -straightforward method calls (see -<a href="http://golang.org/pkg/reflect/">package reflect</a> for details). -Note that we extract the names of the fields from the struct type, -but the fields themselves are regular <code>reflect.Value</code> -objects. -</p> - -{{code "progs/interface2.go" `/START f8/` `/STOP/`}} - -<p> -The output of this program is -</p> - -<pre> -0: A int = 23 -1: B string = skidoo -</pre> - -<p> -There's one more point about settability introduced in -passing here: the field names of <code>T</code> are upper case -(exported) because only exported fields of a struct are -settable. -</p> - -<p> -Because <code>s</code> contains a settable reflection object, we -can modify the fields of the structure. -</p> - -{{code "progs/interface2.go" `/START f8b/` `/STOP/`}} - -<p> -And here's the result: -</p> - -<pre> -t is now {77 Sunset Strip} -</pre> - -<p> -If we modified the program so that <code>s</code> was created from -<code>t</code>, not <code>&t</code>, the calls to -<code>SetInt</code> and <code>SetString</code> would fail as the -fields of <code>t</code> would not be settable. -</p> - -<p><b>Conclusion</b></p> - -<p> -Here again are the laws of reflection: -</p> - -<ol> -<li>Reflection goes from interface value to reflection -object.</li> -<li>Reflection goes from reflection object to interface -value.</li> -<li>To modify a reflection object, the value must be settable.</li> -</ol> - -<p> -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. -</p> - -<p> -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. -</p>
\ No newline at end of file diff --git a/doc/articles/slices_usage_and_internals.html b/doc/articles/slices_usage_and_internals.html index c10dfe0ca..810b0a41f 100644 --- a/doc/articles/slices_usage_and_internals.html +++ b/doc/articles/slices_usage_and_internals.html @@ -1,11 +1,7 @@ <!--{ - "Title": "Slices: usage and internals" + "Title": "Slices: usage and internals", + "Template": true }--> -<!-- - DO NOT EDIT: created by - tmpltohtml articles/slices_usage_and_internals.tmpl ---> - <p> Go's slice type provides a convenient and efficient means of working with @@ -326,20 +322,7 @@ appends byte elements to a slice of bytes, growing the slice if necessary, and returns the updated slice value: </p> -<pre><!--{{code "progs/slices.go" `/AppendByte/` `/STOP/`}} --->func AppendByte(slice []byte, data ...byte) []byte { - m := len(slice) - n := m + len(data) - if n > cap(slice) { // if necessary, reallocate - // allocate double what's needed, for future growth. - newSlice := make([]byte, (n+1)*2) - copy(newSlice, slice) - slice = newSlice - } - slice = slice[0:n] - copy(slice[m:n], data) - return slice -}</pre> +{{code "/doc/progs/slices.go" `/AppendByte/` `/STOP/`}} <p> One could use <code>AppendByte</code> like this: @@ -398,18 +381,7 @@ Since the zero value of a slice (<code>nil</code>) acts like a zero-length slice, you can declare a slice variable and then append to it in a loop: </p> -<pre><!--{{code "progs/slices.go" `/Filter/` `/STOP/`}} --->// Filter returns a new slice holding only -// the elements of s that satisfy f() -func Filter(s []int, fn func(int) bool) []int { - var p []int // == nil - for _, i := range s { - if fn(i) { - p = append(p, i) - } - } - return p -}</pre> +{{code "/doc/progs/slices.go" `/Filter/` `/STOP/`}} <p> <b>A possible "gotcha"</b> @@ -428,13 +400,7 @@ searches it for the first group of consecutive numeric digits, returning them as a new slice. </p> -<pre><!--{{code "progs/slices.go" `/digit/` `/STOP/`}} --->var digitRegexp = regexp.MustCompile("[0-9]+") - -func FindDigits(filename string) []byte { - b, _ := ioutil.ReadFile(filename) - return digitRegexp.Find(b) -}</pre> +{{code "/doc/progs/slices.go" `/digit/` `/STOP/`}} <p> This code behaves as advertised, but the returned <code>[]byte</code> points @@ -449,14 +415,7 @@ To fix this problem one can copy the interesting data to a new slice before returning it: </p> -<pre><!--{{code "progs/slices.go" `/CopyDigits/` `/STOP/`}} --->func CopyDigits(filename string) []byte { - b, _ := ioutil.ReadFile(filename) - b = digitRegexp.Find(b) - c := make([]byte, len(b)) - copy(c, b) - return c -}</pre> +{{code "/doc/progs/slices.go" `/CopyDigits/` `/STOP/`}} <p> A more concise version of this function could be constructed by using diff --git a/doc/articles/slices_usage_and_internals.tmpl b/doc/articles/slices_usage_and_internals.tmpl deleted file mode 100644 index d2f8fb7f5..000000000 --- a/doc/articles/slices_usage_and_internals.tmpl +++ /dev/null @@ -1,438 +0,0 @@ -<!