## Introduction

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 language specification. After you've read this tutorial, you should look at Effective Go, 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: Day 1, Day 2, Day 3.

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 `/doc/progs/`.

## Hello, World

Let's start in the usual way:

    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")
    }

Every Go source file declares, using a `package` statement, which package it's part of. It may also import other packages to use their facilities. This program imports the package `fmt` to gain access to our old, now capitalized and package-qualified, friend, `fmt.Printf`.

Functions are introduced with the `func` keyword. The `main` package's `main` function is where the program starts running (after any initialization).

String constants can contain Unicode characters, encoded in UTF-8. (In fact, Go source files are defined to be encoded in UTF-8.)

The comment convention is the same as in C++:

    /* ... */
    // ...

Later we'll have much more to say about printing.

## Semicolons

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 `for` loops and the like; they are not necessary after every statement.

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.

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.

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 `if` statement on the same line as the `if`; 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.

## Compiling

Go is a compiled language. At the moment there are two compilers. `Gccgo` 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: `6g` for the 64-bit x86, `8g` for the 32-bit x86, and more. These compilers run significantly faster but generate less efficient code than `gccgo`. At the time of writing (late 2009), they also have a more robust run-time system although `gccgo` is catching up.

Here's how to compile and run our program. With `6g`, say,

    $ 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
    $

With `gccgo` it looks a little more traditional.

    $ gccgo helloworld.go
    $ a.out
    Hello, world; or Καλημέρα κόσμε; or こんにちは 世界
    $

## Echo

Next up, here's a version of the Unix utility `echo(1)`:

    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)
    }

This program is small but it's doing a number of new things. In the last example, we saw `func` introduce a function. The keywords `var`, `const`, and `type` (not used yet) also introduce declarations, as does `import`. Notice that we can group declarations of the same sort into parenthesized lists, one item per line, as in the `import` and `const` clauses here. But it's not necessary to do so; we could have said

    const Space = " "
    const Newline = "\n"

This program imports the `"os"` package to access its `Stdout` variable, of type `*os.File`. The `import` statement is actually a declaration: in its general form, as used in our ``hello world'' program, it names the identifier (`fmt`) that will be used to access members of the package imported from the file (`"fmt"`), 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 `import "fmt"`.

You can specify your own import names if you want but it's only necessary if you need to resolve a naming conflict.

Given `os.Stdout` we can use its `WriteString` method to print the string.

After importing the `flag` package, we use a `var` declaration to create and initialize a global variable, called `omitNewline`, to hold the value of echo's `-n` flag. The variable has type `*bool`, pointer to `bool`.

In `main.main`, we parse the arguments (the call to `flag.Parse`) and then create a local string variable with which to build the output.

The declaration statement has the form

    var s string = ""

This is the `var` keyword, followed by the name of the variable, followed by its type, followed by an equals sign and an initial value for the variable.

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

    var s = ""

or we could go even shorter and write the idiom

    s := ""

The `:=` operator is used a lot in Go to represent an initializing declaration. There's one in the `for` clause on the next line:

        for i := 0; i < flag.NArg(); i++ {

The `flag` package has parsed the arguments and left the non-flag arguments in a list that can be iterated over in the obvious way.

The Go `for` statement differs from that of C in a number of ways. First, it's the only looping construct; there is no `while` or `do`. Second, there are no parentheses on the clause, but the braces on the body are mandatory. The same applies to the `if` and `switch` statements. Later examples will show some other ways `for` can be written.

The body of the loop builds up the string `s` by appending (using `+=`) the arguments and separating spaces. After the loop, if the `-n` flag is not set, the program appends a newline. Finally, it writes the result.

Notice that `main.main` is a niladic function with no return type. It's defined that way. Falling off the end of `main.main` means ''success''; if you want to signal an erroneous return, call

    os.Exit(1)

The `os` package contains other essentials for getting started; for instance, `os.Args` is a slice used by the `flag` package to access the command-line arguments.

## An Interlude about Types

Go has some familiar types such as `int` and `uint` (unsigned `int`), which represent values of the ''appropriate'' size for the machine. It also defines explicitly-sized types such as `int8`, `float64`, and so on, plus unsigned integer types such as `uint`, `uint32`, etc. These are distinct types; even if `int` and `int32` are both 32 bits in size, they are not the same type. There is also a `byte` synonym for `uint8`, which is the element type for strings.

Floating-point types are always sized: `float32` and `float64`, plus `complex64` (two `float32s`) and `complex128` (two `float64s`). Complex numbers are outside the scope of this tutorial.

