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diff --git a/doc/go_tutorial.html b/doc/go_tutorial.html deleted file mode 100644 index 8f6e07b06..000000000 --- a/doc/go_tutorial.html +++ /dev/null @@ -1,1454 +0,0 @@ -<!-- A Tutorial for the Go Programming Language --> -<h2>Introduction</h2> -<p> -This document is a tutorial introduction to the basics of the Go programming -language, intended for programmers familiar with C or C++. It is not a comprehensive -guide to the language; at the moment the document closest to that is the -<a href='/doc/go_spec.html'>language specification</a>. -After you've read this tutorial, you should look at -<a href='/doc/effective_go.html'>Effective Go</a>, -which digs deeper into how the language is used and -talks about the style and idioms of programming in Go. -Also, slides from a 3-day course about Go are available. -They provide some background and a lot of examples: -<a href='/doc/GoCourseDay1.pdf'>Day 1</a>, -<a href='/doc/GoCourseDay2.pdf'>Day 2</a>, -<a href='/doc/GoCourseDay3.pdf'>Day 3</a>. -<p> -The presentation here proceeds through a series of modest programs to illustrate -key features of the language. All the programs work (at time of writing) and are -checked into the repository in the directory <a href='/doc/progs'><code>/doc/progs/</code></a>. -<p> -<h2>Hello, World</h2> -<p> -Let's start in the usual way: -<p> -<pre><!--{{code "progs/helloworld.go" `/package/` "$"}} --->package main - -import fmt "fmt" // Package implementing formatted I/O. - -func main() { - fmt.Printf("Hello, world; or Καλημέρα κόσμε; or こんにちは 世界\n") -} -</pre> -<p> -Every Go source file declares, using a <code>package</code> statement, which package it's part of. -It may also import other packages to use their facilities. -This program imports the package <code>fmt</code> to gain access to -our old, now capitalized and package-qualified, friend, <code>fmt.Printf</code>. -<p> -Functions are introduced with the <code>func</code> keyword. -The <code>main</code> package's <code>main</code> function is where the program starts running (after -any initialization). -<p> -String constants can contain Unicode characters, encoded in UTF-8. -(In fact, Go source files are defined to be encoded in UTF-8.) -<p> -The comment convention is the same as in C++: -<p> -<pre> -/* ... */ -// ... -</pre> -<p> -Later we'll have much more to say about printing. -<p> -<h2>Semicolons</h2> -<p> -You might have noticed that our program has no semicolons. In Go -code, the only place you typically see semicolons is separating the -clauses of <code>for</code> loops and the like; they are not necessary after -every statement. -<p> -In fact, what happens is that the formal language uses semicolons, -much as in C or Java, but they are inserted automatically -at the end of every line that looks like the end of a statement. You -don't need to type them yourself. -<p> -For details about how this is done you can see the language -specification, but in practice all you need to know is that you -never need to put a semicolon at the end of a line. (You can put -them in if you want to write multiple statements per line.) As an -extra help, you can also leave out a semicolon immediately before -a closing brace. -<p> -This approach makes for clean-looking, semicolon-free code. The -one surprise is that it's important to put the opening -brace of a construct such as an <code>if</code> statement on the same line as -the <code>if</code>; if you don't, there are situations that may not compile -or may give the wrong result. The language forces the brace style -to some extent. -<p> -<h2>Compiling</h2> -<p> -Go is a compiled language. At the moment there are two compilers. -<code>Gccgo</code> is a Go compiler that uses the GCC back end. There is also a -suite of compilers with different (and odd) names for each architecture: -<code>6g</code> for the 64-bit x86, <code>8g</code> for the 32-bit x86, and more. These -compilers run significantly faster but generate less efficient code -than <code>gccgo</code>. At the time of writing (late 2009), they also have -a more robust run-time system although <code>gccgo</code> is catching up. -<p> -Here's how to compile and run our program. With <code>6g</code>, say, -<p> -<pre> -$ 6g helloworld.go # compile; object goes into helloworld.6 -$ 6l helloworld.6 # link; output goes into 6.out -$ ./6.out -Hello, world; or Καλημέρα κόσμε; or こんにちは 世界 -$ -</pre> -<p> -With <code>gccgo</code> it looks a little more traditional. -<p> -<pre> -$ gccgo helloworld.go -$ a.out -Hello, world; or Καλημέρα κόσμε; or こんにちは 世界 -$ -</pre> -<p> -<h2>Echo</h2> -<p> -Next up, here's a version of the Unix utility <code>echo(1)</code>: -<p> -<pre><!--{{code "progs/echo.go" `/package/` "$"}} --->package main - -import ( - "os" - "flag" // command line option parser -) - -var omitNewline = flag.Bool("n", false, "don't print final newline") - -const ( - Space = " " - Newline = "\n" -) - -func main() { - flag.Parse() // Scans the arg list and sets up flags - var s string = "" - for i := 0; i < flag.NArg(); i++ { - if i > 0 { - s += Space - } - s += flag.Arg(i) - } - if !*omitNewline { - s += Newline - } - os.Stdout.WriteString(s) -} -</pre> -<p> -This program is small but it's doing a number of new things. In the last example, -we saw <code>func</code> introduce a function. The keywords <code>var</code>, <code>const</code>, and <code>type</code> -(not used yet) also introduce declarations, as does <code>import</code>. -Notice that we can group declarations of the same sort into -parenthesized lists, one item per line, as in the <code>import</code> and <code>const</code> clauses here. -But it's not necessary to do so; we could have said -<p> -<pre> -const Space = " " -const Newline = "\n" -</pre> -<p> -This program imports the <code>"os"</code> package to access its <code>Stdout</code> variable, of type -<code>*os.File</code>. The <code>import</code> statement is actually a declaration: in its general form, -as used in our ``hello world'' program, -it names the identifier (<code>fmt</code>) -that will be used to access members of the package imported from the file (<code>"fmt"</code>), -found in the current directory or in a standard location. -In this program, though, we've dropped the explicit name from the imports; by default, -packages are imported using the name defined by the imported package, -which by convention is of course the file name itself. Our ``hello world'' program -could have said just <code>import "fmt"</code>. -<p> -You can