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doc: expand code.html to discuss the go tool in more depth 

R=golang-dev, balasanjay, r
CC=golang-dev
https://golang.org/cl/5640045
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<h2 id="Introduction">Introduction</h2> <h2 id="Introduction">Introduction</h2>
<p> <p>
This document explains how to write a new package This document demonstrates the development of a simple Go package and
and how to test code. introduces the <a href="/cmd/go/">go command</a>, the standard way to fetch,
It assumes you have installed Go using the build, and install Go packages and commands.
<a href="install.html">installation instructions</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>
<p> <p>
Before embarking on a change to an existing The one thing you must do is set a <code>GOPATH</code> environment variable.
package or the creation of a new package, <code>GOPATH</code> tells the <code>go</code> command (and other related tools)
be sure to send mail to the where to find and install the Go packages on your system.
<a href="http://groups.google.com/group/golang-nuts">mailing list</a>
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.
</p> </p>
<h2 id="Community_resources">Community resources</h2> <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> <p>
For real-time help, there may be users or developers on (On a Windows system use semicolons as the path separator instead of colons.)
<code>#go-nuts</code> on the <a href="http://freenode.net/">Freenode</a> IRC server.
</p> </p>
<p> <p>
The official mailing list for discussion of the Go language is Each path in the list (in this case <code>/home/user/ext</code> or
<a href="http://groups.google.com/group/golang-nuts">Go Nuts</a>. <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> </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.</code>
</ul>
<p> <p>
Bugs can be reported using the <a href="http://code.google.com/p/go/issues/list">Go issue tracker</a>. 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>
<p> <p>
For those who wish to keep up with development, When building a program that imports the package "<code>widget</code>" the
there is another mailing list, <a href="http://groups.google.com/group/golang-checkins">golang-checkins</a>, <code>go</code> command looks for <code>src/pkg/widget</code> inside the Go root,
that receives a message summarizing each checkin to the Go repository. 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>
<p>
Multiple workspaces can offer some flexibility and convenience, but for now
we'll concern ourselves with only a single workspace.
</p>
<h2 id="New_package">Creating a new package</h2> <p>
Let's work through a simple example. First, create a <code>$HOME/mygo</code>
directory and its <code>src</code> subdirectory:
</p>
<h3>Choosing an import path</h3> <pre>
$ mkdir -p $HOME/mygo/src # create a place to put source code
</pre>
<p> <p>
The standard packages are given short names like <code>fmt</code> and Next, set it as the <code>GOPATH</code>. You should also add the
<code>net/http</code> for convenience. <code>bin</code> subdirectory to your <code>PATH</code> environment variable so
For your own projects, choose a name space that is unlikely that you can run the commands therein without specifying their full path.
to collide with future additions to the standard library or other To do this, add the following lines to <code>$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. external libraries.
</p> </p>
<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> 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 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 paths with that URL, as in "<code>example.com/foo/bar</code>" or
"<code>code.google.com/p/example/foo/bar</code>". "<code>code.google.com/p/example/foo/bar</code>".
This way the <a href="/cmd/go/"><code>go</code> tool</a> can automatically Using this convention, the <code>go</code> command can automatically check out and
check out and build the source code from its import path. build the source code by its import path alone.
</p> </p>
<p> <p>
If you don't intend your code to be installed in this way, you should at 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 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 "<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. company or project name, since it is unlikely to be used by another group.
</p> </p>
<p>
We'll use <code>example/</code> as our base import path:
</p>
<h3>The <code>go</code> tool and <code>GOPATH</code></h3> <pre>
$ mkdir -p $GOPATH/src/example
</pre>
<h3>Package names</h3>
<p> <p>
The <a href="/cmd/go/"><code>go</code> tool</a> is the standard means of The first statement in a Go source file should be
building and installing Go libraries and programs. It is a "zero configuration"
tool; it determines how to build Go packages from their source code alone.
