## 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 might want to look at Effective Go, which digs deeper into how the language is used.

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/`.

Program snippets are annotated with the line number in the original file; for cleanliness, blank lines remain blank.

## Hello, World

Let's start in the usual way:

    05    package main

    07    import fmt "fmt"  // Package implementing formatted I/O.

    09    func main() {
    10        fmt.Printf("Hello, world; or Καλημέρα κόσμε; or こんにちは 世界\n");
    11    }

Every Go source file declares, using a `package` statement, which package it's part of. The `main` package's `main` function is where the program starts running (after any initialization). 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`.

Function declarations are introduced with the `func` keyword.

Notice that string constants can contain Unicode characters, encoded in UTF-8. Go is defined to accept UTF-8 input. Strings are arrays of bytes, usually used to store Unicode strings represented in UTF-8.

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

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

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

## 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)`:

    05    package main

    07    import (
    08        "os";
    09        "flag";  // command line option parser
    10    )

    12    var n_flag = flag.Bool("n", false, "don't print final newline")

    14    const (
    15        Space = " ";
    16        Newline = "\n";
    17    )

    19    func main() {
    20        flag.Parse();   // Scans the arg list and sets up flags
    21        var s string = "";
    22        for i := 0; i < flag.NArg(); i++ {
    23            if i > 0 {
    24                s += Space
    25            }
    26            s += flag.Arg(i)
    27        }
    28        if !*n_flag {
    29            s += Newline
    30        }
    31        os.Stdout.WriteString(s);
    32    }

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, semicolon-separated lists if we want, as on lines 7-10 and 14-17. But it's not necessary to do so; we could have said

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

Semicolons aren't needed here; in fact, semicolons are unnecessary after any top-level declaration, even though they are needed as separators within a parenthesized list of declarations.

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.

Having imported the `flag` package, line 12 creates a global variable to hold the value of echo's `-n` flag. The variable `n_flag` has type `*bool`, pointer to `bool`.

In `main.main`, we parse the arguments (line 20) and then create a local string variable we will use 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. (For those who know Limbo, its `:=` construct is the same, but notice that Go has no colon after the name in a full `var` declaration. Also, for simplicity of parsing, `:=` only works inside functions, not at the top level.) There's one in the `for` clause on the next line:

    22        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 flags 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 an array used by the `flag` package to access the command-line arguments.

## An Interlude about Types

Go has some familiar types such as `int` and `float`, which represent values of the ''appropriate'' size for the machine. It also defines specifically-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.

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:

    11        s := "hello";
    12        if s[1] != 'e' { os.Exit(1) }
    13        s = "good bye";
    14        var p *string = &s;
    15        *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 array_of_int [10]int;

Arrays, like strings, are values, but they are mutable. This differs from C, in which `array_of_int` 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 which one can assign a pointer to any array with the same element type or—much more commonly—a slice expression of the form `a[low : high]`, representing the subarray indexed by `low` through `high-1`. Slices look a lot like arrays but have no explicit size (`[]` vs. `[10]`) and they reference a segment of an underlying, often anonymous, regular array. Multiple slices can share data if they represent pieces of the same array; multiple arrays can never share data.

Slices are actually 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.

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, take the address of the array and Go will automatically create (efficiently) a slice reference and pass that.

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

    09    func sum(a []int) int {   // returns an int
    10        s := 0;
    11        for i := 0; i < len(a); i++ {
    12            s += a[i]
    13        }
    14        return s
    15    }

and invoke it like this:

    19        s := sum(&[3]int{1,2,3});  // a slice of the array is passed to sum

Note how the return type (`int`) is defined for `sum()` by stating it after the parameter list. The expression `[3]int{1,2,3}` -- a type followed by a brace-bounded expression -- is a constructor for a value, in this case an array of 3 `ints`. Putting an `&` in front gives us the address of a unique instance of the value. We pass the pointer to `sum()` by (automatically) promoting it to a slice.

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});

In practice, though, unless you're meticulous about storage layout within a data structure, a slice itself - using empty brackets and no `&` - 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, and maps.