Speaking of `string`, that's a built-in type as well. Strings are immutable values—they are not just arrays of `byte` values. Once you've built a string value, you can't change it, although of course you can change a string variable simply by reassigning it. This snippet from `strings.go` is legal code:

        s := "hello"
        if s[1] != 'e' {
            os.Exit(1)
        }
        s = "good bye"
        var p *string = &s
        *p = "ciao"

However the following statements are illegal because they would modify a `string` value:

    s[0] = 'x'
    (*p)[1] = 'y'

In C++ terms, Go strings are a bit like `const strings`, while pointers to strings are analogous to `const string` references.

Yes, there are pointers. However, Go simplifies their use a little; read on.

Arrays are declared like this:

    var arrayOfInt [10]int

Arrays, like strings, are values, but they are mutable. This differs from C, in which `arrayOfInt` would be usable as a pointer to `int`. In Go, since arrays are values, it's meaningful (and useful) to talk about pointers to arrays.

The size of the array is part of its type; however, one can declare a slice variable to hold a reference to any array, of any size, with the same element type. A slice expression has the form `a[low : high]`, representing the internal array indexed from `low` through `high-1`; the resulting slice is indexed from `0` through `high-low-1`. In short, slices look a lot like arrays but with no explicit size (`[]` vs. `[10]`) 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.

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 constructor—an expression formed from a type followed by a brace-bounded expression like this:

    [3]int{1,2,3}

In this case the constructor builds an array of 3 `ints`.

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 `[:]` will slice the whole array.

Using slices one can write this function (from `sum.go`):

    func sum(a []int) int { // returns an int
        s := 0
        for i := 0; i < len(a); i++ {
            s += a[i]
        }
        return s
    }

Note how the return type (`int`) is defined for `sum` by stating it after the parameter list.

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:

    s := sum([3]int{1,2,3}[:])

If you are creating a regular array but want the compiler to count the elements for you, use `...` as the array size:

    s := sum([...]int{1,2,3}[:])

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:

    s := sum([]int{1,2,3})

There are also maps, which you can initialize like this:

    m := map[string]int{"one":1 , "two":2}

The built-in function `len`, which returns number of elements, makes its first appearance in `sum`. It works on strings, arrays, slices, maps, and channels.

By the way, another thing that works on strings, arrays, slices, maps and channels is the `range` clause on `for` loops. Instead of writing

    for i := 0; i < len(a); i++ { ... }

to loop over the elements of a slice (or map or ...) , we could write

    for i, v := range a { ... }

This assigns `i` to the index and `v` to the value of the successive elements of the target of the range. See Effective Go for more examples of its use.

## An Interlude about Allocation

Most types in Go are values. If you have an `int` or a `struct` or an array, assignment copies the contents of the object. To allocate a new variable, use the built-in function `new`, which returns a pointer to the allocated storage.

    type T struct { a, b int }
    var t *T = new(T)

or the more idiomatic

    t := new(T)

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 `make`:

    m := make(map[string]int)

This statement initializes a new map ready to store entries. If you just declare the map, as in

    var m map[string]int

it creates a `nil` reference that cannot hold anything. To use the map, you must first initialize the reference using `make` or by assignment from an existing map.

Note that `new(T)` returns type `*T` while `make(T)` returns type `T`. If you (mistakenly) allocate a reference object with `new` rather than `make`, you receive a pointer to a nil reference, equivalent to declaring an uninitialized variable and taking its address.

## An Interlude about Constants

Although integers come in lots of sizes in Go, integer constants do not. There are no constants like `0LL` or `0x0UL`. 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.

    const hardEight = (1 << 100) >> 97  // legal

There are nuances that deserve redirection to the legalese of the language specification but here are some illustrative examples:

    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

Conversions only work for simple cases such as converting `ints` 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.

## An I/O Package

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 `file.go`:

    package file

    import (
        "os"
        "syscall"
    )

    type File struct {
        fd   int    // file descriptor number
        name string // file name at Open time
    }

The first few lines declare the name of the package—`file`—and then import two packages. The `os` 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.

The other item is the low-level, external `syscall` package, which provides a primitive interface to the underlying operating system's calls.

Next is a type definition: the `type` keyword introduces a type declaration, in this case a data structure called `File`. To make things a little more interesting, our `File` includes the name of the file that the file descriptor refers to.

Because `File` 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''.

In the case of `File`, all its fields are lower case and so invisible to users, but we will soon give it some exported, upper-case methods.