</p> </p>
<pre>
package <i>name</i>
</pre>
<p> <p>
To use the <code>go</code> tool effectively you must set the where <code><i>name</i></code> is the package's default name for imports.
<code>GOPATH</code> variable. (All files in a package must use the same <code><i>name</i></code>.)
<code>GOPATH</code> specifies a list of paths that contain Go source code
and package binaries. Source code, package objects, and command binaries are
located inside the <code>GOPATH</code>s' <code>src</code>, <code>pkg</code>,
and <code>bin</code> subdirectories respectively.
</p> </p>
<p> <p>
You should set <code>GOPATH</code> in your shell profile Go's convention is that the package name is the last element of the
(<code>$HOME/.bashrc</code>, <code>$HOME/.profile</code>, or equivalent). 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>
<p> <p>
This shell session demonstrates setting <code>GOPATH</code>, creating a trivial Create a new package under <code>example</code> called <code>newmath</code>:
<code>widgets/foo</code> package, and building and installing the package.
</p> </p>
<pre> <pre>
$ export GOPATH=$HOME/gocode $ cd $GOPATH/src/example
$ mkdir -p $GOPATH/src/widgets/foo $ mkdir newmath
$ cat &gt; $GOPATH/src/widgets/foo/foo.go
package foo
const String = "Go rules!"
^D
$ go install widgets/foo
$ ls $GOPATH/pkg/*/widgets
foo.a
</pre> </pre>
<p>(<code>^D</code> means to type Control-D.)</p> <p>
Then create a file named <code>$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 < 1000; i++ {
z -= (z*z - x) / (2 * x)
}
return z
}
</pre>
<p> <p>
Type <code>go help gopath</code> on the command line for more information This package is imported by the path name of the directory it's in, starting
about <code>GOPATH</code>. 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> </p>
<h3>Go source files</h3> <h2>Building and installing</h3>
<p> <p>
The first statement in a Go source file should be <code>package The <code>go</code> command comprises several subcommands, the most central being
<i>name</i></code>, where <code><i>name</i></code> is the package's default <code>install</code>. Running <code>go install <i>importpath</i></code> builds
name for imports. and installs a package and its dependencies.
(All files in a package must use the same <code><i>name</i></code>.) </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>" <p>
should be named <code>rot13</code>. To "install a package" means to write the package object or executable command
There is no requirement that package names be unique to the <code>pkg</code> or <code>bin</code> subdirectory of the workspace in
across all packages linked into a single binary, which the source resides.
only that the import paths (their full file names) be unique.
</p> </p>
<h3>Building a package</h3>
<p> <p>
Go compiles all the source files in a package at once, so one file To build and install the <code>newmath</code> package, type
can refer to constants, variables, types, and functions in another
file without special arrangement or declarations.
</p> </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 affect as the one above:
</p>
<pre>
$ cd $GOPATH/src/example/newmath
$ go install
</pre>
<p> <p>
Writing clean, idiomatic Go code is beyond the scope of this document. The resulting workspace directory tree (assuimg we're running Linux on a 64-bit
<a href="effective_go.html">Effective Go</a> is an introduction to system) looks like this:
that topic.
</p> </p>
<h2 id="Building_programs">Building programs</h2> <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> <p>
The <a href="/cmd/go/"><code>go</code> tool</a> treats code belonging to Add a command named <code>hello</code> to the source tree.
<code>package main</code> as an executable command, and installs the package First create the <code>example/hello</code> directory:
binary to the <code>GOPATH</code>'s <code>bin</code> subdirectory.
</p> </p>
<pre>
$ cd $GOPATH/src/example
$ mkdir hello
</pre>
<p> <p>
Building executable commands is the same as building packages. Then create the file <code>$GOPATH/src/example/hello/hello.go</code>
Use "<code>go install</code>": containing the following Go code.
</p> </p>
<pre> <pre>
$ mkdir -p $GOPATH/src/widgets/bar // Hello is a trivial example of a main package.