specify your -own import names if you want but it's only necessary if you need to resolve -a naming conflict. -<p> -Given <code>os.Stdout</code> we can use its <code>WriteString</code> method to print the string. -<p> -After importing the <code>flag</code> package, we use a <code>var</code> declaration -to create and initialize a global variable, called <code>omitNewline</code>, -to hold the value of echo's <code>-n</code> flag. -The variable has type <code>*bool</code>, pointer to <code>bool</code>. -<p> -In <code>main.main</code>, we parse the arguments (the call to <code>flag.Parse</code>) and then create a local -string variable with which to build the output. -<p> -The declaration statement has the form -<p> -<pre> -var s string = "" -</pre> -<p> -This is the <code>var</code> keyword, followed by the name of the variable, followed by -its type, followed by an equals sign and an initial value for the variable. -<p> -Go tries to be terse, and this declaration could be shortened. Since the -string constant is of type string, we don't have to tell the compiler that. -We could write -<p> -<pre> -var s = "" -</pre> -<p> -or we could go even shorter and write the idiom -<p> -<pre> -s := "" -</pre> -<p> -The <code>:=</code> operator is used a lot in Go to represent an initializing declaration. -There's one in the <code>for</code> clause on the next line: -<p> -<pre><!--{{code "progs/echo.go" `/for/`}} ---> for i := 0; i < flag.NArg(); i++ { -</pre> -<p> -The <code>flag</code> package has parsed the arguments and left the non-flag arguments -in a list that can be iterated over in the obvious way. -<p> -The Go <code>for</code> statement differs from that of C in a number of ways. First, -it's the only looping construct; there is no <code>while</code> or <code>do</code>. Second, -there are no parentheses on the clause, but the braces on the body -are mandatory. The same applies to the <code>if</code> and <code>switch</code> statements. -Later examples will show some other ways <code>for</code> can be written. -<p> -The body of the loop builds up the string <code>s</code> by appending (using <code>+=</code>) -the arguments and separating spaces. After the loop, if the <code>-n</code> flag is not -set, the program appends a newline. Finally, it writes the result. -<p> -Notice that <code>main.main</code> is a niladic function with no return type. -It's defined that way. Falling off the end of <code>main.main</code> means -''success''; if you want to signal an erroneous return, call -<p> -<pre> -os.Exit(1) -</pre> -<p> -The <code>os</code> package contains other essentials for getting -started; for instance, <code>os.Args</code> is a slice used by the -<code>flag</code> package to access the command-line arguments. -<p> -<h2>An Interlude about Types</h2> -<p> -Go has some familiar types such as <code>int</code> and <code>uint</code> (unsigned <code>int</code>), which represent -values of the ''appropriate'' size for the machine. It also defines -explicitly-sized types such as <code>int8</code>, <code>float64</code>, and so on, plus -unsigned integer types such as <code>uint</code>, <code>uint32</code>, etc. -These are distinct types; even if <code>int</code> and <code>int32</code> are both 32 bits in size, -they are not the same type. There is also a <code>byte</code> synonym for -<code>uint8</code>, which is the element type for strings. -<p> -Floating-point types are always sized: <code>float32</code> and <code>float64</code>, -plus <code>complex64</code> (two <code>float32s</code>) and <code>complex128</code> -(two <code>float64s</code>). Complex numbers are outside the -scope of this tutorial. -<p> -Speaking of <code>string</code>, that's a built-in type as well. Strings are -<i>immutable values</i>—they are not just arrays of <code>byte</code> values. -Once you've built a string <i>value</i>, you can't change it, although -of course you can change a string <i>variable</i> simply by -reassigning it. This snippet from <code>strings.go</code> is legal code: -<p> -<pre><!--{{code "progs/strings.go" `/hello/` `/ciao/`}} ---> s := "hello" - if s[1] != 'e' { - os.Exit(1) - } - s = "good bye" - var p *string = &s - *p = "ciao" -</pre> -<p> -However the following statements are illegal because they would modify -a <code>string</code> value: -<p> -<pre> -s[0] = 'x' -(*p)[1] = 'y' -</pre> -<p> -In C++ terms, Go strings are a bit like <code>const strings</code>, while pointers -to strings are analogous to <code>const string</code> references. -<p> -Yes, there are pointers. However, Go simplifies their use a little; -read on. -<p> -Arrays are declared like this: -<p> -<pre> -var arrayOfInt [10]int -</pre> -<p> -Arrays, like strings, are values, but they are mutable. This differs -from C, in which <code>arrayOfInt</code> would be usable as a pointer to <code>int</code>. -In Go, since arrays are values, it's meaningful (and useful) to talk -about pointers to arrays. -<p> -The size of the array is part of its type; however, one can declare -a <i>slice</i> variable to hold a reference to any array, of any size, -with the same element type. -A <i>slice -expression</i> has the form <code>a[low : high]</code>, representing -the internal array indexed from <code>low</code> through <code>high-1</code>; the resulting -slice is indexed from <code>0</code> through <code>high-low-1</code>. -In short, slices look a lot like arrays but with -no explicit size (<code>[]</code> vs. <code>[10]</code>) and they reference a segment of -an underlying, usually anonymous, regular array. Multiple slices -can share data if they represent pieces of the same array; -multiple arrays can never share data. -<p> -Slices are much more common in Go programs than -regular arrays; they're more flexible, have reference semantics, -and are efficient. What they lack is the precise control of storage -layout of a regular array; if you want to have a hundred elements -of an array stored within your structure, you should use a regular -array. To create one, use a compound value <i>constructor</i>—an -expression formed -from a type followed by a brace-bounded expression like this: -<p> -<pre> -[3]int{1,2,3} -</pre> -<p> -In this case the constructor builds an array of 3 <code>ints</code>. -<p> -When passing an array to a function, you almost always want -to declare the formal parameter to be a slice. When you call -the function, slice the array to create -(efficiently) a slice reference and pass that. -By default, the lower and upper bounds of a slice match the -ends of the existing object, so the concise notation <code>[:]</code> -will slice the whole array. -<p> -Using slices one can write this function (from <code>sum.go</code>): -<p> -<pre><!