## 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 something on the stack, just declare a variable. To allocate it on the heap, use `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 to 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()`, you receive a pointer to an uninitialized 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 hard_eight = (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; use a "conversion"
        i := 0x1234       // i gets default type: int
        var j int = 1e6   // legal - 1000000 is representable in an int
        x := 1.5          // a float
        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 `ints` and `floats`, plus a few other simple cases. 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 the usual sort of open/close/read/write interface. Here's the start of `file.go`:

    05    package file

    07    import (
    08        "os";
    09        "syscall";
    10    )

    12    type File struct {
    13        fd      int;    // file descriptor number
    14        name    string; // file name at Open time
    15    }

The first line declares the name of the package -- `file` -- and then we 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 only 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 them:

    17    func newFile(fd int, name string) *File {
    18        if fd < 0 {
    19            return nil
    20        }
    21        return &File{fd, name}
    22    }

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 nonce composite literal, as is done here on line 21.

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

    24    var (
    25        Stdin  = newFile(0, "/dev/stdin");
    26        Stdout = newFile(1, "/dev/stdout");
    27        Stderr = newFile(2, "/dev/stderr");
    28    )

The `newFile` function was not exported because it's internal. The proper, exported factory to use is `Open`:

    30    func Open(name string, mode int, perm int) (file *File, err os.Error) {
    31        r, e := syscall.Open(name, mode, perm);
    32        if e != 0 {
    33            err = os.Errno(e);
    34        }
    35        return newFile(r, name), err
    36    }

There are a number of new things in these few lines. First, `Open` returns multiple values, an `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 line 31; it declares `r` and `e` to hold the two values, both of type `int64` (although you'd have to look at the `syscall` package to see that). Finally, line 35 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 string, maintaining a unique set of errors throughout the program. 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 `os.Errno` to translate Unix's integer `errno` value into an error value, which will be stored in a unique instance of type `os.Error`.

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`.

    38    func (file *File) Close() os.Error {
    39        if file == nil {
    40            return os.EINVAL
    41        }
    42        e := syscall.Close(file.fd);
    43        file.fd = -1;  // so it can't be closed again
    44        if e != 0 {
    45            return os.Errno(e);
    46        }
    47        return nil
    48    }

    50    func (file *File) Read(b []byte) (ret int, err os.Error) {
    51        if file == nil {
    52            return -1, os.EINVAL
    53        }
    54        r, e := syscall.Read(file.fd, b);
    55        if e != 0 {
    56            err = os.Errno(e);
    57        }
    58        return int(r), err
    59    }

    61    func (file *File) Write(b []byte) (ret int, err os.Error) {
    62        if file == nil {
    63            return -1, os.EINVAL
    64        }
    65        r, e := syscall.Write(file.fd, b);
    66        if e != 0 {
    67            err = os.Errno(e);
    68        }
    69        return int(r), err
    70    }

    72    func (file *File) String() string {
    73        return file.name
    74    }

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 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:

    05    package main

    07    import (
    08        "./file";
    09        "fmt";
    10        "os";
    11    )

    13    func main() {
    14        hello := []byte{'h', 'e', 'l', 'l', 'o', ',', ' ', 'w', 'o', 'r', 'l', 'd', '\n'};
    15        file.Stdout.Write(hello);
    16        file, err := file.Open("/does/not/exist",  0,  0);
    17        if file == nil {
    18            fmt.Printf("can't open file; err=%s\n",  err.String());
    19            os.Exit(1);
    20        }
    21    }

The import of ''`./file`'' tells the compiler to use our own package rather than something from the directory of installed packages.

Finally we can run the program:

        % 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`:

    05    package main

    07    import (
    08        "./file";
    09        "flag";
    10        "fmt";
    11        "os";
    12    )

    14    func cat(f *file.File) {
    15        const NBUF = 512;
    16        var buf [NBUF]byte;
    17        for {
    18            switch nr, er := f.Read(&buf); true {
    19            case nr < 0:
    20                fmt.Fprintf(os.Stderr, "cat: error reading from %s: %s\n", f.String(), er.String());
    21                os.Exit(1);
    22            case nr == 0:  // EOF
    23                return;
    24            case nr > 0:
    25                if nw, ew := file.Stdout.Write(buf[0:nr]); nw != nr {
    26                    fmt.Fprintf(os.Stderr, "cat: error writing from %s: %s\n", f.String(), ew.String());
    27                }
    28            }
    29        }
    30    }

    32    func main() {
    33        flag.Parse();   // Scans the arg list and sets up flags
    34        if flag.NArg() == 0 {
    35            cat(file.Stdin);
    36        }
    37        for i := 0; i < flag.NArg(); i++ {
    38            f, err := file.Open(flag.Arg(i), 0, 0);
    39            if f == nil {
    40                fmt.Fprintf(os.Stderr, "cat: can't open %s: error %s\n", flag.Arg(i), err);
    41                os.Exit(1);
    42            }
    43            cat(f);
    44            f.Close();
    45        }
    46    }

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` on line 18 uses one to create variables `nr` and `er` to hold the return values from `f.Read()`. (The `if` on line 25 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`.

