doc/code.html - go - Git at Google

 <!--{
 	"Title": "How to Write Go Code"
 }-->

 <h2 id="Introduction">Introduction</h2>

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

 <p>
 The <code>go</code> tool requires you to organize your code in a specific
 way. Please read this document carefully.
 It explains the simplest way to get up and running with your Go installation.
 </p>

 <p>
 A similar explanation is available as a
 <a href="//www.youtube.com/watch?v=XCsL89YtqCs">screencast</a>.
 </p>


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

 <h3 id="Workspaces">Workspaces</h3>

 <p>
 The <code>go</code> tool is designed to work with open source code maintained
 in public repositories. Although you don't need to publish your code, the model
 for how the environment is set up works the same whether you do or not.
 </p>

 <p>
 Go code must be kept inside a <i>workspace</i>.
 A workspace is a directory hierarchy with three directories at its root:
 </p>

 <ul>
 <li><code>src</code> contains Go source files organized into packages (one package per directory),
 <li><code>pkg</code> contains package objects, and
 <li><code>bin</code> contains executable commands.
 </ul>

 <p>
 The <code>go</code> tool builds source packages and installs the resulting
 binaries to the <code>pkg</code> and <code>bin</code> directories.
 </p>

 <p>
 The <code>src</code> subdirectory typically contains multiple version control
 repositories (such as for Git or Mercurial) that track the development of one
 or more source packages.
 </p>

 <p>
 To give you an idea of how a workspace looks in practice, here's an example:
 </p>

 <pre>
 bin/
     hello                          # command executable
     outyet                         # command executable
 pkg/
     linux_amd64/
         github.com/golang/example/
             stringutil.a           # package object
 src/
     <a href="https://github.com/golang/example/">github.com/golang/example/</a>
         .git/                      # Git repository metadata
 	hello/
 	    hello.go               # command source
 	outyet/
 	    main.go                # command source
 	    main_test.go           # test source
 	stringutil/
 	    reverse.go             # package source
 	    reverse_test.go        # test source
 </pre>

 <p>
 This workspace contains one repository (<code>example</code>)
 comprising two commands (<code>hello</code> and <code>outyet</code>)
 and one library (<code>stringutil</code>).
 </p>

 <p>
 A typical workspace would contain many source repositories containing many
 packages and commands. Most Go programmers keep <i>all</i> their Go source code
 and dependencies in a single workspace.
 </p>

 <p>
 Commands and libraries are built from different kinds of source packages.
 We will discuss the distinction <a href="#PackageNames">later</a>.
 </p>


 <h3 id="GOPATH">The <code>GOPATH</code> environment variable</h3>

 <p>
 The <code>GOPATH</code> environment variable specifies the location of your
 workspace. It is likely the only environment variable you'll need to set
 when developing Go code.
 </p>

 <p>
 To get started, create a workspace directory and set <code>GOPATH</code>
 accordingly. Your workspace can be located wherever you like, but we'll use
 <code>$HOME/work</code> in this document. Note that this must <b>not</b> be the
 same path as your Go installation.
 (Another common setup is to set <code>GOPATH=$HOME</code>.)
 </p>

 <pre>
 $ <b>mkdir $HOME/work</b>
 $ <b>export GOPATH=$HOME/work</b>
 </pre>

 <p>
 For convenience, add the workspace's <code>bin</code> subdirectory
 to your <code>PATH</code>:
 </p>

 <pre>
 $ <b>export PATH=$PATH:$GOPATH/bin</b>
 </pre>

 <p>
 To learn more about setting up the <code>GOPATH</code> environment variable,
 please see
 <a href="/cmd/go/#hdr-GOPATH_environment_variable"><code>go help gopath</code></a>
 </p>

 <h3 id="PackagePaths">Package paths</h3>

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

 <p>
 If you keep your code in a source repository somewhere, then you should use the
 root of that source repository as your base path.
 For instance, if you have a <a href="https://github.com/">GitHub</a> account at
 <code>github.com/user</code>, that should be your base path.
 </p>