--{{code "progs/sum.go" `/sum/` `/^}/`}} --->func sum(a []int) int { // returns an int - s := 0 - for i := 0; i < len(a); i++ { - s += a[i] - } - return s -} -</pre> -<p> -Note how the return type (<code>int</code>) is defined for <code>sum</code> by stating it -after the parameter list. -<p> -To call the function, we slice the array. This intricate call (we'll show -a simpler way in a moment) constructs -an array and slices it: -<p> -<pre> -s := sum([3]int{1,2,3}[:]) -</pre> -<p> -If you are creating a regular array but want the compiler to count the -elements for you, use <code>...</code> as the array size: -<p> -<pre> -s := sum([...]int{1,2,3}[:]) -</pre> -<p> -That's fussier than necessary, though. -In practice, unless you're meticulous about storage layout within a -data structure, a slice itself—using empty brackets with no size—is all you need: -<p> -<pre> -s := sum([]int{1,2,3}) -</pre> -<p> -There are also maps, which you can initialize like this: -<p> -<pre> -m := map[string]int{"one":1 , "two":2} -</pre> -<p> -The built-in function <code>len</code>, which returns number of elements, -makes its first appearance in <code>sum</code>. It works on strings, arrays, -slices, maps, and channels. -<p> -By the way, another thing that works on strings, arrays, slices, maps -and channels is the <code>range</code> clause on <code>for</code> loops. Instead of writing -<p> -<pre> -for i := 0; i < len(a); i++ { ... } -</pre> -<p> -to loop over the elements of a slice (or map or ...) , we could write -<p> -<pre> -for i, v := range a { ... } -</pre> -<p> -This assigns <code>i</code> to the index and <code>v</code> to the value of the successive -elements of the target of the range. See -<a href='/doc/effective_go.html'>Effective Go</a> -for more examples of its use. -<p> -<p> -<h2>An Interlude about Allocation</h2> -<p> -Most types in Go are values. If you have an <code>int</code> or a <code>struct</code> -or an array, assignment -copies the contents of the object. -To allocate a new variable, use the built-in function <code>new</code>, which -returns a pointer to the allocated storage. -<p> -<pre> -type T struct { a, b int } -var t *T = new(T) -</pre> -<p> -or the more idiomatic -<p> -<pre> -t := new(T) -</pre> -<p> -Some types—maps, slices, and channels (see below)—have reference semantics. -If you're holding a slice or a map and you modify its contents, other variables -referencing the same underlying data will see the modification. For these three -types you want to use the built-in function <code>make</code>: -<p> -<pre> -m := make(map[string]int) -</pre> -<p> -This statement initializes a new map ready to store entries. -If you just declare the map, as in -<p> -<pre> -var m map[string]int -</pre> -<p> -it creates a <code>nil</code> reference that cannot hold anything. To use the map, -you must first initialize the reference using <code>make</code> or by assignment from an -existing map. -<p> -Note that <code>new(T)</code> returns type <code>*T</code> while <code>make(T)</code> returns type -<code>T</code>. If you (mistakenly) allocate a reference object with <code>new</code> rather than <code>make</code>, -you receive a pointer to a nil reference, equivalent to -declaring an uninitialized variable and taking its address. -<p> -<h2>An Interlude about Constants</h2> -<p> -Although integers come in lots of sizes in Go, integer constants do not. -There are no constants like <code>0LL</code> or <code>0x0UL</code>. Instead, integer -constants are evaluated as large-precision values that -can overflow only when they are assigned to an integer variable with -too little precision to represent the value. -<p> -<pre> -const hardEight = (1 << 100) >> 97 // legal -</pre> -<p> -There are nuances that deserve redirection to the legalese of the -language specification but here are some illustrative examples: -<p> -<pre> -var a uint64 = 0 // a has type uint64, value 0 -a := uint64(0) // equivalent; uses a "conversion" -i := 0x1234 // i gets default type: int -var j int = 1e6 // legal - 1000000 is representable in an int -x := 1.5 // a float64, the default type for floating constants -i3div2 := 3/2 // integer division - result is 1 -f3div2 := 3./2. // floating-point division - result is 1.5 -</pre> -<p> -Conversions only work for simple cases such as converting <code>ints</code> of one -sign or size to another and between integers and floating-point numbers, -plus a couple of other instances outside the scope of a tutorial. -There are no automatic numeric conversions of any kind in Go, -other than that of making constants have concrete size and type when -assigned to a variable. -<p> -<h2>An I/O Package</h2> -<p> -Next we'll look at a simple package for doing file I/O with an -open/close/read/write interface. Here's the start of <code>file.go</code>: -<p> -<pre><!--{{code "progs/file.go" `/package/` `/^}/`}} --->package file - -import ( - "os" - "syscall" -) - -type File struct { - fd int // file descriptor number - name string // file name at Open time -} -</pre> -<p> -The first few lines declare the name of the -package—<code>file</code>—and then import two packages. The <code>os</code> -package hides the differences -between various operating systems to give a consistent view of files and -so on; here we're going to use its error handling utilities -and reproduce the rudiments of its file I/O. -<p> -The other item is the low-level, external <code>syscall</code> package, which provides -a primitive interface to the underlying operating system's calls. -<p> -Next is a type definition: the <code>type</code> keyword introduces a type declaration, -in this case a data structure called <code>File</code>. -To make things a little more interesting, our <code>File</code> includes the name of the file -that the file descriptor refers to. -<p> -Because <code>File</code> starts with a capital letter, the type is available outside the package, -that is, by users of the package. In Go the rule about visibility of information is -simple: if a name (of a top-level type, function, method, constant or variable, or of -a structure field or method) is capitalized, users of the package may see it. Otherwise, the -name and hence the thing being named is visible only inside the package in which -it is declared. This is more than a convention; the rule is enforced by the compiler. -In Go, the term for publicly visible names is ''exported''. -<p> -In the case of <code>File</code>, all its fields are lower case and so invisible to users, but we -will soon give it some exported, upper-case methods. -<p> -First, though, here is a factory to create a <code>File</code>: -<p> -<pre><!