go/doc/effective_go.html

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

<p>
Go is a new language.  Although it's in the C family
it has some unusual properties that make effective Go programs
different in character from programs in existing languages.
A straightforward translation of a C++ or Java program into Go
is unlikely to produce a satisfactory result—Java programs
are written in Java, not Go.
On the other hand, thinking about the problem from a Go
perspective could produce a successful but quite different
program.
In other words,
to write Go well, it's important to understand its properties
and idioms.
It's also important to know the established conventions for
programming in Go, such as naming, formatting, program
construction, and so on, so that programs you write
will be easy for other Go programmers to understand.
</p>

<p>
This document gives tips for writing clear, idiomatic Go code.
It augments the <a href="go_spec.html">language specification</a>
and the <a href="go_tutorial.html">tutorial</a>, both of which you
should read first.
</p>

<h3 id="read">Examples</h3>

<p>
The <a href="/src/pkg/">Go package sources</a>
are intended to serve not
only as the core library but also as examples of how to
use the language.
If you have a question about how to approach a problem or how something
might be implemented they can provide answers, ideas and
background.
</p>


<h2 id="formatting">Formatting</h2>

<p>
Formatting issues are the most contentious
but the least consequential.
People can adapt to different formatting styles
but it's better if they don't have to, and
less time is devoted to the topic
if everyone adheres to the same style.
The problem is how to approach this Utopia without a long
prescriptive style guide.
</p>

<p>
With Go we take an unusual
approach and let the machine
take care of most formatting issues.
A program, <code>gofmt</code>, reads a Go program
and emits the source in a standard style of indentation
and vertical alignment, retaining and if necessary
reformatting comments.
If you want to know how to handle some new layout
situation, run <code>gofmt</code>; if the answer doesn't
seem right, fix the program (or file a bug), don't work around it.
</p>

<p>
As an example, there's no need to spend time lining up
the comments on the fields of a structure.
<code>Gofmt</code> will do that for you.  Given the
declaration
</p>

<pre>
type T struct {
    name string; // name of the object
    value int; // its value
}
</pre>

<p>
<code>gofmt</code> will make the columns line up:
</p>

<pre>
type T struct {
    name    string; // name of the object
    value   int;    // its value
}
</pre>

<p>
All code in the libraries has been formatted with <code>gofmt</code>.
<font color=red>TODO</font>
</p>


<p>
Some formatting details remain.  Very briefly:
</p>

<dl>
	<dt>Indentation</dt>
	<dd>We use tabs for indentation and <code>gofmt</code> emits them by default.
	Use spaces if you must.
	</dd>
	<dt>Line length</dt>
	<dd>
	Go has no line length limit.  Don't worry about overflowing a punched card.
	If a line feels too long, wrap it and indent with an extra tab.
	</dd>
	<dt>Parentheses</dt>
	<dd>
	Go needs fewer parentheses: control structures (<code>if</code>,
	<code>for</code>, <code>switch</code>) do not have parentheses in
	their syntax.
	Also, the operator precedence hierarchy is shorter and clearer, so
<pre>
x&lt;&lt;8 + y&lt;&lt;16
</pre>
	means what the spacing implies.
	</dd>
</dl>

<h2>Commentary</h2>

<p>
Go provides C-style <code>/* */</code> block comments 
and C++-style <code>//</code> line comments.
Line comments are the norm;
block comments appear mostly as package comments and
are also useful to disable large swaths of code.
</p>

<p>
The program—and web server—<code>godoc</code> processes
Go source files to extract documentation about the contents of the
package.
Comments that appear before top-level declarations, with no intervening newlines,
are extracted along with the declaration to serve as explanatory text for the item.
The nature and style of these comments determines the
quality of the documentation <code>godoc</code> produces.
</p>

<p>
Every package should have a <i>package comment</i>, a block
comment preceding the package clause.
For multi-file packages, the package comment only needs to be
present in one file, and any one will do.
The package comment should introduce the package and
provide information relevant to the package as a whole.
It will appear first on the <code>godoc</code> page and
should set up the detailed documentation that follows.
</p>

<pre>
/*
	The regexp package implements a simple library for
	regular expressions.