$ cat &gt; $GOPATH/src/widgets/bar/bar.go
package main package main
import ( import (
"fmt" "example/newmath"
"widgets/foo" "fmt"
) )
func main() { func main() {
fmt.Println(foo.String) fmt.Printf("Hello, world. Sqrt(2) = %v\n", newmath.Sqrt(2))
} }
^D
$ go install widgets/bar
$ $GOPATH/bin/bar
Go rules!
</pre> </pre>
<p> <p>
Run <code>go help build</code> and <code>go help install</code> for more Next, run <code>go install</code>, which builds and installs the binary to
about building and installing Go binaries. <code>$GOPATH/bin</code>:
</p> </p>
<h2 id="Testing">Testing</h2> <pre>
$ go install example/hello
</pre>
<p> <p>
Go has a lightweight test framework composed of the <code>go</code> tool and To run the program, invoke it by name as you would any other command:
the <code>testing</code> package.
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.
Run <code>go help test</code> and see the
<a href="/pkg/testing/">testing package documentation</a> for more detail.
</p> </p>
<pre>
$ $GOPATH/bin/hello
Hello, world. Sqrt(2) = 1.414213562373095
</pre>
<p> <p>
To run the test, run "<code>go test</code>": If you added <code>$HOME/mygo/bin</code> to your <code>PATH</code>, you may omit
the path to the executable:
</p> </p>
<pre> <pre>
$ cat &gt; $GOPATH/src/widgets/foo/foo_test.go $ hello
package foo Hello, world. Sqrt(2) = 1.414213562373095
</pre>
import "testing" <p>
The workspace directory tree now looks like this:
</p>
func TestString(t *testing.T) { <pre>
const expect = "Go rules!" bin/
if String != expect { hello # command executable
t.Errorf("String == %q, want %q", String, expect) pkg/
} linux_amd64/
} example/
^D newmath.a # package object
$ go test widgets/foo src/
ok widgets/foo 0.018s example/
hello/
hello.go # command source
newmath/
sqrt.go # package source
</pre> </pre>
<p> <p>
If your change affects performance, add a <code>Benchmark</code> function The <code>go</code> command also provides a <code>build</code> command, which is
(run <code>go help testfunc</code>) and run it using <code>go test like <code>install</code> except it builds all objects in a temporary directory
-test.bench=.*</code>. 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> </p>
<h2 id="pkg_example">An example package with tests</h2>
<h2 id="Testing">Testing</h2>
<p> <p>
This example package, <code>numbers</code>, consists of the function Go has a lightweight test framework composed of the <code>go test</code>
<code>Double</code>, which takes an <code>int</code> and returns that value command and the <code>testing</code> package.
multiplied by 2. It consists of two files.
</p> </p>
<p> <p>
First, the package implementation, <code>numbers.go</code>: 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> </p>
<pre> <pre>
package numbers package newmath
import "testing"
func Double(i int) int { func TestSqrt(t *testing.T) {
return i * 2 const in, out = 9, 3
if x := Sqrt(in); x != out {
t.Errorf("Sqrt(%v) = %v, want %v", in, x, out)
}
} }
</pre> </pre>
<p> <p>
Next, the tests, <code>numbers_test.go</code>: Now run the test with <code>go test</code>:
</p> </p>
<pre> <pre>
package numbers $ go test example/newmath
ok example/newmath
import ( </pre>
"testing"
)
type doubleTest struct { <p>
in, out int Run <code><a href="/cmd/go/#Test_packages">go help test</a></code> and see the
} <a href="/pkg/testing/">testing package documentation</a> for more detail.