 <p>
 Note that you don't need to publish your code to a remote repository before you
 can build it. It's just a good habit to organize your code as if you will
 publish it someday. In practice you can choose any arbitrary path name,
 as long as it is unique to the standard library and greater Go ecosystem.
 </p>

 <p>
 We'll use <code>github.com/user</code> as our base path. Create a directory
 inside your workspace in which to keep source code:
 </p>

 <pre>
 $ <b>mkdir -p $GOPATH/src/github.com/user</b>
 </pre>


 <h3 id="Command">Your first program</h3>

 <p>
 To compile and run a simple program, first choose a package path (we'll use
 <code>github.com/user/hello</code>) and create a corresponding package directory
 inside your workspace:
 </p>

 <pre>
 $ <b>mkdir $GOPATH/src/github.com/user/hello</b>
 </pre>

 <p>
 Next, create a file named <code>hello.go</code> inside that directory,
 containing the following Go code.
 </p>

 <pre>
 package main

 import "fmt"

 func main() {
 	fmt.Printf("Hello, world.\n")
 }
 </pre>

 <p>
 Now you can build and install that program with the <code>go</code> tool:
 </p>

 <pre>
 $ <b>go install github.com/user/hello</b>
 </pre>

 <p>
 Note that you can run this command from anywhere on your system. The
 <code>go</code> tool finds the source code by looking for the
 <code>github.com/user/hello</code> package inside the workspace specified by
 <code>GOPATH</code>.
 </p>

 <p>
 You can also omit the package path if you run <code>go install</code> from the
 package directory:
 </p>

 <pre>
 $ <b>cd $GOPATH/src/github.com/user/hello</b>
 $ <b>go install</b>
 </pre>

 <p>
 This command builds the <code>hello</code> command, producing an executable
 binary. It then installs that binary to the workspace's <code>bin</code>
 directory as <code>hello</code> (or, under Windows, <code>hello.exe</code>).
 In our example, that will be <code>$GOPATH/bin/hello</code>, which is
 <code>$HOME/work/bin/hello</code>.
 </p>

 <p>
 The <code>go</code> tool will only print output when an error occurs, so if
 these commands produce no output they have executed successfully.
 </p>

 <p>
 You can now run the program by typing its full path at the command line:
 </p>

 <pre>
 $ <b>$GOPATH/bin/hello</b>
 Hello, world.
 </pre>

 <p>
 Or, as you have added <code>$GOPATH/bin</code> to your <code>PATH</code>,
 just type the binary name:
 </p>

 <pre>
 $ <b>hello</b>
 Hello, world.
 </pre>

 <p>
 If you're using a source control system, now would be a good time to initialize
 a repository, add the files, and commit your first change. Again, this step is
 optional: you do not need to use source control to write Go code.
 </p>

 <pre>
 $ <b>cd $GOPATH/src/github.com/user/hello</b>
 $ <b>git init</b>
 Initialized empty Git repository in /home/user/work/src/github.com/user/hello/.git/
 $ <b>git add hello.go</b>
 $ <b>git commit -m "initial commit"</b>
 [master (root-commit) 0b4507d] initial commit
  1 file changed, 1 insertion(+)
   create mode 100644 hello.go
 </pre>

 <p>
 Pushing the code to a remote repository is left as an exercise for the reader.
 </p>


 <h3 id="Library">Your first library</h3>

 <p>
 Let's write a library and use it from the <code>hello</code> program.
 </p>

 <p>
 Again, the first step is to choose a package path (we'll use
 <code>github.com/user/stringutil</code>) and create the package directory:
 </p>

 <pre>
 $ <b>mkdir $GOPATH/src/github.com/user/stringutil</b>
 </pre>

 <p>
 Next, create a file named <code>reverse.go</code> in that directory with the
 following contents.
 </p>

 <pre>
 // Package stringutil contains utility functions for working with strings.
 package stringutil

