Introduction

This document explains how to write a new package and how to test code. It assumes you have installed Go using the installation instructions.

Before embarking on a change to an existing package or the creation of a new package, be sure to send mail to the mailing list to let people know what you are thinking of doing. Doing so helps avoid duplication of effort and enables discussions about design before any code has been written.

Community resources

For real-time help, there may be users or developers on #go-nuts on the Freenode IRC server.

The official mailing list for discussion of the Go language is Go Nuts.

Bugs can be reported using the Go issue tracker.

For those who wish to keep up with development, there is another mailing list, golang-checkins, that receives a message summarizing each checkin to the Go repository.

Creating a new package

Choosing an import path

The standard packages are given short names like fmt and net/http for convenience. For your own projects, choose a name space that is unlikely to collide with future additions to the standard library or other external libraries.

For instance, if your source repository is at example.com or code.google.com/p/example, you should begin your package paths with that URL, as in "example.com/foo/bar" or "code.google.com/p/example/foo/bar". This way the go tool can automatically check out and build the source code from its import path.

If you don't intend your code to be installed in this way, you should at least use a unique prefix like "widgets/", as in "widgets/foo/bar". A good rule is to use a prefix such as your company or project name since it is unlikely to be used by another group.

The go tool and GOPATH

The go tool is the standard means of building and installing Go libraries and programs. It is a "zero configuration" tool; it determines how to build Go packages from their source code alone.

To use the go tool effectively you must set the GOPATH variable. GOPATH specifies a list of paths that contain Go source code and package binaries. Source code, package objects, and command binaries are located inside the GOPATHs' src, pkg, and bin subdirectories respectively.

You should set GOPATH in your shell profile ($HOME/.bashrc, $HOME/.profile, or equivalent).

This shell session demonstrates setting GOPATH, creating a trivial widgets/foo package, and building and installing the package.

$ export GOPATH=$HOME/gocode
$ mkdir -p $GOPATH/src/widgets/foo
$ cat > $GOPATH/src/widgets/foo/foo.go
package foo
const String = "Go rules!"
^D
$ go install widgets/foo
$ ls $GOPATH/pkg/*/widgets
foo.a

(^D means to type Control-D.)

Type go help gopath on the command line for more information about GOPATH.

Go source files

The first statement in a Go source file should be package name, where name is the package's default name for imports. (All files in a package must use the same name.) Go's convention is that the package name is the last element of the import path: the package imported as "crypto/rot13" should be named rot13. There is no requirement that package names be unique across all packages linked into a single binary, only that the import paths (their full file names) be unique.

Go compiles all the source files in a package at once, so one file can refer to constants, variables, types, and functions in another file without special arrangement or declarations.

Writing clean, idiomatic Go code is beyond the scope of this document. Effective Go is an introduction to that topic.

Building programs

The go tool treats code belonging to package main as an executable command, and installs the package binary to the GOPATH's bin subdirectory.

Building executable commands is the same as building packages. Use "go install":

$ mkdir -p $GOPATH/src/widgets/bar
$ cat > $GOPATH/src/widgets/bar/bar.go
package main

import (
    "fmt"
    "widgets/foo"
)

func main() {
    fmt.Println(foo.String)
}
^D
$ go install widgets/bar
$ $GOPATH/bin/bar
Go rules!

Run go help build and go help install for more about building and installing Go binaries.

Testing

Go has a lightweight test framework composed of the go tool and the testing package. You write a test by creating a file with a name ending in _test.go that contains functions named TestXXX with signature func (t *testing.T). The test framework runs each such function; if the function calls a failure function such as t.Error or t.Fail, the test is considered to have failed. Run go help test and see the testing package documentation for more detail.

To run the test, run "go test":

$ cat > $GOPATH/src/widgets/foo/foo_test.go
package foo

import "testing"

func TestString(t *testing.T) {
    const expect = "Go rules!"
    if String != expect {
        t.Errorf("String == %q, want %q", String, expect)
    }
}
^D
$ go test widgets/foo
ok  	widgets/foo	0.018s

If your change affects performance, add a Benchmark function (run go help testfunc) and run it using go test -test.bench=.*.

An example package with tests

This example package, numbers, consists of the function Double, which takes an int and returns that value multiplied by 2. It consists of two files.

First, the package implementation, numbers.go:

package numbers

func Double(i int) int {
	return i * 2
}

Next, the tests, numbers_test.go:

package numbers

import (
	"testing"
)

type doubleTest struct {
	in, out int
}

var doubleTests = []doubleTest{
	doubleTest{1, 2},
	doubleTest{2, 4},
	doubleTest{-5, -10},
}

func TestDouble(t *testing.T) {
	for _, dt := range doubleTests {
		v := Double(dt.in)
		if v != dt.out {
			t.Errorf("Double(%d) = %d, want %d.", dt.in, v, dt.out)
		}
	}
}

Running go install will build and install the package to the GOPATH's pkg directory (it can then be imported by any other Go program).

Running go test will rebuild the package, including the numbers_test.go file, and then run the TestDouble function. The output "ok" indicates that all tests passed successfully. Breaking the implementation by changing the multiplier from 2 to 3 will allow you to see how failing tests are reported.

Run go help test, go help testfunc, and go help testflag and see the testing package documentation for more detail.

Architecture- and operating system-specific code

First, a disclaimer: very few Go packages should need to know about the hardware and operating system they run on. In the vast majority of cases the language and standard library handle most portability issues. This section is a guide for experienced systems programmers who have a good reason to write platform-specific code, such as assembly-language support for fast trigonometric functions or code that implements a common interface above different operating systems.

To compile such code, use the $GOOS and $GOARCH environment variables in your source file names.

For example, consider the package foo that consists of four files:

foo.go
foo_386.go
foo_amd64.go
foo_arm.go

The general code goes in foo.go, while architecture-specific code goes in foo_386.go, foo_amd64.go, and foo_arm.go.

If you follow these conventional parameterizations, tools such as the go tool will work seamlessly with your package:

foo_$GOOS.go
foo_$GOARCH.go
foo_$GOOS_$GOARCH.go

The same holds for .s (assembly) and .c files.