	The syntax of the regular expressions accepted is:

	regexp:
		concatenation { '|' concatenation }
	concatenation:
		{ closure }
	closure:
		term [ '*' | '+' | '?' ]
	term:
		'^'
		'$'
		'.'
		character
		'[' [ '^' ] character-ranges ']'
		'(' regexp ')'
*/
package regexp
</pre>

<p>
If the package is simple, the package comment can be brief.
</p>

<pre>
// The path package implements utility routines for
// manipulating slash-separated filename paths.
</pre>

<p>
Comments do not need extra formatting such as banners of stars.
The generated output may not even be presented in a fixed-width font, so don't depend
on spacing for alignment—<code>godoc</code>, like <code>gofmt</code>,
takes care of that.
Finally, the comments are uninterpreted plain text, so HTML and other
annotations such as <code>_this_</code> will reproduce <i>verbatim</i> and should
not be used.
</p>

<p>
Inside a package, any comment immediately preceding a top-level declaration
serves as a <i>doc comment</i> for that declaration.
Every exported (capitalized) name in a program should
have a doc comment.
</p>

<p>
Doc comments work best as complete English sentences, which allow
a wide variety of automated presentations.
The first sentence should be a one-sentence summary that
starts with the name being declared:
</p>

<pre>
// Compile parses a regular expression and returns, if successful, a Regexp
// object that can be used to match against text.
func Compile(str string) (regexp *Regexp, error os.Error) {
</pre>

<p>
Go's declaration syntax allows grouping of declarations.
A single doc comment can introduce a group of related constants or variables.
Since the whole declaration is presented, such a comment can often be perfunctory.
</p>

<pre>
// Error codes returned by failures to parse an expression.
var (
	ErrInternal = os.NewError("internal error");
	ErrUnmatchedLpar = os.NewError("unmatched '('");
	ErrUnmatchedRpar = os.NewError("unmatched ')'");
	...
)
</pre>

<p>
Even for private names, grouping can also indicate relationships between items,
such as the fact that a set of variables is controlled by a mutex.
</p>

<pre>
var (
	countLock	sync.Mutex;
	inputCount	uint32;
	outputCount	uint32;
	errorCount	uint32;
)
</pre>

<h2 id="names">Names</h2>

<p>
Names are as important in Go as in any other language.
In some cases they even have semantic effect: for instance,
the visibility of a name outside a package is determined by whether its
first character is an upper case letter,
while methods are looked up by name alone (although the type must match too).
It's therefore worth spending a little time talking about naming conventions
in Go programs.
</p>


<h3 id="package-names">Package names</h3>

<p>
When a package is imported, the package name becomes an accessor for the
contents.  After
</p>

<pre>
import "bytes"
</pre>

<p>
the importing package can talk about <code>bytes.Buffer</code>.  It's
helpful if everyone using the package can use the same name to refer to
its contents, which implies that the package name should be good:
short, concise, evocative.  By convention, packages are given
lower case, single-word names; there should be no need for underscores
or mixedCaps.
Err on the side of brevity, since everyone using your
package will be typing that name.
And don't worry about collisions <i>a priori</i>.
The package name is only the default name for imports; it need not be unique
across all source code, and in the rare case of a collision the
importing package can choose a different name to use locally.
</p>

<p>
Another convention is that the package name is the base name of
its source directory;
the package in <code>src/pkg/container/vector</code>
is installed as <code>"container/vector"</code> but has name <code>vector</code>,
not <code>container_vector</code> and not <code>containerVector</code>.
</p>

<p>
The importer of a package will use the name to refer to its contents
(the <code>import .</code> notation is intended mostly for tests and other
unusual situations), and exported names in the package can use that fact
to avoid stutter.
For instance, the buffered reader type in the <code>bufio</code> package is called <code>Reader</code>,
not <code>BufReader</code>, because users see it as <code>bufio.Reader</code>,
which is a clear, concise name.
Moreover,
because imported entities are always addressed with their package name, <code>bufio.Reader</code>
does not conflict with <code>io.Reader</code>.
Use the package structure to help you choose good names.
</p>

<p>
Another short example is <code>once.Do</code>;
<code>once.Do(setup)</code> reads well and would not be improved by
writing <code>once.DoOrWaitUntilDone(setup)</code>.
Long names don't automatically make things more readable.
If the name represents something intricate or subtle, it's usually better
to write a helpful doc comment than to attempt to put all the information
into the name.
</p>

<h3 id="interface-names">Interface names</h3>

<p>
By convention, one-method interfaces are named by
the method name plus the -er suffix: <code>Reader</code>,
<code>Writer</code>, <code>Formatter</code> etc.
</p>

<p>
There are a number of such names and it's productive to honor them and the function
names they capture.
<code>Read</code>, <code>Write</code>, <code>Close</code>, <code>Flush</code>,
<code>String</code> and so on have
canonical signatures and meanings.  To avoid confusion,
don't give your method one of those names unless it
has the same signature and meaning.
Conversely, if your type implements a method with the
same meaning as a method on a well-known type,
give it the same name and signature;
call your string-converter method <code>String</code> not <code>ToString</code>.
</p>

<h3 id="mixed-caps">MixedCaps</h3>

<p>
Finally, the convention in Go is to used <code>MixedCaps</code>
or <code>mixedCaps</code> rather than underscores to write
multiword names.
</p>