 // Reverse returns its argument string reversed rune-wise left to right.
 func Reverse(s string) string {
 	r := []rune(s)
 	for i, j := 0, len(r)-1; i &lt; len(r)/2; i, j = i+1, j-1 {
 		r[i], r[j] = r[j], r[i]
 	}
 	return string(r)
 }
 </pre>

 <p>
 Now, test that the package compiles with <code>go build</code>:
 </p>

 <pre>
 $ <b>go build github.com/user/stringutil</b>
 </pre>

 <p>
 Or, if you are working in the package's source directory, just:
 </p>

 <pre>
 $ <b>go build</b>
 </pre>

 <p>
 This won't produce an output file. To do that, you must use <code>go
 install</code>, which places the package object inside the <code>pkg</code>
 directory of the workspace.
 </p>

 <p>
 After confirming that the <code>stringutil</code> package builds,
 modify your original <code>hello.go</code> (which is in
 <code>$GOPATH/src/github.com/user/hello</code>) to use it:
 </p>

 <pre>
 package main

 import (
 	"fmt"

 	<b>"github.com/user/stringutil"</b>
 )

 func main() {
 	fmt.Printf(stringutil.Reverse("!oG ,olleH"))
 }
 </pre>

 <p>
 Whenever the <code>go</code> tool installs a package or binary, it also
 installs whatever dependencies it has.
 So when you install the <code>hello</code> program
 </p>

 <pre>
 $ <b>go install github.com/user/hello</b>
 </pre>

 <p>
 the <code>stringutil</code> package will be installed as well, automatically.
 </p>

 <p>
 Running the new version of the program, you should see a new, reversed message:
 </p>

 <pre>
 $ <b>hello</b>
 Hello, Go!
 </pre>

 <p>
 After the steps above, your workspace should look like this:
 </p>

 <pre>
 bin/
     hello                 # command executable
 pkg/
     linux_amd64/          # this will reflect your OS and architecture
         github.com/user/
             stringutil.a  # package object
 src/
     github.com/user/
         hello/
             hello.go      # command source
         stringutil/
             reverse.go    # package source
 </pre>

 <p>
 Note that <code>go install</code> placed the <code>stringutil.a</code> object
 in a directory inside <code>pkg/linux_amd64</code> that mirrors its source
 directory.
 This is so that future invocations of the <code>go</code> tool can find the
 package object and avoid recompiling the package unnecessarily.
 The <code>linux_amd64</code> part is there to aid in cross-compilation,
 and will reflect the operating system and architecture of your system.
 </p>

 <p>
 Go command executables are statically linked; the package objects need not
 be present to run Go programs.
 </p>


 <h3 id="PackageNames">Package names</h3>

 <p>
 The first statement in a Go source file must 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>.
 </p>

 <p>
 Executable commands must always use <code>package main</code>.
 </p>

 <p>
 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>
 See <a href="/doc/effective_go.html#names">Effective Go</a> to learn more about
 Go's naming conventions.
 </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>stringutil</code> package by creating the file
 <code>$GOPATH/src/github.com/user/stringutil/reverse_test.go</code> containing
 the following Go code.
 </p>

 <pre>
 package stringutil

 import "testing"

 func TestReverse(t *testing.T) {
 	cases := []struct {
 		in, want string
 	}{
 		{"Hello, world", "dlrow ,olleH"},
 		{"Hello, 世界", "界世 ,olleH"},
 		{"", ""},
 	}
 	for _, c := range cases {
 		got := Reverse(c.in)
 		if got != c.want {
 			t.Errorf("Reverse(%q) == %q, want %q", c.in, got, c.want)
 		}
 	}
 }
 </pre>

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

 <pre>
 $ <b>go test github.com/user/stringutil</b>
 ok  	github.com/user/stringutil 0.165s
 </pre>

 <p>
 As always, if you are running the <code>go</code> tool from the package
 directory, you can omit the package path:
 </p>