First, though, here is a factory to create a `File`:

    func newFile(fd int, name string) *File {
        if fd < 0 {
            return nil
        }
        return &File{fd, name}
    }

This returns a pointer to a new `File` 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

    n := new(File)
    n.fd = fd
    n.name = name
    return n

but for simple structures like `File` it's easier to return the address of a composite literal, as is done here in the `return` statement from `newFile`.

We can use the factory to construct some familiar, exported variables of type `*File`:

    var (
        Stdin  = newFile(syscall.Stdin, "/dev/stdin")
        Stdout = newFile(syscall.Stdout, "/dev/stdout")
        Stderr = newFile(syscall.Stderr, "/dev/stderr")
    )

The `newFile` function was not exported because it's internal. The proper, exported factory to use is `OpenFile` (we'll explain that name in a moment):

    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
    }

There are a number of new things in these few lines. First, `OpenFile` returns multiple values, a `File` 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 `syscall.Open` also has a multi-value return, which we can grab with the multi-variable declaration on the first line; it declares `r` and `e` to hold the two values, both of type `int` (although you'd have to look at the `syscall` package to see that). Finally, `OpenFile` returns two values: a pointer to the new `File` and the error. If `syscall.Open` fails, the file descriptor `r` will be negative and `newFile` will return `nil`.

About those errors: The `os` 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 `Open` we use a conversion to translate Unix's integer `errno` value into the integer type `os.Errno`, which implements `os.Error`.

Why `OpenFile` and not `Open`? To mimic Go's `os` package, which our exercise is emulating. The `os` package takes the opportunity to make the two commonest cases - open for read and create for write - the simplest, just `Open` and `Create`. `OpenFile` is the general case, analogous to the Unix system call `Open`. Here is the implementation of our `Open` and `Create`; they're trivial wrappers that eliminate common errors by capturing the tricky standard arguments to open and, especially, to create a file:

    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)
    }

    func Create(name string) (file *File, err os.Error) {
        return OpenFile(name, O_RDWR|O_CREATE|O_TRUNC, 0666)
    }

Back to our main story. Now that we can build `Files`, 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 `*File`, each of which declares a receiver variable `file`.

    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
    }

There is no implicit `this` and the receiver variable must be used to access members of the structure. Methods are not declared within the `struct` declaration itself. The `struct` 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 `structs`. We'll see an example with arrays later.

The `String` method is so called because of a printing convention we'll describe later.

The methods use the public variable `os.EINVAL` to return the (`os.Error` version of the) Unix error code `EINVAL`. The `os` library defines a standard set of such error values.

We can now use our new 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)
        }
    }

The ''`./`'' in the import of ''`./file`'' tells the compiler to use our own package rather than something from the directory of installed packages. (Also, ''`file.go`'' must be compiled before we can import the package.)

Now we can compile and run the program. On Unix, this would be the result:

    $ 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
    $

## Rotting cats

Building on the `file` package, here's a simple version of the Unix utility `cat(1)`, `progs/cat.go`:

    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()
        }
    }

By now this should be easy to follow, but the `switch` statement introduces some new features. Like a `for` loop, an `if` or `switch` can include an initialization statement. The `switch` statement in `cat` uses one to create variables `nr` and `er` to hold the return values from the call to `f.Read`. (The `if` a few lines later has the same idea.) The `switch` 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.

Since the `switch` value is just `true`, we could leave it off—as is also the situation in a `for` statement, a missing value means `true`. In fact, such a `switch` is a form of `if-else` chain. While we're here, it should be mentioned that in `switch` statements each `case` has an implicit `break`.

The argument to `file.Stdout.Write` is created by slicing the array `buf`. Slices provide the standard Go way to handle I/O buffers.

Now let's make a variant of `cat` that optionally does `rot13` on its input. It's easy to do by just processing the bytes, but instead we will exploit Go's notion of an interface.

The `cat` subroutine uses only two methods of `f`: `Read` and `String`, so let's start by defining an interface that has exactly those two methods. Here is code from `progs/cat_rot13.go`:

    type reader interface {
        Read(b []byte) (ret int, err os.Error)
        String() string
    }

Any type that has the two methods of `reader`—regardless of whatever other methods the type may also have—is said to implement the interface. Since `file.File` implements these methods, it implements the `reader` interface. We could tweak the `cat` subroutine to accept a `reader` instead of a `*file.File` and it would work just fine, but let's embellish a little first by writing a second type that implements `reader`, one that wraps an existing `reader` and does `rot13` 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 `reader` interface.

    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

(The `rot13` function called in `Read` is trivial and not worth reproducing here.)