<h2 id="semicolons">Semicolons</h2>

<p>
Go needs fewer semicolons between statements than do other C variants.
Semicolons are never required at the top level.
Also they are separators, not terminators, so they
can be left off the last element of a statement or declaration list,
a convenience
for one-line <code>funcs</code> and the like:
</p>

<pre>
func CopyInBackground(src, dst chan Item) {
    go func() { for { dst &lt;- &lt;-src } }()
}
</pre>

<p>
In fact, semicolons can omitted at the end of any "StatementList" in the
grammar, which includes things like cases in <code>switch</code>
statements:
</p>

<pre>
switch {
case a &lt; b:
    return -1
case a == b:
    return 0
case a &gt; b:
    return 1
}

</pre>

<p>
The grammar admits an empty statement after any statement list, which
means a terminal semicolon is always OK.  As a result,
it's fine to put semicolons everywhere you'd put them in a
C program—they would be fine after those return statements,
for instance—but they can often be omitted.
By convention, they're always left off top-level declarations (for
instance, they don't appear after the closing brace of <code>struct</code>
declarations, or of <code>funcs</code> for that matter)
and often left off one-liners.  But within functions, place them
as you see fit.
</p>

<h2 id="control-structures">Control structures</h2>

<p>
The control structures of Go are related to those of C but different
in important ways.
There is no <code>do</code> or <code>while</code> loop, only a
slightly generalized
<code>for</code>;
<code>switch</code> is more flexible;
<code>if</code> and <code>switch</code> accept an optional
initialization statement like that of <code>for</code>;
and there are new control structures including a type switch and a
multiway communications multiplexer, <code>select</code>.
The syntax is also slightly different: parentheses are not part of the syntax
and the bodies must always be brace-delimited.
</p>

<h3 id="if">If</h3>

<p>
In Go a simple <code>if</code> looks like this:
</p>
<pre>
if x > 0 {
    return y
}
</pre>

<p>
Mandatory braces encourage writing simple <code>if</code> statements
on multiple lines.  It's good style to do so anyway,
especially when the body contains a control statement such as a
<code>return</code> or <code>break</code>.
</p>

<p>
Since <code>if</code> and <code>switch</code> accept an initialization
statement, it's common to see one used to set up a local variable:
</p>

<pre>
if err := file.Chmod(0664); err != nil {
    log.Stderr(err)
}
</pre>

<p id="else">
In the Go libraries, you'll find that
when an <code>if</code> statement doesn't flow into the next statement—that is,
the body ends in <code>break</code>, <code>continue</code>,
<code>goto</code>, or <code>return</code>—the unnecessary
<code>else</code> is omitted.
</p>

<pre>
f, err := os.Open(name, os.O_RDONLY, 0);
if err != nil {
    return err;
}
codeUsing(f);
</pre>

<p>
This is a example of a common situation where code must analyze a
sequence of error possibilities.  The code reads well if the
successful flow of control runs down the page, eliminating error cases
as they arise.  Since error cases tend to end in <code>return</code>
statements, the resulting code needs no <code>else</code> statements:
</p>

<pre>
f, err := os.Open(name, os.O_RDONLY, 0);
if err != nil {
    return err;
}
d, err := f.Stat();
if err != nil {
    return err;
}
codeUsing(f, d);
</pre>


<h3 id="switch">Switch</h3>

<p>
Go's <code>switch</code> is more general than C's.
The expressions need not be constants or even integers,
the cases are evaluated top to bottom until a match is found,
and if the <code>switch</code> has no expression it switches on
<code>true</code>.
It's therefore possible—and idiomatic—to write an
<code>if</code>-<code>else</code>-<code>if</code>-<code>else</code>
chain as a <code>switch</code>:
</p>

<pre>
func unhex(c byte) byte {
    switch {
    case '0' &lt;= c &amp;&amp; c &lt;= '9':
        return c - '0'
    case 'a' &lt;= c &amp;&amp; c &lt;= 'f':
        return c - 'a' + 10
    case 'A' &lt;= c &amp;&amp; c &lt;= 'F':
        return c - 'A' + 10
    }
    return 0
}
</pre>

<p>
There is no automatic fall through, but cases can be presented
in comma-separated lists:
<pre>
func shouldEscape(c byte) bool {
    switch c {
    case ' ', '?', '&amp;', '=', '#', '+', '%':
        return true
    }
    return false
}
</pre>

<p>
Here's a comparison routine for byte arrays that uses two
<code>switch</code> statements:
<pre>
// Compare returns an integer comparing the two byte arrays
// lexicographically.
// The result will be 0 if a == b, -1 if a &lt; b, and +1 if a &gt; b
func Compare(a, b []byte) int {
    for i := 0; i &lt; len(a) &amp;&amp; i &lt; len(b); i++ {
        switch {
        case a[i] &gt; b[i]:
            return 1
        case a[i] &lt; b[i]:
            return -1
        }
    }
    switch {
    case len(a) &lt; len(b):
        return -1
    case len(a) &gt; len(b):
        return 1
    }
    return 0
}
</pre>