--{{code "progs/file.go" `/newFile/` `/^}/`}} --->func newFile(fd int, name string) *File { - if fd < 0 { - return nil - } - return &File{fd, name} -} -</pre> -<p> -This returns a pointer to a new <code>File</code> structure with the file descriptor and name -filled in. This code uses Go's notion of a ''composite literal'', analogous to -the ones used to build maps and arrays, to construct a new heap-allocated -object. We could write -<p> -<pre> -n := new(File) -n.fd = fd -n.name = name -return n -</pre> -<p> -but for simple structures like <code>File</code> it's easier to return the address of a -composite literal, as is done here in the <code>return</code> statement from <code>newFile</code>. -<p> -We can use the factory to construct some familiar, exported variables of type <code>*File</code>: -<p> -<pre><!--{{code "progs/file.go" `/var/` `/^\)/`}} --->var ( - Stdin = newFile(syscall.Stdin, "/dev/stdin") - Stdout = newFile(syscall.Stdout, "/dev/stdout") - Stderr = newFile(syscall.Stderr, "/dev/stderr") -) -</pre> -<p> -The <code>newFile</code> function was not exported because it's internal. The proper, -exported factory to use is <code>OpenFile</code> (we'll explain that name in a moment): -<p> -<pre><!--{{code "progs/file.go" `/func.OpenFile/` `/^}/`}} --->func OpenFile(name string, mode int, perm uint32) (file *File, err os.Error) { - r, e := syscall.Open(name, mode, perm) - if e != 0 { - err = os.Errno(e) - } - return newFile(r, name), err -} -</pre> -<p> -There are a number of new things in these few lines. First, <code>OpenFile</code> returns -multiple values, a <code>File</code> and an error (more about errors in a moment). -We declare the -multi-value return as a parenthesized list of declarations; syntactically -they look just like a second parameter list. The function -<code>syscall.Open</code> -also has a multi-value return, which we can grab with the multi-variable -declaration on the first line; it declares <code>r</code> and <code>e</code> to hold the two values, -both of type <code>int</code> (although you'd have to look at the <code>syscall</code> package -to see that). Finally, <code>OpenFile</code> returns two values: a pointer to the new <code>File</code> -and the error. If <code>syscall.Open</code> fails, the file descriptor <code>r</code> will -be negative and <code>newFile</code> will return <code>nil</code>. -<p> -About those errors: The <code>os</code> library includes a general notion of an error. -It's a good idea to use its facility in your own interfaces, as we do here, for -consistent error handling throughout Go code. In <code>Open</code> we use a -conversion to translate Unix's integer <code>errno</code> value into the integer type -<code>os.Errno</code>, which implements <code>os.Error</code>. -<p> -Why <code>OpenFile</code> and not <code>Open</code>? To mimic Go's <code>os</code> package, which -our exercise is emulating. The <code>os</code> package takes the opportunity -to make the two commonest cases - open for read and create for -write - the simplest, just <code>Open</code> and <code>Create</code>. <code>OpenFile</code> is the -general case, analogous to the Unix system call <code>Open</code>. Here is -the implementation of our <code>Open</code> and <code>Create</code>; they're trivial -wrappers that eliminate common errors by capturing -the tricky standard arguments to open and, especially, to create a file: -<p> -<pre><!--{{code "progs/file.go" `/^const/` `/^}/`}} --->const ( - O_RDONLY = syscall.O_RDONLY - O_RDWR = syscall.O_RDWR - O_CREATE = syscall.O_CREAT - O_TRUNC = syscall.O_TRUNC -) - -func Open(name string) (file *File, err os.Error) { - return OpenFile(name, O_RDONLY, 0) -} -</pre> -<p> -<pre><!--{{code "progs/file.go" `/func.Create/` `/^}/`}} --->func Create(name string) (file *File, err os.Error) { - return OpenFile(name, O_RDWR|O_CREATE|O_TRUNC, 0666) -} -</pre> -<p> -Back to our main story. -Now that we can build <code>Files</code>, we can write methods for them. To declare -a method of a type, we define a function to have an explicit receiver -of that type, placed -in parentheses before the function name. Here are some methods for <code>*File</code>, -each of which declares a receiver variable <code>file</code>. -<p> -<pre><!--{{code "progs/file.go" `/Close/` "$"}} --->func (file *File) Close() os.Error { - if file == nil { - return os.EINVAL - } - e := syscall.Close(file.fd) - file.fd = -1 // so it can't be closed again - if e != 0 { - return os.Errno(e) - } - return nil -} - -func (file *File) Read(b []byte) (ret int, err os.Error) { - if file == nil { - return -1, os.EINVAL - } - r, e := syscall.Read(file.fd, b) - if e != 0 { - err = os.Errno(e) - } - return int(r), err -} - -func (file *File) Write(b []byte) (ret int, err os.Error) { - if file == nil { - return -1, os.EINVAL - } - r, e := syscall.Write(file.fd, b) - if e != 0 { - err = os.Errno(e) - } - return int(r), err -} - -func (file *File) String() string { - return file.name -} -</pre> -<p> -There is no implicit <code>this</code> and the receiver variable must be used to access -members of the structure. Methods are not declared within -the <code>struct</code> declaration itself. The <code>struct</code> declaration defines only data members. -In fact, methods can be created for almost any type you name, such as an integer or -array, not just for <code>structs</code>. We'll see an example with arrays later. -<p> -The <code>String</code> method is so called because of a printing convention we'll -describe later. -<p> -The methods use the public variable <code>os.EINVAL</code> to return the (<code>os.Error</code> -version of the) Unix error code <code>EINVAL</code>. The <code>os</code> library defines a standard -set of such error values. -<p> -We can now use our new package: -<p> -<pre><!