doc/code.html

<!--{
	"Title": "How to Write Go Code"
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<h2 id="Introduction">Introduction</h2>

<p>
This document demonstrates the development of a simple Go package and
introduces the <a href="/cmd/go/">go command</a>, the standard way to fetch,
build, and install Go packages and commands.
</p>

<p>
This content is also available as a <a href="http://www.youtube.com/watch?v=XCsL89YtqCs">screencast</a>.
</p>


<h2 id="GOPATH">Code organization</h2>

<h3><code>GOPATH</code> and workspaces</h3>

<p>
One of Go's design goals is to make writing software easier.  To that end, the
<code>go</code> command doesn't use Makefiles or other configuration files to
guide program construction. Instead, it uses the source code to find
dependencies and determine build conditions. This means your source code and
build scripts are always in sync; they are one and the same.
</p>

<p>
The one thing you must do is set a <code>GOPATH</code> environment variable.
<code>GOPATH</code> tells the <code>go</code> command (and other related tools)
where to find and install the Go packages on your system.
</p>

<p>
<code>GOPATH</code> is a list of paths. It shares the syntax of your system's
<code>PATH</code> environment variable. A typical <code>GOPATH</code> on
a Unix system might look like this:
</p>

<pre>
GOPATH=/home/user/ext:/home/user/mygo
</pre>

<p>
(On a Windows system use semicolons as the path separator instead of colons.)
</p>

<p>
Each path in the list (in this case <code>/home/user/ext</code> or
<code>/home/user/mygo</code>) specifies the location of a <i>workspace</i>.
A workspace contains Go source files and their associated package objects, and
command executables. It has a prescribed structure of three subdirectories:
</p>

<ul>
<li><code>src</code> contains Go source files,
<li><code>pkg</code> contains compiled package objects, and
<li><code>bin</code> contains executable commands.
</ul>

<p>
Subdirectories of the <code>src</code> directory hold independent packages, and
all source files (<code>.go</code>, <code>.c</code>, <code>.h</code>, and
<code>.s</code>) in each subdirectory are elements of that subdirectory's
package.
</p>

<p>
When building a program that imports the package "<code>widget</code>" the
<code>go</code> command looks for <code>src/pkg/widget</code> inside the Go root,
and then&mdash;if the package source isn't found there&mdash;it searches
for <code>src/widget</code> inside each workspace in order.
</p>

<p>
Multiple workspaces can offer some flexibility and convenience, but for now
we'll concern ourselves with only a single workspace.
</p>

<p>
Let's work through a simple example. First, create a <code>$HOME/mygo</code>
directory and its <code>src</code> subdirectory:
</p>

<pre>
$ mkdir -p $HOME/mygo/src # create a place to put source code
</pre>

<p>
Next, set it as the <code>GOPATH</code>. You should also add the
<code>bin</code> subdirectory to your <code>PATH</code> environment variable so
that you can run the commands therein without specifying their full path.
To do this, add the following lines to <code>$HOME/.profile</code> (or
equivalent):
</p>

<pre>
export GOPATH=$HOME/mygo
export PATH=$PATH:$HOME/mygo/bin
</pre>


<h3>Import paths</h3>

<p>
The standard packages are given short import paths such as <code>"fmt"</code>
and <code>"net/http"</code> for convenience. 
For your own projects, it is important to choose a base import path that is
unlikely to collide with future additions to the standard library or other
external libraries.
</p>

<p>
The best way to choose an import path is to use the location of your version
control repository.
For instance, if your source repository is at <code>example.com</code> 
or <code>code.google.com/p/example</code>, you should begin your package
paths with that URL, as in "<code>example.com/foo/bar</code>" or
"<code>code.google.com/p/example/foo/bar</code>".
Using this convention, the <code>go</code> command can automatically check out and
build the source code by its import path alone.
</p>

<p>
If you don't intend to install your code in this way, you should at
least use a unique prefix like "<code>widgets/</code>", as in
"<code>widgets/foo/bar</code>". 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.
</p>

<p>
We'll use <code>example/</code> as our base import path:
</p>

<pre>
$ mkdir -p $GOPATH/src/example
</pre>


<h3>Package names</h3>

<p>
The first statement in a Go source file should be
</p>

<pre>
package <i>name</i>
</pre>

<p>
where <code><i>name</i></code> is the package's default name for imports.
(All files in a package must use the same <code><i>name</i></code>.)
</p>