 <pre>
 $ <b>go test</b>
 ok  	github.com/user/stringutil 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> tool uses
 this property to automatically fetch packages from remote repositories.
 For instance, the examples described in this document are also kept in a
 Git repository hosted at GitHub
 <code><a href="https://github.com/golang/example">github.com/golang/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>
 $ <b>go get github.com/golang/example/hello</b>
 $ <b>$GOPATH/bin/hello</b>
 Hello, Go examples!
 </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 look like this:
 </p>

 <pre>
 bin/
     hello                           # command executable
 pkg/
     linux_amd64/
         github.com/golang/example/
             stringutil.a            # package object
         github.com/user/
             stringutil.a            # package object
 src/
     github.com/golang/example/
 	.git/                       # Git repository metadata
         hello/
             hello.go                # command source
         stringutil/
             reverse.go              # package source
             reverse_test.go         # test source
     github.com/user/
         hello/
             hello.go                # command source
         stringutil/
             reverse.go              # package source
             reverse_test.go         # test source
 </pre>

 <p>
 The <code>hello</code> command hosted at GitHub depends on the
 <code>stringutil</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 "github.com/golang/example/stringutil"
 </pre>

 <p>
 This convention is the easiest way to make your Go packages available for
 others to use.
 The <a href="//golang.org/wiki/Projects">Go Wiki</a>
 and <a href="//godoc.org/">godoc.org</a>
 provide lists of external Go projects.
 </p>

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


 <h2 id="next">What's next</h2>

 <p>
 Subscribe to the
 <a href="//groups.google.com/group/golang-announce">golang-announce</a>
 mailing list to be notified when a new stable version of Go is released.
 </p>

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

 <p>
 Take <a href="//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>


 <h2 id="help">Getting help</h2>

 <p>
 For real-time help, ask the helpful gophers in <code>#go-nuts</code> on the
 <a href="http://freenode.net/">Freenode</a> IRC server.
 </p>

 <p>
 The official mailing list for discussion of the Go language is
 <a href="//groups.google.com/group/golang-nuts">Go Nuts</a>.
 </p>

 <p>
 Report bugs using the
 <a href="//golang.org/issue">Go issue tracker</a>.
 </p>
	<!--{
	"Title": "How to Write Go Code"
	}-->

	<h2 id="Introduction">Introduction</h2>

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

	<p>
	The <code>go</code> tool requires you to organize your code in a specific
	way. Please read this document carefully.
	It explains the simplest way to get up and running with your Go installation.
	</p>

	<p>
	A similar explanation is available as a
	<a href="//www.youtube.com/watch?v=XCsL89YtqCs">screencast</a>.
	</p>


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

	<h3 id="Workspaces">Workspaces</h3>

	<p>
	The <code>go</code> tool is designed to work with open source code maintained
	in public repositories. Although you don't need to publish your code, the model
	for how the environment is set up works the same whether you do or not.
	</p>

	<p>
	Go code must be kept inside a <i>workspace</i>.
	A workspace is a directory hierarchy with three directories at its root:
	</p>

	<ul>
	<li><code>src</code> contains Go source files organized into packages (one package per directory),
	<li><code>pkg</code> contains package objects, and
	<li><code>bin</code> contains executable commands.
	</ul>

	<p>
	The <code>go</code> tool builds source packages and installs the resulting
	binaries to the <code>pkg</code> and <code>bin</code> directories.
	</p>

	<p>
	The <code>src</code> subdirectory typically contains multiple version control
	repositories (such as for Git or Mercurial) that track the development of one
	or more source packages.
	</p>

	<p>
	To give you an idea of how a workspace looks in practice, here's an example:
	</p>

	<pre>
	bin/
	hello # command executable
	outyet # command executable
	pkg/
	linux_amd64/
	github.com/golang/example/
	stringutil.a # package object
	src/
	<a href="https://github.com/golang/example/">github.com/golang/example/</a>
	.git/ # Git repository metadata
	hello/
	hello.go # command source
	outyet/
	main.go # command source
	main_test.go # test source
	stringutil/
	reverse.go # package source
	reverse_test.go # test source
	</pre>