--{{code "progs/helloworld3.go" `/package/` "$"}} --->package main - -import ( - "./file" - "fmt" - "os" -) - -func main() { - hello := []byte("hello, world\n") - file.Stdout.Write(hello) - f, err := file.Open("/does/not/exist") - if f == nil { - fmt.Printf("can't open file; err=%s\n", err.String()) - os.Exit(1) - } -} -</pre> -<p> -The ''<code>./</code>'' in the import of ''<code>./file</code>'' tells the compiler -to use our own package rather than -something from the directory of installed packages. -(Also, ''<code>file.go</code>'' must be compiled before we can import the -package.) -<p> -Now we can compile and run the program. On Unix, this would be the result: -<p> -<pre> -$ 6g file.go # compile file package -$ 6g helloworld3.go # compile main package -$ 6l -o helloworld3 helloworld3.6 # link - no need to mention "file" -$ helloworld3 -hello, world -can't open file; err=No such file or directory -$ -</pre> -<p> -<h2>Rotting cats</h2> -<p> -Building on the <code>file</code> package, here's a simple version of the Unix utility <code>cat(1)</code>, -<code>progs/cat.go</code>: -<p> -<pre><!--{{code "progs/cat.go" `/package/` "$"}} --->package main - -import ( - "./file" - "flag" - "fmt" - "os" -) - -func cat(f *file.File) { - const NBUF = 512 - var buf [NBUF]byte - for { - switch nr, er := f.Read(buf[:]); true { - case nr < 0: - fmt.Fprintf(os.Stderr, "cat: error reading from %s: %s\n", f.String(), er.String()) - os.Exit(1) - case nr == 0: // EOF - return - case nr > 0: - if nw, ew := file.Stdout.Write(buf[0:nr]); nw != nr { - fmt.Fprintf(os.Stderr, "cat: error writing from %s: %s\n", f.String(), ew.String()) - os.Exit(1) - } - } - } -} - -func main() { - flag.Parse() // Scans the arg list and sets up flags - if flag.NArg() == 0 { - cat(file.Stdin) - } - for i := 0; i < flag.NArg(); i++ { - f, err := file.Open(flag.Arg(i)) - if f == nil { - fmt.Fprintf(os.Stderr, "cat: can't open %s: error %s\n", flag.Arg(i), err) - os.Exit(1) - } - cat(f) - f.Close() - } -} -</pre> -<p> -By now this should be easy to follow, but the <code>switch</code> statement introduces some -new features. Like a <code>for</code> loop, an <code>if</code> or <code>switch</code> can include an -initialization statement. The <code>switch</code> statement in <code>cat</code> uses one to create variables -<code>nr</code> and <code>er</code> to hold the return values from the call to <code>f.Read</code>. (The <code>if</code> a few lines later -has the same idea.) The <code>switch</code> statement is general: it evaluates the cases -from top to bottom looking for the first case that matches the value; the -case expressions don't need to be constants or even integers, as long as -they all have the same type. -<p> -Since the <code>switch</code> value is just <code>true</code>, we could leave it off—as is also -the situation -in a <code>for</code> statement, a missing value means <code>true</code>. In fact, such a <code>switch</code> -is a form of <code>if-else</code> chain. While we're here, it should be mentioned that in -<code>switch</code> statements each <code>case</code> has an implicit <code>break</code>. -<p> -The argument to <code>file.Stdout.Write</code> is created by slicing the array <code>buf</code>. -Slices provide the standard Go way to handle I/O buffers. -<p> -Now let's make a variant of <code>cat</code> that optionally does <code>rot13</code> on its input. -It's easy to do by just processing the bytes, but instead we will exploit -Go's notion of an <i>interface</i>. -<p> -The <code>cat</code> subroutine uses only two methods of <code>f</code>: <code>Read</code> and <code>String</code>, -so let's start by defining an interface that has exactly those two methods. -Here is code from <code>progs/cat_rot13.go</code>: -<p> -<pre><!--{{code "progs/cat_rot13.go" `/type.reader/` `/^}/`}} --->type reader interface { - Read(b []byte) (ret int, err os.Error) - String() string -} -</pre> -<p> -Any type that has the two methods of <code>reader</code>—regardless of whatever -other methods the type may also have—is said to <i>implement</i> the -interface. Since <code>file.File</code> implements these methods, it implements the -<code>reader</code> interface. We could tweak the <code>cat</code> subroutine to accept a <code>reader</code> -instead of a <code>*file.File</code> and it would work just fine, but let's embellish a little -first by writing a second type that implements <code>reader</code>, one that wraps an -existing <code>reader</code> and does <code>rot13</code> on the data. To do this, we just define -the type and implement the methods and with no other bookkeeping, -we have a second implementation of the <code>reader</code> interface. -<p> -<pre><!--{{code "progs/cat_rot13.go" `/type.rotate13/` `/end.of.rotate13/`}} --->type rotate13 struct { - source reader -} - -func newRotate13(source reader) *rotate13 { - return &rotate13{source} -} - -func (r13 *rotate13) Read(b []byte) (ret int, err os.Error) { - r, e := r13.source.Read(b) - for i := 0; i < r; i++ { - b[i] = rot13(b[i]) - } - return r, e -} - -func (r13 *rotate13) String() string { - return r13.source.String() -} -// end of rotate13 implementation -</pre> -<p> -(The <code>rot13</code> function called in <code>Read</code> is trivial and not worth reproducing here.) -<p> -To use the new feature, we define a flag: -<p> -<pre><!--{{code "progs/cat_rot13.go" `/rot13Flag/`}} --->var rot13Flag = flag.Bool("rot13", false, "rot13 the input") -</pre> -<p> -and use it from within a mostly unchanged <code>cat</code> function: -<p> -<pre><!--{{code "progs/cat_rot13.go" `/func.cat/` `/^}/`}} --->func cat(r reader) { - const NBUF = 512 - var buf [NBUF]byte - - if *rot13Flag { - r = newRotate13(r) - } - for { - switch nr, er := r.Read(buf[:]); { - case nr < 0: - fmt.Fprintf(os.Stderr, "cat: error reading from %s: %s\n", r.String(), er.String()) - os.Exit(1) - case nr == 0: // EOF - return - case nr > 0: - nw, ew := file.Stdout.Write(buf[0:nr]) - if nw != nr { - fmt.Fprintf(os.Stderr, "cat: error writing from %s: %s\n", r.String(), ew.String()) - os.Exit(1) - } - } - } -} -</pre> -<p> -(We could also do the wrapping in <code>main</code> and leave <code>cat</code> mostly alone, except -for changing the type of the argument; consider that an exercise.) -The <code>if</code> at the top of <code>cat</code> sets it all up: If the <code>rot13</code> flag is true, wrap the <code>reader</code> -we received into a <code>rotate13</code> and proceed. Note that the interface variables -are values, not pointers: the argument is of type <code>reader</code>, not <code>*reader</code>, -even though under the covers it holds a pointer to a <code>struct</code>. -<p> -Here it is in action: -<p> -<pre> -$ echo abcdefghijklmnopqrstuvwxyz | ./cat -abcdefghijklmnopqrstuvwxyz -$ echo abcdefghijklmnopqrstuvwxyz | ./cat --rot13 -nopqrstuvwxyzabcdefghijklm -$ -</pre> -<p> -Fans of dependency injection may take cheer from how easily interfaces -allow us to substitute the implementation of a file