<p>
Go's convention is that the package name is the last element of the
import path: the package imported as "<code>crypto/rot13</code>"
should be named <code>rot13</code>.
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.
</p>

<p>
Create a new package under <code>example</code> called <code>newmath</code>:
</p>

<pre>
$ cd $GOPATH/src/example
$ mkdir newmath
</pre>

<p>
Then create a file named <code>$GOPATH/src/example/newmath/sqrt.go</code>
containing the following Go code:
</p>

<pre>
// Package newmath is a trivial example package.
package newmath

// Sqrt returns an approximation to the square root of x.
func Sqrt(x float64) float64 {
        // This is a terrible implementation.
        // Real code should import "math" and use math.Sqrt.
        z := 0.0
        for i := 0; i &lt; 1000; i++ {
                z -= (z*z - x) / (2 * x)
        }
        return z
}
</pre>

<p>
This package is imported by the path name of the directory it's in, starting
after the <code>src</code> component:
</p>

<pre>
import "example/newmath"
</pre>

<p>
See <a href="/doc/effective_go.html#names">Effective Go</a> to learn more about
Go's naming conventions.
</p>


<h2>Building and installing</h2>

<p>
The <code>go</code> command comprises several subcommands, the most central being
<code>install</code>. Running <code>go install <i>importpath</i></code> builds
and installs a package and its dependencies.
</p>

<p>
To "install a package" means to write the package object or executable command
to the <code>pkg</code> or <code>bin</code> subdirectory of the workspace in
which the source resides.
</p>

<h3>Building a package</h3>

<p>
To build and install the <code>newmath</code> package, type
</p>

<pre>
$ go install example/newmath
</pre>

<p>
This command will produce no output if the package and its dependencies
are built and installed correctly.
</p>

<p>
As a convenience, the <code>go</code> command will assume the current directory
if no import path is specified on the command line. This sequence of commands
has the same effect as the one above:
</p>

<pre>
$ cd $GOPATH/src/example/newmath
$ go install
</pre>

<p>
The resulting workspace directory tree (assuming we're running Linux on a 64-bit
system) looks like this:
</p>

<pre>
pkg/
    linux_amd64/
        example/
            newmath.a  # package object
src/
    example/
        newmath/
            sqrt.go    # package source
</pre>


<h3>Building a command</h3>

<p>
The <code>go</code> command treats code belonging to <code>package main</code> as
an executable command and installs the package binary to the
<code>GOPATH</code>'s <code>bin</code> subdirectory.
</p>

<p>
Add a command named <code>hello</code> to the source tree.
First create the <code>example/hello</code> directory:
</p>

<pre>
$ cd $GOPATH/src/example
$ mkdir hello
</pre>

<p>
Then create the file <code>$GOPATH/src/example/hello/hello.go</code>
containing the following Go code.
</p>

<pre>
// Hello is a trivial example of a main package.
package main

import (
        "example/newmath"
        "fmt"
)

func main() {
        fmt.Printf("Hello, world.  Sqrt(2) = %v\n", newmath.Sqrt(2))
}
</pre>

<p>
Next, run <code>go install</code>, which builds and installs the binary to
<code>$GOPATH/bin</code> (or <code>$GOBIN</code>, if set; to simplify
presentation, this document assumes <code>GOBIN</code> is unset):
</p>

<pre>
$ go install example/hello
</pre>

<p>
To run the program, invoke it by name as you would any other command:
</p>

<pre>
$ $GOPATH/bin/hello
Hello, world.  Sqrt(2) = 1.414213562373095
</pre>

<p>
If you added <code>$HOME/mygo/bin</code> to your <code>PATH</code>, you may omit
the path to the executable:
</p>

<pre>
$ hello
Hello, world.  Sqrt(2) = 1.414213562373095
</pre>

<p>
The workspace directory tree now looks like this:
</p>

<pre>
bin/
    hello              # command executable
pkg/
    linux_amd64/ 
        example/
            newmath.a  # package object
src/
    example/
        hello/
            hello.go   # command source
        newmath/
            sqrt.go    # package source
</pre>