</p>
var doubleTests = []doubleTest{
doubleTest{1, 2},
doubleTest{2, 4},
doubleTest{-5, -10},
}
func TestDouble(t *testing.T) { <h2 id="remote">Remote packages</h3>
for _, dt := range doubleTests {
v := Double(dt.in)
if v != dt.out {
t.Errorf("Double(%d) = %d, want %d.", dt.in, v, dt.out)
}
}
}
</pre>
<p> <p>
Running <code>go install</code> will build and install the package to An import path can describe how to obtain the package source code using a
the <code>GOPATH</code>'s <code>pkg</code> directory revision control system such as Git or Mercurial. The <code>go</code> command uses
(it can then be imported by any other Go program). 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> </p>
<pre>
$ go get code.google.com/p/go.example/hello
$ $GOPATH/bin/hello
Hello, world. Sqrt(2) = 1.414213562373095
</pre>
<p> <p>
Running <code>go test</code> will rebuild the package, including the If the specified package is not present in a workspace, <code>go get</code>
<code>numbers_test.go</code> file, and then run the <code>TestDouble</code> will place it inside the first workspace specified by <code>GOPATH</code>.
function. The output "<code>ok</code>" indicates that all tests passed (If the package does already exist, <code>go get</code> skips the remote
successfully. Breaking the implementation by changing the multiplier from fetch and behaves the same as <code>go install</code>.)
<code>2</code> to <code>3</code> will allow you to see how failing tests are
reported.
</p> </p>
<p> <p>
Run <code>go help test</code>, <code>go help testfunc</code>, After issuing the above <code>go get</code> command, the workspace directory
and <code>go help testflag</code> and see the tree should now now look like this:
<a href="/pkg/testing/">testing package documentation</a> for more detail.
</p> </p>
<h2 id="arch_os_specific">Architecture- and operating system-specific code</h2> <pre>
bin/
<p>First, a disclaimer: very few Go packages should need to know about the hello # command executable
hardware and operating system they run on. In the vast majority of cases the pkg/
language and standard library handle most portability issues. This section is linux_amd64/
a guide for experienced systems programmers who have a good reason to write code.google.com/p/go.example/
platform-specific code, such as assembly-language support for fast newmath.a # package object
trigonometric functions or code that implements a common interface above example/
different operating systems.</p> newmath.a # package object
src/
<p>To compile such code, use the <code>$GOOS</code> and <code>$GOARCH</code> code.google.com/p/go.example/
<a href="/doc/install.html#environment">environment variables</a> in your hello/
source file names.</p> hello.go # command source
newmath/
sqrt.go # package source
sqrt_test.go # test source
example/
hello/
hello.go # command source
newmath/
sqrt.go # package source
sqrt_test.go # test source
</pre>
<p>For example, consider the package <code>foo</code> that consists of four <p>
files:</p> The <code>hello</code> command hosted at Google Code depends on the
<code>newmath</code> package within the same repository. The imports in
<code>hello.go</code> file use the same import path convention, so the <code>go
get</code> command is able to locate and install the dependent package, too.
</p>
<pre> <pre>
foo.go import "code.google.com/p/go.example/newmath"
foo_386.go
foo_amd64.go
foo_arm.go
</pre> </pre>
<p>describes a package that builds on <p>
different architectures by parameterizing the file name with This convention is the easiest way to make your Go packages available for
<code>$GOARCH</code>.</p> others to use.
The <a href="http://godashboard.appspot.com/package">Go Package Dashboard</a>
displays a list of packages recently installed with the <code>go</code> command.
</p>
<p>The general code goes in <code>foo.go</code>, while architecture-specific <p>
code goes in <code>foo_386.go</code>, <code>foo_amd64.go</code>, and For more information on using remote repositories with the <code>go</code> command, see
<code>foo_arm.go</code>.</p> <code><a href="/cmd/go/#Remote_import_path_syntax">go help remote</a></code>.
</p>
<p>If you follow these conventional parameterizations, tools such as the <a
href="/cmd/go/"><code>go</code> tool</a> will work seamlessly with your
package:</p>
<pre> <h2 id="more">Further reading</h2>
foo_$GOOS.go
foo_$GOARCH.go
foo_$GOOS_$GOARCH.go
</pre>
<p>The same holds for <code>.s</code> (assembly) and <code>.c</code> files.</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="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>
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