--{ - "Title": "Slices: usage and internals" -}--> -{{donotedit}} - -<p> -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. -</p> - -<p> -<b>Arrays</b> -</p> - -<p> -The slice type is an abstraction built on top of Go's array type, and so to -understand slices we must first understand arrays. -</p> - -<p> -An array type definition specifies a length and an element type. For example, -the type <code>[4]int</code> represents an array of four integers. An array's -size is fixed; its length is part of its type (<code>[4]int</code> and -<code>[5]int</code> are distinct, incompatible types). Arrays can be indexed in -the usual way, so the expression <code>s[n]</code> accesses the <i>n</i>th -element: -</p> - -<pre> -var a [4]int -a[0] = 1 -i := a[0] -// i == 1 -</pre> - -<p> -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: -</p> - -<pre> -// a[2] == 0, the zero value of the int type -</pre> - -<p> -The in-memory representation of <code>[4]int</code> is just four integer values laid out sequentially: -</p> - -<p> -<img src="slice-array.png"> -</p> - -<p> -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 <i>pointer</i> 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. -</p> - -<p> -An array literal can be specified like so: -</p> - -<pre> -b := [2]string{"Penn", "Teller"} -</pre> - -<p> -Or, you can have the compiler count the array elements for you: -</p> - -<pre> -b := [...]string{"Penn", "Teller"} -</pre> - -<p> -In both cases, the type of <code>b</code> is <code>[2]string</code>. -</p> - -<p> -<b>Slices</b> -</p> - -<p> -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. -</p> - -<p> -The type specification for a slice is <code>[]T</code>, where <code>T</code> is -the type of the elements of the slice. Unlike an array type, a slice type has -no specified length. -</p> - -<p> -A slice literal is declared just like an array literal, except you leave out -the element count: -</p> - -<pre> -letters := []string{"a", "b", "c", "d"} -</pre> - -<p> -A slice can be created with the built-in function called <code>make</code>, -which has the signature, -</p> - -<pre> -func make([]T, len, cap) []T -</pre> - -<p> -where T stands for the element type of the slice to be created. The -<code>make</code> function takes a type, a length, and an optional capacity. -When called, <code>make</code> allocates an array and returns a slice that -refers to that array. -</p> - -<pre> -var s []byte -s = make([]byte, 5, 5) -// s == []byte{0, 0, 0, 0, 0} -</pre> - -<p> -When the capacity argument is omitted, it defaults to the specified length. -Here's a more succinct version of the same code: -</p> - -<pre> -s := make([]byte, 5) -</pre> - -<p> -The length and capacity of a slice can be inspected using the built-in -<code>len</code> and <code>cap</code> functions. -</p> - -<pre> -len(s) == 5 -cap(s) == 5 -</pre> - -<p> -The next two sections discuss the relationship between length and capacity. -</p> - -<p> -The zero value of a slice is <code>nil</code>. The <code>len</code> and -<code>cap</code> functions will both return 0 for a nil slice. -</p> - -<p> -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 <code>b[1:4]</code> creates a slice including elements -1 through 3 of <code>b</code> (the indices of the resulting slice will be 0 -through 2). -</p> - -<pre> -b := []byte{'g', 'o', 'l', 'a', 'n', 'g'} -// b[1:4] == []byte{'o', 'l', 'a'}, sharing the same storage as b -</pre> - -<p> -The start and end indices of a slice expression are optional; they default to zero and the slice's length respectively: -</p> - -<pre> -// b[:2] == []byte{'g', 'o'} -// b[2:] == []byte{'l', 'a', 'n', 'g'} -// b[:] == b -</pre> - -<p> -This is also the syntax to create a slice given an array: -</p> - -<pre> -x := [3]string{"Лайка", "Белка", "Стрелка"} -s := x[:] // a slice referencing the storage of x -</pre> - -<p> -<b>Slice internals</b> -</p> - -<p> -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). -</p> - -<p> -<img src="slice-struct.png"> -</p> - -<p> -Our variable <code>s</code>, created earlier by <code>make([]byte, 5)</code>, -is structured like this: -</p> - -<p> -<img src="slice-1.png"> -</p> - -<p> -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. -</p> - -<p> -As we slice <code>s</code>, observe the changes in the slice data structure and -their relation to the underlying array: -</p> - -<pre> -s = s[2:4] -</pre> - -<p> -<img src="slice-2.png"> -</p> - -<p> -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 <i>elements</i> (not the -slice itself) of a re-slice modifies the elements