Line 25 calls `Write()` by slicing the incoming buffer, which is itself a slice. 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`:

    26    type reader interface {
    27        Read(b []byte) (ret int, err os.Error);
    28        String() string;
    29    }

Any type that implements the two methods of `reader` -- regardless of whatever other methods the type may also contain -- 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.

    31    type rotate13 struct {
    32        source    reader;
    33    }

    35    func newRotate13(source reader) *rotate13 {
    36        return &rotate13{source}
    37    }

    39    func (r13 *rotate13) Read(b []byte) (ret int, err os.Error) {
    40        r, e := r13.source.Read(b);
    41        for i := 0; i < r; i++ {
    42            b[i] = rot13(b[i])
    43        }
    44        return r, e
    45    }

    47    func (r13 *rotate13) String() string {
    48        return r13.source.String()
    49    }
    50    // end of rotate13 implementation

(The `rot13` function called on line 42 is trivial and not worth reproducing here.)

To use the new feature, we define a flag:

    14    var rot13_flag = flag.Bool("rot13", false, "rot13 the input")

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

    52    func cat(r reader) {
    53        const NBUF = 512;
    54        var buf [NBUF]byte;

    56        if *rot13_flag {
    57            r = newRotate13(r)
    58        }
    59        for {
    60            switch nr, er := r.Read(&buf); {
    61            case nr < 0:
    62                fmt.Fprintf(os.Stderr, "cat: error reading from %s: %s\n", r.String(), er.String());
    63                os.Exit(1);
    64            case nr == 0:  // EOF
    65                return;
    66            case nr > 0:
    67                nw, ew := file.Stdout.Write(buf[0:nr]);
    68                if nw != nr {
    69                    fmt.Fprintf(os.Stderr, "cat: error writing from %s: %s\n", r.String(), ew.String());
    70                }
    71            }
    72        }
    73    }

(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.) Lines 56 through 58 set 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 distinct 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 since 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`:

    13    func Sort(data Interface) {
    14        for i := 1; i < data.Len(); i++ {
    15            for j := i; j > 0 && data.Less(j, j-1); j-- {
    16                data.Swap(j, j-1);
    17            }
    18        }
    19    }

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

    07    type Interface interface {
    08        Len() int;
    09        Less(i, j int) bool;
    10        Swap(i, j int);
    11    }

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`

    33    type IntArray []int

    35    func (p IntArray) Len() int            { return len(p); }
    36    func (p IntArray) Less(i, j int) bool  { return p[i] < p[j]; }
    37    func (p IntArray) 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.

    12    func ints() {
    13        data := []int{74, 59, 238, -784, 9845, 959, 905, 0, 0, 42, 7586, -5467984, 7586};
    14        a := sort.IntArray(data);
    15        sort.Sort(a);
    16        if !sort.IsSorted(a) {
    17            panic()
    18        }
    19    }

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:

    30    type day struct {
    31        num        int;
    32        shortName  string;
    33        longName   string;
    34    }

    36    type dayArray struct {
    37        data []*day;
    38    }

    40    func (p *dayArray) Len() int            { return len(p.data); }
    41    func (p *dayArray) Less(i, j int) bool  { return p.data[i].num < p.data[j].num; }
    42    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 ...) (n int, errno os.Error)

That `...` represents the variadic argument list that in C would be handled using the `stdarg.h` macros, but in Go is passed using an empty interface variable (`interface {}`) that is then unpacked using the reflection library. It's off topic here but the use of reflection 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

    10        var u64 uint64 = 1<<64-1;
    11        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

    14        type T struct { a int; b string };
    15        t := T{77, "Sunset Strip"};
    16        a := []int{1, 2, 3, 4};
    17        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` automatically 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.

    18        fmt.Print(u64, " ", t, " ", a, "\n");
    19        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.