descriptor. -<p> -Interfaces are a distinctive feature of Go. An interface is implemented by a -type if the type implements all the methods declared in the interface. -This means -that a type may implement an arbitrary number of different interfaces. -There is no type hierarchy; things can be much more <i>ad hoc</i>, -as we saw with <code>rot13</code>. The type <code>file.File</code> implements <code>reader</code>; it could also -implement a <code>writer</code>, or any other interface built from its methods that -fits the current situation. Consider the <i>empty interface</i> -<p> -<pre> -type Empty interface {} -</pre> -<p> -<i>Every</i> type implements the empty interface, which makes it -useful for things like containers. -<p> -<h2>Sorting</h2> -<p> -Interfaces provide a simple form of polymorphism. They completely -separate the definition of what an object does from how it does it, allowing -distinct implementations to be represented at different times by the -same interface variable. -<p> -As an example, consider this simple sort algorithm taken from <code>progs/sort.go</code>: -<p> -<pre><!--{{code "progs/sort.go" `/func.Sort/` `/^}/`}} --->func Sort(data Interface) { - for i := 1; i < data.Len(); i++ { - for j := i; j > 0 && data.Less(j, j-1); j-- { - data.Swap(j, j-1) - } - } -} -</pre> -<p> -The code needs only three methods, which we wrap into sort's <code>Interface</code>: -<p> -<pre><!--{{code "progs/sort.go" `/interface/` `/^}/`}} --->type Interface interface { - Len() int - Less(i, j int) bool - Swap(i, j int) -} -</pre> -<p> -We can apply <code>Sort</code> to any type that implements <code>Len</code>, <code>Less</code>, and <code>Swap</code>. -The <code>sort</code> package includes the necessary methods to allow sorting of -arrays of integers, strings, etc.; here's the code for arrays of <code>int</code> -<p> -<pre><!--{{code "progs/sort.go" `/type.*IntSlice/` `/Swap/`}} --->type IntSlice []int - -func (p IntSlice) Len() int { return len(p) } -func (p IntSlice) Less(i, j int) bool { return p[i] < p[j] } -func (p IntSlice) Swap(i, j int) { p[i], p[j] = p[j], p[i] } -</pre> -<p> -Here we see methods defined for non-<code>struct</code> types. You can define methods -for any type you define and name in your package. -<p> -And now a routine to test it out, from <code>progs/sortmain.go</code>. This -uses a function in the <code>sort</code> package, omitted here for brevity, -to test that the result is sorted. -<p> -<pre><!--{{code "progs/sortmain.go" `/func.ints/` `/^}/`}} --->func ints() { - data := []int{74, 59, 238, -784, 9845, 959, 905, 0, 0, 42, 7586, -5467984, 7586} - a := sort.IntSlice(data) - sort.Sort(a) - if !sort.IsSorted(a) { - panic("fail") - } -} -</pre> -<p> -If we have a new type we want to be able to sort, all we need to do is -to implement the three methods for that type, like this: -<p> -<pre><!--{{code "progs/sortmain.go" `/type.day/` `/Swap/`}} --->type day struct { - num int - shortName string - longName string -} - -type dayArray struct { - data []*day -} - -func (p *dayArray) Len() int { return len(p.data) } -func (p *dayArray) Less(i, j int) bool { return p.data[i].num < p.data[j].num } -func (p *dayArray) Swap(i, j int) { p.data[i], p.data[j] = p.data[j], p.data[i] } -</pre> -<p> -<p> -<h2>Printing</h2> -<p> -The examples of formatted printing so far have been modest. In this section -we'll talk about how formatted I/O can be done well in Go. -<p> -We've seen simple uses of the package <code>fmt</code>, which -implements <code>Printf</code>, <code>Fprintf</code>, and so on. -Within the <code>fmt</code> package, <code>Printf</code> is declared with this signature: -<p> -<pre> -Printf(format string, v ...interface{}) (n int, errno os.Error) -</pre> -<p> -The token <code>...</code> introduces a variable-length argument list that in C would -be handled using the <code>stdarg.h</code> macros. -In Go, variadic functions are passed a slice of the arguments of the -specified type. In <code>Printf</code>'s case, the declaration says <code>...interface{}</code> -so the actual type is a slice of empty interface values, <code>[]interface{}</code>. -<code>Printf</code> can examine the arguments by iterating over the slice -and, for each element, using a type switch or the reflection library -to interpret the value. -It's off topic here but such run-time type analysis -helps explain some of the nice properties of Go's <code>Printf</code>, -due to the ability of <code>Printf</code> to discover the type of its arguments -dynamically. -<p> -For example, in C each format must correspond to the type of its -argument. It's easier in many cases in Go. Instead of <code>%llud</code> you -can just say <code>%d</code>; <code>Printf</code> knows the size and signedness of the -integer and can do the right thing for you. The snippet -<p> -<pre><!--{{code "progs/print.go" 10 11}} ---> var u64 uint64 = 1<<64 - 1 - fmt.Printf("%d %d\n", u64, int64(u64)) -</pre> -<p> -prints -<p> -<pre> -18446744073709551615 -1 -</pre> -<p> -In fact, if you're lazy the format <code>%v</code> will print, in a simple -appropriate style, any value, even an array or structure. The output of -<p> -<pre><!--{{code "progs/print.go" 14 20}} ---> type T struct { - a int - b string - } - t := T{77, "Sunset Strip"} - a := []int{1, 2, 3, 4} - fmt.Printf("%v %v %v\n", u64, t, a) -</pre> -<p> -is -<p> -<pre> -18446744073709551615 {77 Sunset Strip} [1 2 3 4] -</pre> -<p> -You can drop the formatting altogether if you use <code>Print</code> or <code>Println</code> -instead of <code>Printf</code>. Those routines do fully automatic formatting. -The <code>Print</code> function just prints its elements out using the equivalent -of <code>%v</code> while <code>Println</code> inserts spaces between arguments -and adds a newline. The output of each of these two lines is identical -to that of the <code>Printf</code> call above. -<p> -<pre><!--{{code "progs/print.go" 21 22}} ---> fmt.Print(u64, " ", t, " ", a, "\n") - fmt.Println(u64, t, a) -</pre> -<p> -If you have your own type you'd like <code>Printf</code> or <code>Print</code> to format, -just give it a <code>String</code> method that returns a string. The print -routines will examine the value to inquire whether it implements -the method and if so, use it rather than some other formatting. -Here's a simple example. -<p> -<pre><!