of the original slice: -</p> - -<pre> -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'} -</pre> - -<p> -Earlier we sliced <code>s</code> to a length shorter than its capacity. We can -grow s to its capacity by slicing it again: -</p> - -<pre> -s = s[:cap(s)] -</pre> - -<p> -<img src="slice-3.png"> -</p> - -<p> -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. -</p> - -<p> -<b>Growing slices (the copy and append functions)</b> -</p> - -<p> -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 <code>s</code> by making a new slice, -<code>t</code>, copying the contents of <code>s</code> into <code>t</code>, and -then assigning the slice value <code>t</code> to <code>s</code>: -</p> - -<pre> -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 -</pre> - -<p> -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. -</p> - -<pre> -func copy(dst, src []T) int -</pre> - -<p> -The <code>copy</code> function supports copying between slices of different -lengths (it will copy only up to the smaller number of elements). In addition, -<code>copy</code> can handle source and destination slices that share the same -underlying array, handling overlapping slices correctly. -</p> - -<p> -Using <code>copy</code>, we can simplify the code snippet above: -</p> - -<pre> -t := make([]byte, len(s), (cap(s)+1)*2) -copy(t, s) -s = t -</pre> - -<p> -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: -</p> - -{{code "progs/slices.go" `/AppendByte/` `/STOP/`}} - -<p> -One could use <code>AppendByte</code> like this: -</p> - -<pre> -p := []byte{2, 3, 5} -p = AppendByte(p, 7, 11, 13) -// p == []byte{2, 3, 5, 7, 11, 13} -</pre> - -<p> -Functions like <code>AppendByte</code> 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. -</p> - -<p> -But most programs don't need complete control, so Go provides a built-in -<code>append</code> function that's good for most purposes; it has the -signature -</p> - -<pre> -func append(s []T, x ...T) []T -</pre> - -<p> -The <code>append</code> function appends the elements <code>x</code> to the end -of the slice <code>s</code>, and grows the slice if a greater capacity is -needed. -</p> - -<pre> -a := make([]int, 1) -// a == []int{0} -a = append(a, 1, 2, 3) -// a == []int{0, 1, 2, 3} -</pre> - -<p> -To append one slice to another, use <code>...</code> to expand the second -argument to a list of arguments. -</p> - -<pre> -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"} -</pre> - -<p> -Since the zero value of a slice (<code>nil</code>) acts like a zero-length -slice, you can declare a slice variable and then append to it in a loop: -</p> - -{{code "progs/slices.go" `/Filter/` `/STOP/`}} - -<p> -<b>A possible "gotcha"</b> -</p> - -<p> -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. -</p> - -<p> -For example, this <code>FindDigits</code> function loads a file into memory and -searches it for the first group of consecutive numeric digits, returning them -as a new slice. -</p> - -{{code "progs/slices.go" `/digit/` `/STOP/`}} - -<p> -This code behaves as advertised, but the returned <code>[]byte</code> 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. -</p> - -<p> -To fix this problem one can copy the interesting data to a new slice before -returning it: -</p> - -{{code "progs/slices.go" `/CopyDigits/` `/STOP/`}} - -<p> -A more concise version of this function could be constructed by using -<code>append</code>. This is left as an exercise for the reader. -</p> - -<p> -<b>Further Reading</b> -</p> - -<p> -<a href="/doc/effective_go.html">Effective Go</a> contains an -in-depth treatment of <a href="/doc/effective_go.html#slices">slices</a> -and <a href="/doc/effective_go.html#arrays">arrays</a>, -and the Go <a href="/doc/go_spec.html">language specification</a> -defines <a href="/doc/go_spec.html#Slice_types">slices</a> and their -<a href="/doc/go_spec.html#Length_and_capacity">associated</a> -<a href="/doc/go_spec.html#Making_slices_maps_and_channels">helper</a> -<a href="/doc/go_spec.html#Appending_and_copying_slices">functions</a>. -</p> diff --git a/doc/articles/wiki/test.sh b/doc/articles/wiki/test.bash index 58b218a78..5c2cb60dc 100755 --- a/doc/articles/wiki/test.sh +++ b/doc/articles/wiki/test.bash @@ -1,4 +1,7 @@ #!/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= @@ -8,10 +11,10 @@ cleanup() { } trap cleanup 0 INT -make get.bin +go build -o get.bin get.go addr=$(./get.bin -addr) sed s/:8080/$addr/ < final.go > final-test.go -make final-test.bin +go build -o final-test.bin final-test.go (./final-test.bin) & wiki_pid=$! |