	<p>
	This workspace contains one repository (<code>example</code>)
	comprising two commands (<code>hello</code> and <code>outyet</code>)
	and one library (<code>stringutil</code>).
	</p>

	<p>
	A typical workspace would contain many source repositories containing many
	packages and commands. Most Go programmers keep <i>all</i> their Go source code
	and dependencies in a single workspace.
	</p>

	<p>
	Commands and libraries are built from different kinds of source packages.
	We will discuss the distinction <a href="#PackageNames">later</a>.
	</p>


	<h3 id="GOPATH">The <code>GOPATH</code> environment variable</h3>

	<p>
	The <code>GOPATH</code> environment variable specifies the location of your
	workspace. It is likely the only environment variable you'll need to set
	when developing Go code.
	</p>

	<p>
	To get started, create a workspace directory and set <code>GOPATH</code>
	accordingly. Your workspace can be located wherever you like, but we'll use
	<code>$HOME/work</code> in this document. Note that this must <b>not</b> be the
	same path as your Go installation.
	(Another common setup is to set <code>GOPATH=$HOME</code>.)
	</p>

	<pre>
	$ <b>mkdir $HOME/work</b>
	$ <b>export GOPATH=$HOME/work</b>
	</pre>

	<p>
	For convenience, add the workspace's <code>bin</code> subdirectory
	to your <code>PATH</code>:
	</p>

	<pre>
	$ <b>export PATH=$PATH:$GOPATH/bin</b>
	</pre>

	<p>
	To learn more about setting up the <code>GOPATH</code> environment variable,
	please see
	<a href="/cmd/go/#hdr-GOPATH_environment_variable"><code>go help gopath</code></a>
	</p>

	<h3 id="PackagePaths">Package paths</h3>

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

	<p>
	If you keep your code in a source repository somewhere, then you should use the
	root of that source repository as your base path.
	For instance, if you have a <a href="https://github.com/">GitHub</a> account at
	<code>github.com/user</code>, that should be your base path.
	</p>

	<p>
	Note that you don't need to publish your code to a remote repository before you
	can build it. It's just a good habit to organize your code as if you will
	publish it someday. In practice you can choose any arbitrary path name,
	as long as it is unique to the standard library and greater Go ecosystem.
	</p>

	<p>
	We'll use <code>github.com/user</code> as our base path. Create a directory
	inside your workspace in which to keep source code:
	</p>

	<pre>
	$ <b>mkdir -p $GOPATH/src/github.com/user</b>
	</pre>


	<h3 id="Command">Your first program</h3>

	<p>
	To compile and run a simple program, first choose a package path (we'll use
	<code>github.com/user/hello</code>) and create a corresponding package directory
	inside your workspace:
	</p>

	<pre>
	$ <b>mkdir $GOPATH/src/github.com/user/hello</b>
	</pre>

	<p>
	Next, create a file named <code>hello.go</code> inside that directory,
	containing the following Go code.
	</p>

	<pre>
	package main

	import "fmt"

	func main() {
	fmt.Printf("Hello, world.\n")
	}
	</pre>

	<p>
	Now you can build and install that program with the <code>go</code> tool:
	</p>

	<pre>
	$ <b>go install github.com/user/hello</b>
	</pre>

	<p>
	Note that you can run this command from anywhere on your system. The
	<code>go</code> tool finds the source code by looking for the
	<code>github.com/user/hello</code> package inside the workspace specified by
	<code>GOPATH</code>.
	</p>

	<p>
	You can also omit the package path if you run <code>go install</code> from the
	package directory:
	</p>

	<pre>
	$ <b>cd $GOPATH/src/github.com/user/hello</b>
	$ <b>go install</b>
	</pre>

	<p>
	This command builds the <code>hello</code> command, producing an executable
	binary. It then installs that binary to the workspace's <code>bin</code>
	directory as <code>hello</code> (or, under Windows, <code>hello.exe</code>).
	In our example, that will be <code>$GOPATH/bin/hello</code>, which is
	<code>$HOME/work/bin/hello</code>.
	</p>