To use the new feature, we define a flag:

    var rot13Flag = flag.Bool("rot13", false, "rot13 the input")

and use it from within a mostly unchanged `cat` function:

    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)
                }
            }
        }
    }

(We could also do the wrapping in `main` and leave `cat` mostly alone, except for changing the type of the argument; consider that an exercise.) The `if` at the top of `cat` sets it all up: If the `rot13` flag is true, wrap the `reader` we received into a `rotate13` and proceed. Note that the interface variables are values, not pointers: the argument is of type `reader`, not `*reader`, even though under the covers it holds a pointer to a `struct`.

Here it is in action:

    $ echo abcdefghijklmnopqrstuvwxyz | ./cat
    abcdefghijklmnopqrstuvwxyz
    $ echo abcdefghijklmnopqrstuvwxyz | ./cat --rot13
    nopqrstuvwxyzabcdefghijklm
    $

Fans of dependency injection may take cheer from how easily interfaces allow us to substitute the implementation of a file descriptor.

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 ad hoc, as we saw with `rot13`. The type `file.File` implements `reader`; it could also implement a `writer`, or any other interface built from its methods that fits the current situation. Consider the empty interface

    type Empty interface {}

Every type implements the empty interface, which makes it useful for things like containers.

## Sorting

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.

As an example, consider this simple sort algorithm taken from `progs/sort.go`:

    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)
            }
        }
    }

The code needs only three methods, which we wrap into sort's `Interface`:

    type Interface interface {
        Len() int
        Less(i, j int) bool
        Swap(i, j int)
    }

We can apply `Sort` to any type that implements `Len`, `Less`, and `Swap`. The `sort` package includes the necessary methods to allow sorting of arrays of integers, strings, etc.; here's the code for arrays of `int`

    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] }

Here we see methods defined for non-`struct` types. You can define methods for any type you define and name in your package.

And now a routine to test it out, from `progs/sortmain.go`. This uses a function in the `sort` package, omitted here for brevity, to test that the result is sorted.

    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")
        }
    }

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:

    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] }

## Printing

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.

We've seen simple uses of the package `fmt`, which implements `Printf`, `Fprintf`, and so on. Within the `fmt` package, `Printf` is declared with this signature:

    Printf(format string, v ...interface{}) (n int, errno os.Error)

The token `...` introduces a variable-length argument list that in C would be handled using the `stdarg.h` macros. In Go, variadic functions are passed a slice of the arguments of the specified type. In `Printf`'s case, the declaration says `...interface{}` so the actual type is a slice of empty interface values, `[]interface{}`. `Printf` 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 `Printf`, due to the ability of `Printf` to discover the type of its arguments dynamically.

For example, in C each format must correspond to the type of its argument. It's easier in many cases in Go. Instead of `%llud` you can just say `%d`; `Printf` knows the size and signedness of the integer and can do the right thing for you. The snippet

        var u64 uint64 = 1<<64 - 1
        fmt.Printf("%d %d\n", u64, int64(u64))

prints

    18446744073709551615 -1

In fact, if you're lazy the format `%v` will print, in a simple appropriate style, any value, even an array or structure. The output of

        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)

is

    18446744073709551615 {77 Sunset Strip} [1 2 3 4]

You can drop the formatting altogether if you use `Print` or `Println` instead of `Printf`. Those routines do fully automatic formatting. The `Print` function just prints its elements out using the equivalent of `%v` while `Println` inserts spaces between arguments and adds a newline. The output of each of these two lines is identical to that of the `Printf` call above.

        fmt.Print(u64, " ", t, " ", a, "\n")
        fmt.Println(u64, t, a)

If you have your own type you'd like `Printf` or `Print` to format, just give it a `String` 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.

    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)
    }

Since `*testType` has a `String` method, the default formatter for that type will use it and produce the output

    77 Sunset Strip

Observe that the `String` method calls `Sprint` (the obvious Go variant that returns a string) to do its formatting; special formatters can use the `fmt` library recursively.

Another feature of `Printf` is that the format `%T` will print a string representation of the type of a value, which can be handy when debugging polymorphic code.

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.

You might ask, though, how `Printf` can tell whether a type implements the `String` 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 `v`, it does this:

    type Stringer interface {
        String() string
    }

    s, ok := v.(Stringer)  // Test whether v implements "String()"
    if ok {
        result = s.String()
    } else {
        result = defaultOutput(v)
    }

The code uses a ``type assertion'' (`v.(Stringer)`) to test if the value stored in `v` satisfies the `Stringer` interface; if it does, `s` will become an interface variable implementing the method and `ok` will be `true`. 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, `ok` will be false.