<p>
The <code>go</code> command also provides a <code>build</code> command, which is
like <code>install</code> except it builds all objects in a temporary directory
and does not install them under <code>pkg</code> or <code>bin</code>.
When building a command an executable named after the last element of the
import path is written to the current directory. When building a package, 
<code>go build</code> serves merely to test that the package and its
dependencies can be built. (The resulting package object is thrown away.)
</p>


<h2 id="Testing">Testing</h2>

<p>
Go has a lightweight test framework composed of the <code>go test</code>
command and the <code>testing</code> package.
</p>

<p>
You write a test by creating a file with a name ending in <code>_test.go</code>
that contains functions named <code>TestXXX</code> with signature
<code>func (t *testing.T)</code>.
The test framework runs each such function;
if the function calls a failure function such as <code>t.Error</code> or
<code>t.Fail</code>, the test is considered to have failed.
</p>

<p>
Add a test to the <code>newmath</code> package by creating the file
<code>$GOPATH/src/example/newmath/sqrt_test.go</code> containing the following
Go code.
</p>

<pre>
package newmath

import "testing"

func TestSqrt(t *testing.T) {
	const in, out = 4, 2
	if x := Sqrt(in); x != out {
		t.Errorf("Sqrt(%v) = %v, want %v", in, x, out)
        }
}
</pre>

<p>
Now run the test with <code>go test</code>:
</p>

<pre>
$ go test example/newmath
ok  	example/newmath 0.165s
</pre>

<p>
Run <code><a href="/cmd/go/#hdr-Test_packages">go help test</a></code> and see the
<a href="/pkg/testing/">testing package documentation</a> for more detail.
</p>


<h2 id="remote">Remote packages</h2>

<p>
An import path can describe how to obtain the package source code using a
revision control system such as Git or Mercurial. The <code>go</code> command uses
this property to automatically fetch packages from remote repositories.
For instance, the examples described in this document are also kept in a
Mercurial repository hosted at Google Code,
<code><a href="http://code.google.com/p/go.example">code.google.com/p/go.example</a></code>.
If you include the repository URL in the package's import path,
<code>go get</code> will fetch, build, and install it automatically:
</p>

<pre>
$ go get code.google.com/p/go.example/hello
$ $GOPATH/bin/hello
Hello, world.  Sqrt(2) = 1.414213562373095
</pre>

<p>
If the specified package is not present in a workspace, <code>go get</code>
will place it inside the first workspace specified by <code>GOPATH</code>.
(If the package does already exist, <code>go get</code> skips the remote
fetch and behaves the same as <code>go install</code>.)
</p>

<p>
After issuing the above <code>go get</code> command, the workspace directory
tree should now now look like this:
</p>

<pre>
bin/
    hello                 # command executable
pkg/
    linux_amd64/ 
        code.google.com/p/go.example/
            newmath.a     # package object
        example/
            newmath.a     # package object
src/
    code.google.com/p/go.example/
        hello/
            hello.go      # command source
        newmath/
            sqrt.go       # package source
            sqrt_test.go  # test source
    example/
        hello/
            hello.go      # command source
        newmath/
            sqrt.go       # package source
            sqrt_test.go  # test source
</pre>

<p>
The <code>hello</code> command hosted at Google Code depends on the
<code>newmath</code> package within the same repository. The imports in
<code>hello.go</code> file use the same import path convention, so the <code>go
get</code> command is able to locate and install the dependent package, too.
</p>

<pre>
import "code.google.com/p/go.example/newmath"
</pre>

<p>
This convention is the easiest way to make your Go packages available for
others to use.
The <a href="http://code.google.com/p/go-wiki/wiki/Projects">Go Community Wiki</a>
has a list of external Go projects including programs and libraries.
</p>

<p>
For more information on using remote repositories with the <code>go</code> command, see
<code><a href="/cmd/go/#hdr-Remote_import_path_syntax">go help remote</a></code>.
</p>


<h2 id="more">Further reading</h2>

<p>
See <a href="/doc/effective_go.html">Effective Go</a> for tips on writing
clear, idiomatic Go code.
</p>

<p>
Take <a href="http://tour.golang.org/">A Tour of Go</a> to learn the language
proper.
</p>

<p>
Visit the <a href="/doc/#articles">documentation page</a> for a set of in-depth
articles about the Go language and its libraries and tools.
</p>