	<p>
	The <code>go</code> tool will only print output when an error occurs, so if
	these commands produce no output they have executed successfully.
	</p>

	<p>
	You can now run the program by typing its full path at the command line:
	</p>

	<pre>
	$ <b>$GOPATH/bin/hello</b>
	Hello, world.
	</pre>

	<p>
	Or, as you have added <code>$GOPATH/bin</code> to your <code>PATH</code>,
	just type the binary name:
	</p>

	<pre>
	$ <b>hello</b>
	Hello, world.
	</pre>

	<p>
	If you're using a source control system, now would be a good time to initialize
	a repository, add the files, and commit your first change. Again, this step is
	optional: you do not need to use source control to write Go code.
	</p>

	<pre>
	$ <b>cd $GOPATH/src/github.com/user/hello</b>
	$ <b>git init</b>
	Initialized empty Git repository in /home/user/work/src/github.com/user/hello/.git/
	$ <b>git add hello.go</b>
	$ <b>git commit -m "initial commit"</b>
	[master (root-commit) 0b4507d] initial commit
	1 file changed, 1 insertion(+)
	create mode 100644 hello.go
	</pre>

	<p>
	Pushing the code to a remote repository is left as an exercise for the reader.
	</p>


	<h3 id="Library">Your first library</h3>

	<p>
	Let's write a library and use it from the <code>hello</code> program.
	</p>

	<p>
	Again, the first step is to choose a package path (we'll use
	<code>github.com/user/stringutil</code>) and create the package directory:
	</p>

	<pre>
	$ <b>mkdir $GOPATH/src/github.com/user/stringutil</b>
	</pre>

	<p>
	Next, create a file named <code>reverse.go</code> in that directory with the
	following contents.
	</p>

	<pre>
	// Package stringutil contains utility functions for working with strings.
	package stringutil

	// Reverse returns its argument string reversed rune-wise left to right.
	func Reverse(s string) string {
	r := []rune(s)
	for i, j := 0, len(r)-1; i < len(r)/2; i, j = i+1, j-1 {
	r[i], r[j] = r[j], r[i]
	}
	return string(r)
	}
	</pre>

	<p>
	Now, test that the package compiles with <code>go build</code>:
	</p>

	<pre>
	$ <b>go build github.com/user/stringutil</b>
	</pre>

	<p>
	Or, if you are working in the package's source directory, just:
	</p>

	<pre>
	$ <b>go build</b>
	</pre>

	<p>
	This won't produce an output file. To do that, you must use <code>go
	install</code>, which places the package object inside the <code>pkg</code>
	directory of the workspace.
	</p>

	<p>
	After confirming that the <code>stringutil</code> package builds,
	modify your original <code>hello.go</code> (which is in
	<code>$GOPATH/src/github.com/user/hello</code>) to use it:
	</p>

	<pre>
	package main

	import (
	"fmt"

	<b>"github.com/user/stringutil"</b>
	)

	func main() {
	fmt.Printf(stringutil.Reverse("!oG ,olleH"))
	}
	</pre>

	<p>
	Whenever the <code>go</code> tool installs a package or binary, it also
	installs whatever dependencies it has.
	So when you install the <code>hello</code> program
	</p>

	<pre>
	$ <b>go install github.com/user/hello</b>
	</pre>

	<p>
	the <code>stringutil</code> package will be installed as well, automatically.
	</p>

	<p>
	Running the new version of the program, you should see a new, reversed message:
	</p>

	<pre>
	$ <b>hello</b>
	Hello, Go!
	</pre>

	<p>
	After the steps above, your workspace should look like this:
	</p>

	<pre>
	bin/
	hello # command executable
	pkg/
	linux_amd64/ # this will reflect your OS and architecture
	github.com/user/
	stringutil.a # package object
	src/
	github.com/user/
	hello/
	hello.go # command source
	stringutil/
	reverse.go # package source
	</pre>