--{{code "progs/print_string.go" 9 "$"}} --->type testType struct { - a int - b string -} - -func (t *testType) String() string { - return fmt.Sprint(t.a) + " " + t.b -} - -func main() { - t := &testType{77, "Sunset Strip"} - fmt.Println(t) -} -</pre> -<p> -Since <code>*testType</code> has a <code>String</code> method, the -default formatter for that type will use it and produce the output -<p> -<pre> -77 Sunset Strip -</pre> -<p> -Observe that the <code>String</code> method calls <code>Sprint</code> (the obvious Go -variant that returns a string) to do its formatting; special formatters -can use the <code>fmt</code> library recursively. -<p> -Another feature of <code>Printf</code> is that the format <code>%T</code> will print a string -representation of the type of a value, which can be handy when debugging -polymorphic code. -<p> -It's possible to write full custom print formats with flags and precisions -and such, but that's getting a little off the main thread so we'll leave it -as an exploration exercise. -<p> -You might ask, though, how <code>Printf</code> can tell whether a type implements -the <code>String</code> method. Actually what it does is ask if the value can -be converted to an interface variable that implements the method. -Schematically, given a value <code>v</code>, it does this: -<p> -<p> -<pre> -type Stringer interface { - String() string -} -</pre> -<p> -<pre> -s, ok := v.(Stringer) // Test whether v implements "String()" -if ok { - result = s.String() -} else { - result = defaultOutput(v) -} -</pre> -<p> -The code uses a ``type assertion'' (<code>v.(Stringer)</code>) to test if the value stored in -<code>v</code> satisfies the <code>Stringer</code> interface; if it does, <code>s</code> -will become an interface variable implementing the method and <code>ok</code> will -be <code>true</code>. We then use the interface variable to call the method. -(The ''comma, ok'' pattern is a Go idiom used to test the success of -operations such as type conversion, map update, communications, and so on, -although this is the only appearance in this tutorial.) -If the value does not satisfy the interface, <code>ok</code> will be false. -<p> -In this snippet the name <code>Stringer</code> follows the convention that we add ''[e]r'' -to interfaces describing simple method sets like this. -<p> -One last wrinkle. To complete the suite, besides <code>Printf</code> etc. and <code>Sprintf</code> -etc., there are also <code>Fprintf</code> etc. Unlike in C, <code>Fprintf</code>'s first argument is -not a file. Instead, it is a variable of type <code>io.Writer</code>, which is an -interface type defined in the <code>io</code> library: -<p> -<pre> -type Writer interface { - Write(p []byte) (n int, err os.Error) -} -</pre> -<p> -(This interface is another conventional name, this time for <code>Write</code>; there are also -<code>io.Reader</code>, <code>io.ReadWriter</code>, and so on.) -Thus you can call <code>Fprintf</code> on any type that implements a standard <code>Write</code> -method, not just files but also network channels, buffers, whatever -you want. -<p> -<h2>Prime numbers</h2> -<p> -Now we come to processes and communication—concurrent programming. -It's a big subject so to be brief we assume some familiarity with the topic. -<p> -A classic program in the style is a prime sieve. -(The sieve of Eratosthenes is computationally more efficient than -the algorithm presented here, but we are more interested in concurrency than -algorithmics at the moment.) -It works by taking a stream of all the natural numbers and introducing -a sequence of filters, one for each prime, to winnow the multiples of -that prime. At each step we have a sequence of filters of the primes -so far, and the next number to pop out is the next prime, which triggers -the creation of the next filter in the chain. -<p> -Here's a flow diagram; each box represents a filter element whose -creation is triggered by the first number that flowed from the -elements before it. -<p> -<br> -<p> - <img src='sieve.gif'> -<p> -<br> -<p> -To create a stream of integers, we use a Go <i>channel</i>, which, -borrowing from CSP's descendants, represents a communications -channel that can connect two concurrent computations. -In Go, channel variables are references to a run-time object that -coordinates the communication; as with maps and slices, use -<code>make</code> to create a new channel. -<p> -Here is the first function in <code>progs/sieve.go</code>: -<p> -<pre><!--{{code "progs/sieve.go" `/Send/` `/^}/`}} --->// Send the sequence 2, 3, 4, ... to channel 'ch'. -func generate(ch chan int) { - for i := 2; ; i++ { - ch <- i // Send 'i' to channel 'ch'. - } -} -</pre> -<p> -The <code>generate</code> function sends the sequence 2, 3, 4, 5, ... to its -argument channel, <code>ch</code>, using the binary communications operator <code><-</code>. -Channel operations block, so if there's no recipient for the value on <code>ch</code>, -the send operation will wait until one becomes available. -<p> -The <code>filter</code> function has three arguments: an input channel, an output -channel, and a prime number. It copies values from the input to the -output, discarding anything divisible by the prime. The unary communications -operator <code><-</code> (receive) retrieves the next value on the channel. -<p> -<pre><!--{{code "progs/sieve.go" `/Copy.the/` `/^}/`}} --->// Copy the values from channel 'in' to channel 'out', -// removing those divisible by 'prime'. -func filter(in, out chan int, prime int) { - for { - i := <-in // Receive value of new variable 'i' from 'in'. - if i%prime != 0 { - out <- i // Send 'i' to channel 'out'. - } - } -} -</pre> -<p> -The generator and filters execute concurrently. Go has -its own model of process/threads/light-weight processes/coroutines, -so to avoid notational confusion we call concurrently executing -computations in Go <i>goroutines</i>. To start a goroutine, -invoke the function, prefixing the call with the keyword <code>go</code>; -this starts the function running in parallel with the current -computation but in the same address space: -<p> -<pre> -go sum(hugeArray) // calculate sum in the background -</pre> -<p> -If you want to know when the calculation is done, pass a channel -on which it can report back: -<p> -<pre> -ch := make(chan int) -go sum(hugeArray, ch) -// ... do something else for a while -result := <-ch // wait for, and retrieve, result -</pre> -<p> -Back to our prime sieve. Here's how the sieve pipeline is stitched -together: -<p> -<pre><!