In this snippet the name `Stringer` follows the convention that we add ''[e]r'' to interfaces describing simple method sets like this.

One last wrinkle. To complete the suite, besides `Printf` etc. and `Sprintf` etc., there are also `Fprintf` etc. Unlike in C, `Fprintf`'s first argument is not a file. Instead, it is a variable of type `io.Writer`, which is an interface type defined in the `io` library:

    type Writer interface {
        Write(p []byte) (n int, err os.Error)
    }

(This interface is another conventional name, this time for `Write`; there are also `io.Reader`, `io.ReadWriter`, and so on.) Thus you can call `Fprintf` on any type that implements a standard `Write` method, not just files but also network channels, buffers, whatever you want.

## Prime numbers

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.

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.

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.

To create a stream of integers, we use a Go channel, 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 `make` to create a new channel.

Here is the first function in `progs/sieve.go`:

    // 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'.
        }
    }

The `generate` function sends the sequence 2, 3, 4, 5, ... to its argument channel, `ch`, using the binary communications operator `<-`. Channel operations block, so if there's no recipient for the value on `ch`, the send operation will wait until one becomes available.

The `filter` 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 `<-` (receive) retrieves the next value on the channel.

    // 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'.
            }
        }
    }

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 goroutines. To start a goroutine, invoke the function, prefixing the call with the keyword `go`; this starts the function running in parallel with the current computation but in the same address space:

    go sum(hugeArray) // calculate sum in the background

If you want to know when the calculation is done, pass a channel on which it can report back:

    ch := make(chan int)
    go sum(hugeArray, ch)
    // ... do something else for a while
    result := <-ch  // wait for, and retrieve, result

Back to our prime sieve. Here's how the sieve pipeline is stitched together:

    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
        }
    }

The first line of `main` creates the initial channel to pass to `generate`, which it then starts up. As each prime pops out of the channel, a new `filter` is added to the pipeline and its output becomes the new value of `ch`.

The sieve program can be tweaked to use a pattern common in this style of programming. Here is a variant version of `generate`, from `progs/sieve1.go`:

    func generate() chan int {
        ch := make(chan int)
        go func() {
            for i := 2; ; i++ {
                ch <- i
            }
        }()
        return ch
    }

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.

The function literal notation used in the `go` statement allows us to construct an anonymous function and invoke it on the spot. Notice that the local variable `ch` is available to the function literal and lives on even after `generate` returns.

The same change can be made to `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
    }

The `sieve` 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:

    func sieve() chan int {
        out := make(chan int)
        go func() {
            ch := generate()
            for {
                prime := <-ch
                out <- prime
                ch = filter(ch, prime)
            }
        }()
        return out
    }

Now `main`'s interface to the prime sieve is a channel of primes:

    func main() {
        primes := sieve()
        for i := 0; i < 100; i++ { // Print the first hundred primes.
            fmt.Println(<-primes)
        }
    }

## Multiplexing

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 `request` type, which embeds a channel that will be used for the reply.

    type request struct {
        a, b   int
        replyc chan int
    }

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:

    type binOp func(a, b int) int

    func run(op binOp, req *request) {
        reply := op(req.a, req.b)
        req.replyc <- reply
    }

The type declaration makes `binOp` represent a function taking two integers and returning a third.

The `server` routine loops forever, receiving requests and, to avoid blocking due to a long-running operation, starting a goroutine to do the actual work.

    func server(op binOp, service chan *request) {
        for {
            req := <-service
            go run(op, req) // don't wait for it
        }
    }

We construct a server in a familiar way, starting it and returning a channel connected to it:

    func startServer(op binOp) chan *request {
        req := make(chan *request)
        go server(op, req)
        return req
    }

Here's a simple test. It starts a server with an addition operator and sends out `N` requests without waiting for the replies. Only after all the requests are sent does it check the results.

    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")
    }

One annoyance with this program is that it doesn't shut down the server cleanly; when `main` returns there are a number of lingering goroutines blocked on communication. To solve this, we can provide a second, `quit` channel to the server:

    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
    }

It passes the quit channel to the `server` function, which uses it like this:

    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
            }
        }
    }

Inside `server`, the `select` 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 `select` allows the server to honor requests until it receives a quit message, at which point it returns, terminating its execution.

All that's left is to strobe the `quit` channel at the end of main:

        adder, quit := startServer(func(a, b int) int { return a + b })

...

        quit <- true

There's a lot more to Go programming and concurrent programming in general but this quick tour should give you some of the basics.