	<p>
	Note that <code>go install</code> placed the <code>stringutil.a</code> object
	in a directory inside <code>pkg/linux_amd64</code> that mirrors its source
	directory.
	This is so that future invocations of the <code>go</code> tool can find the
	package object and avoid recompiling the package unnecessarily.
	The <code>linux_amd64</code> part is there to aid in cross-compilation,
	and will reflect the operating system and architecture of your system.
	</p>

	<p>
	Go command executables are statically linked; the package objects need not
	be present to run Go programs.
	</p>


	<h3 id="PackageNames">Package names</h3>

	<p>
	The first statement in a Go source file must 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>.
	</p>

	<p>
	Executable commands must always use <code>package main</code>.
	</p>

	<p>
	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>
	See <a href="/doc/effective_go.html#names">Effective Go</a> to learn more about
	Go's naming conventions.
	</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>stringutil</code> package by creating the file
	<code>$GOPATH/src/github.com/user/stringutil/reverse_test.go</code> containing
	the following Go code.
	</p>

	<pre>
	package stringutil

	import "testing"

	func TestReverse(t *testing.T) {
	cases := []struct {
	in, want string
	}{
	{"Hello, world", "dlrow ,olleH"},
	{"Hello, 世界", "界世 ,olleH"},
	{"", ""},
	}
	for _, c := range cases {
	got := Reverse(c.in)
	if got != c.want {
	t.Errorf("Reverse(%q) == %q, want %q", c.in, got, c.want)
	}
	}
	}
	</pre>

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

	<pre>
	$ <b>go test github.com/user/stringutil</b>
	ok github.com/user/stringutil 0.165s
	</pre>

	<p>
	As always, if you are running the <code>go</code> tool from the package
	directory, you can omit the package path:
	</p>

	<pre>
	$ <b>go test</b>
	ok github.com/user/stringutil 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> tool uses
	this property to automatically fetch packages from remote repositories.
	For instance, the examples described in this document are also kept in a
	Git repository hosted at GitHub
	<code><a href="https://github.com/golang/example">github.com/golang/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>
	$ <b>go get github.com/golang/example/hello</b>
	$ <b>$GOPATH/bin/hello</b>
	Hello, Go examples!
	</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 look like this:
	</p>

	<pre>
	bin/
	hello # command executable
	pkg/
	linux_amd64/
	github.com/golang/example/
	stringutil.a # package object
	github.com/user/
	stringutil.a # package object
	src/
	github.com/golang/example/
	.git/ # Git repository metadata
	hello/
	hello.go # command source
	stringutil/
	reverse.go # package source
	reverse_test.go # test source
	github.com/user/
	hello/
	hello.go # command source
	stringutil/
	reverse.go # package source
	reverse_test.go # test source
	</pre>

	<p>
	The <code>hello</code> command hosted at GitHub depends on the
	<code>stringutil</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 "github.com/golang/example/stringutil"
	</pre>

	<p>
	This convention is the easiest way to make your Go packages available for
	others to use.
	The <a href="//golang.org/wiki/Projects">Go Wiki</a>
	and <a href="//godoc.org/">godoc.org</a>
	provide lists of external Go projects.
	</p>

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


	<h2 id="next">What's next</h2>

	<p>
	Subscribe to the
	<a href="//groups.google.com/group/golang-announce">golang-announce</a>
	mailing list to be notified when a new stable version of Go is released.
	</p>

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

	<p>
	Take <a href="//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>


	<h2 id="help">Getting help</h2>

	<p>
	For real-time help, ask the helpful gophers in <code>#go-nuts</code> on the
	<a href="http://freenode.net/">Freenode</a> IRC server.
	</p>

	<p>
	The official mailing list for discussion of the Go language is
	<a href="//groups.google.com/group/golang-nuts">Go Nuts</a>.
	</p>

	<p>
	Report bugs using the
	<a href="//golang.org/issue">Go issue tracker</a>.
	</p>