--{{code "progs/sieve.go" `/func.main/` `/^}/`}} --->func main() { - ch := make(chan int) // Create a new channel. - go generate(ch) // Start generate() as a goroutine. - for i := 0; i < 100; i++ { // Print the first hundred primes. - prime := <-ch - fmt.Println(prime) - ch1 := make(chan int) - go filter(ch, ch1, prime) - ch = ch1 - } -} -</pre> -<p> -The first line of <code>main</code> creates the initial channel to pass to <code>generate</code>, which it -then starts up. As each prime pops out of the channel, a new <code>filter</code> -is added to the pipeline and <i>its</i> output becomes the new value -of <code>ch</code>. -<p> -The sieve program can be tweaked to use a pattern common -in this style of programming. Here is a variant version -of <code>generate</code>, from <code>progs/sieve1.go</code>: -<p> -<pre><!--{{code "progs/sieve1.go" `/func.generate/` `/^}/`}} --->func generate() chan int { - ch := make(chan int) - go func() { - for i := 2; ; i++ { - ch <- i - } - }() - return ch -} -</pre> -<p> -This version does all the setup internally. It creates the output -channel, launches a goroutine running a function literal, and -returns the channel to the caller. It is a factory for concurrent -execution, starting the goroutine and returning its connection. -<p> -The function literal notation used in the <code>go</code> statement allows us to construct an -anonymous function and invoke it on the spot. Notice that the local -variable <code>ch</code> is available to the function literal and lives on even -after <code>generate</code> returns. -<p> -The same change can be made to <code>filter</code>: -<p> -<pre><!--{{code "progs/sieve1.go" `/func.filter/` `/^}/`}} --->func filter(in chan int, prime int) chan int { - out := make(chan int) - go func() { - for { - if i := <-in; i%prime != 0 { - out <- i - } - } - }() - return out -} -</pre> -<p> -The <code>sieve</code> function's main loop becomes simpler and clearer as a -result, and while we're at it let's turn it into a factory too: -<p> -<pre><!--{{code "progs/sieve1.go" `/func.sieve/` `/^}/`}} --->func sieve() chan int { - out := make(chan int) - go func() { - ch := generate() - for { - prime := <-ch - out <- prime - ch = filter(ch, prime) - } - }() - return out -} -</pre> -<p> -Now <code>main</code>'s interface to the prime sieve is a channel of primes: -<p> -<pre><!--{{code "progs/sieve1.go" `/func.main/` `/^}/`}} --->func main() { - primes := sieve() - for i := 0; i < 100; i++ { // Print the first hundred primes. - fmt.Println(<-primes) - } -} -</pre> -<p> -<h2>Multiplexing</h2> -<p> -With channels, it's possible to serve multiple independent client goroutines without -writing an explicit multiplexer. The trick is to send the server a channel in the message, -which it will then use to reply to the original sender. -A realistic client-server program is a lot of code, so here is a very simple substitute -to illustrate the idea. It starts by defining a <code>request</code> type, which embeds a channel -that will be used for the reply. -<p> -<pre><!--{{code "progs/server.go" `/type.request/` `/^}/`}} --->type request struct { - a, b int - replyc chan int -} -</pre> -<p> -The server will be trivial: it will do simple binary operations on integers. Here's the -code that invokes the operation and responds to the request: -<p> -<pre><!--{{code "progs/server.go" `/type.binOp/` `/^}/`}} --->type binOp func(a, b int) int - -func run(op binOp, req *request) { - reply := op(req.a, req.b) - req.replyc <- reply -} -</pre> -<p> -The type declaration makes <code>binOp</code> represent a function taking two integers and -returning a third. -<p> -The <code>server</code> routine loops forever, receiving requests and, to avoid blocking due to -a long-running operation, starting a goroutine to do the actual work. -<p> -<pre><!--{{code "progs/server.go" `/func.server/` `/^}/`}} --->func server(op binOp, service chan *request) { - for { - req := <-service - go run(op, req) // don't wait for it - } -} -</pre> -<p> -We construct a server in a familiar way, starting it and returning a channel -connected to it: -<p> -<pre><!--{{code "progs/server.go" `/func.startServer/` `/^}/`}} --->func startServer(op binOp) chan *request { - req := make(chan *request) - go server(op, req) - return req -} -</pre> -<p> -Here's a simple test. It starts a server with an addition operator and sends out -<code>N</code> requests without waiting for the replies. Only after all the requests are sent -does it check the results. -<p> -<pre><!--{{code "progs/server.go" `/func.main/` `/^}/`}} --->func main() { - adder := startServer(func(a, b int) int { return a + b }) - const N = 100 - var reqs [N]request - for i := 0; i < N; i++ { - req := &reqs[i] - req.a = i - req.b = i + N - req.replyc = make(chan int) - adder <- req - } - for i := N - 1; i >= 0; i-- { // doesn't matter what order - if <-reqs[i].replyc != N+2*i { - fmt.Println("fail at", i) - } - } - fmt.Println("done") -} -</pre> -<p> -One annoyance with this program is that it doesn't shut down the server cleanly; when <code>main</code> returns -there are a number of lingering goroutines blocked on communication. To solve this, -we can provide a second, <code>quit</code> channel to the server: -<p> -<pre><!--{{code "progs/server1.go" `/func.startServer/` `/^}/`}} --->func startServer(op binOp) (service chan *request, quit chan bool) { - service = make(chan *request) - quit = make(chan bool) - go server(op, service, quit) - return service, quit -} -</pre> -<p> -It passes the quit channel to the <code>server</code> function, which uses it like this: -<p> -<pre><!--{{code "progs/server1.go" `/func.server/` `/^}/`}} --->func server(op binOp, service chan *request, quit chan bool) { - for { - select { - case req := <-service: - go run(op, req) // don't wait for it - case <-quit: - return - } - } -} -</pre> -<p> -Inside <code>server</code>, the <code>select</code> statement chooses which of the multiple communications -listed by its cases can proceed. If all are blocked, it waits until one can proceed; if -multiple can proceed, it chooses one at random. In this instance, the <code>select</code> allows -the server to honor requests until it receives a quit message, at which point it -returns, terminating its execution. -<p> -<p> -All that's left is to strobe the <code>quit</code> channel -at the end of main: -<p> -<pre><!--{{code "progs/server1.go" `/adder,.quit/`}} ---> adder, quit := startServer(func(a, b int) int { return a + b }) -</pre> -... -<pre><!--{{code "progs/server1.go" `/quit....true/`}} ---> quit <- true -</pre> -<p> -There's a lot more to Go programming and concurrent programming in general but this -quick tour should give you some of the basics. |