    09    type testType struct { a int; b string }

    11    func (t *testType) String() string {
    12        return fmt.Sprint(t.a) + " " + t.b
    13    }

    15    func main() {
    16        t := &testType{77, "Sunset Strip"};
    17        fmt.Println(t)
    18    }

Since `*T` 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 computationationally 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`:

    09    // Send the sequence 2, 3, 4, ... to channel 'ch'.
    10    func generate(ch chan int) {
    11        for i := 2; ; i++ {
    12            ch <- i  // Send 'i' to channel 'ch'.
    13        }
    14    }

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.

    16    // Copy the values from channel 'in' to channel 'out',
    17    // removing those divisible by 'prime'.
    18    func filter(in, out chan int, prime int) {
    19        for {
    20            i := <-in;  // Receive value of new variable 'i' from 'in'.
    21            if i % prime != 0 {
    22                out <- i  // Send 'i' to channel 'out'.
    23            }
    24        }
    25    }

The generator and filters execute concurrently. Go has its own model of process/threads/light-weight processes/coroutines, so to avoid notational confusion we'll 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(huge_array); // 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(huge_array, 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:

    28    func main() {
    29        ch := make(chan int);  // Create a new channel.
    30        go generate(ch);  // Start generate() as a goroutine.
    31        for {
    32            prime := <-ch;
    33            fmt.Println(prime);
    34            ch1 := make(chan int);
    35            go filter(ch, ch1, prime);
    36            ch = ch1
    37        }
    38    }

Line 29 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`:

    10    func generate() chan int {
    11        ch := make(chan int);
    12        go func(){
    13            for i := 2; ; i++ {
    14                ch <- i
    15            }
    16        }();
    17        return ch;
    18    }

This version does all the setup internally. It creates the output channel, launches a goroutine internally using 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 (lines 12-16) 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`:

    21    func filter(in chan int, prime int) chan int {
    22        out := make(chan int);
    23        go func() {
    24            for {
    25                if i := <-in; i % prime != 0 {
    26                    out <- i
    27                }
    28            }
    29        }();
    30        return out;
    31    }

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:

    33    func sieve() chan int {
    34        out := make(chan int);
    35        go func() {
    36            ch := generate();
    37            for {
    38                prime := <-ch;
    39                out <- prime;
    40                ch = filter(ch, prime);
    41            }
    42        }();
    43        return out;
    44    }

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

    46    func main() {
    47        primes := sieve();
    48        for {
    49            fmt.Println(<-primes);
    50        }
    51    }

## Multiplexing

With channels, it's possible to serve multiple independent client goroutines without writing an actual 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.

    09    type request struct {
    10        a, b    int;
    11        replyc  chan int;
    12    }

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:

    14    type binOp func(a, b int) int

    16    func run(op binOp, req *request) {
    17        reply := op(req.a, req.b);
    18        req.replyc <- reply;
    19    }

Line 18 defines the name `binOp` to be 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.

    21    func server(op binOp, service chan *request) {
    22        for {
    23            req := <-service;
    24            go run(op, req);  // don't wait for it
    25        }
    26    }

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

    28    func startServer(op binOp) chan *request {
    29        req := make(chan *request);
    30        go server(op, req);
    31        return req;
    32    }

Here's a simple test. It starts a server with an addition operator, and sends out lots of requests but doesn't wait for the reply. Only after all the requests are sent does it check the results.

    34    func main() {
    35        adder := startServer(func(a, b int) int { return a + b });
    36        const N = 100;
    37        var reqs [N]request;
    38        for i := 0; i < N; i++ {
    39            req := &reqs[i];
    40            req.a = i;
    41            req.b = i + N;
    42            req.replyc = make(chan int);
    43            adder <- req;
    44        }
    45        for i := N-1; i >= 0; i-- {   // doesn't matter what order
    46            if <-reqs[i].replyc != N + 2*i {
    47                fmt.Println("fail at", i);
    48            }
    49        }
    50        fmt.Println("done");
    51    }

One annoyance with this program is that it doesn't exit 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:

    32    func startServer(op binOp) (service chan *request, quit chan bool) {
    33        service = make(chan *request);
    34        quit = make(chan bool);
    35        go server(op, service, quit);
    36        return service, quit;
    37    }

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

    21    func server(op binOp, service chan *request, quit chan bool) {
    22        for {
    23            select {
    24            case req := <-service:
    25                go run(op, req);  // don't wait for it
    26            case <-quit:
    27                return;
    28            }
    29        }
    30    }

Inside `server`, a `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:

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

...

    55        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.