<h2 id="functions">Functions</h2>

<h3 id="omit-wrappers">Omit needless wrappers</h3>

<p>
Functions are great for factoring out common code, but
if a function is only called once,
ask whether it is necessary,
especially if it is just a short wrapper around another function.
This style is rampant in C++ code: wrappers
call wrappers that call wrappers that call wrappers.
This style hinders people trying to understand the program,
not to mention computers trying to execute it.
</p>

<h3 id="multiple-returns">Return multiple values</h3>

<p>
If a function must return multiple values, it can
do so directly.
There is no need to pass a pointer to a return value.
</p>

<h2 id="idioms">Idioms</h2>

<h3 id="struct-allocation">Allocate using literals</h3>

<p>
A struct literal is an expression that creates a
new instance each time it is evaluated.  The address of such
an expression points to a fresh instance each time.
Use such expressions to avoid the repetition of filling
out a data structure.
</p>

<pre>
length := Point{x, y}.Abs();
</pre>

<pre>
// Prepare RPCMessage to send to server
rpc := &amp;RPCMessage {
	Version: 1,
	Header: &amp;RPCHeader {
		Id: nextId(),
		Signature: sign(body),
		Method: method,
	},
	Body: body,
};
</pre>

<h3 id="buffer-slice">Use parallel assignment to slice a buffer</h3>

<pre>
header, body, checksum := buf[0:20], buf[20:n-4], buf[n-4:n];
</pre>

<h2 id="errors">Errors</h2>

<h3 id="error-returns">Return <code>os.Error</code>, not <code>bool</code></h3>

<p>
Especially in libraries, functions tend to have multiple error modes.
Instead of returning a boolean to signal success,
return an <code>os.Error</code> that describes the failure.
Even if there is only one failure mode now, 
there may be more later.
</p>

<h3 id="error-context">Return structured errors</h3>

Implementations of <code>os.Error</code> should
describe the error and provide context.
For example, <code>os.Open</code> returns an <code>os.PathError</code>:

<a href="/src/pkg/os/file.go">/src/pkg/os/file.go</a>:
<pre>
// PathError records an error and the operation and
// file path that caused it.
type PathError struct {
	Op string;
	Path string;
	Error Error;
}

func (e *PathError) String() string {
	return e.Op + &quot; &quot; + e.Path + &quot;: &quot; + e.Error.String();
}
</pre>

<p>
<code>PathError</code>'s <code>String</code> formats
the error nicely, including the operation and file name
tha failed; just printing the error generates a
message, such as
</p>
<pre>
open /etc/passwx: no such file or directory
</pre>
<p>
that is useful even if printed far from the call that
triggered it.
</p>

<p>
Callers that care about the precise error details can
use a type switch or a type guard to look for specific
errors and extract details.  For <code>PathErrors</code>
this might include examining the internal <code>Error</code>
to see if it is <code>os.EPERM</code> or <code>os.ENOENT</code>,
for instance.
</p>

<h2 id="types">Programmer-defined types</h2>

<h3 id="constructors">Use <code>NewTypeName</code> for constructors</h3>

<p>
The constructor for the type <code>pkg.MyType</code> should
be named <code>pkg.NewMyType</code> and should return <code>*pkg.MyType</code>.
The implementation of <code>NewTypeName</code> often uses the
<a href="#struct-allocation">struct allocation idiom</a>.
</p>

<a href="xxx">go/src/pkg/os/file.go</a>:
<pre>
func NewFile(fd int, name string) *File {
	if file &lt; 0 {
		return nil
	}
	return &amp;File{fd, name, nil, 0}
}
</pre>

<p>Packages that export only a single type can
shorten <code>NewTypeName</code> to <code>New</code>;
the vector constructor is
<code>vector.New</code>, not <code>vector.NewVector</code>.
</p>

<p>
A type that is intended to be allocated
as part of a larger struct may have an <code>Init</code> method
that must be called explicitly.
Conventionally, the <code>Init</code> method returns
the object being initialized, to make the constructor trivial:
</p>

<a href="xxx">go/src/pkg/container/vector/vector.go</a>:
<pre>
func New(len int) *Vector {
	return new(Vector).Init(len)
}
</pre>

<h3 id="zero-value">Make the zero value meaningful</h3>

<p>
In Go, newly allocated memory and newly declared variables are zeroed.
If a type is intended to be allocated without using a constructor
(for example, as part of a larger struct or declared as a local variable),
define the meaning of the zero value and arrange for that meaning
to be useful.
</p>

<p>
For example, <code>sync.Mutex</code> does not
have an explicit constructor or <code>Init</code> method.
Instead, the zero value for a <code>sync.Mutex</code>
is defined to be an unlocked mutex.
</p>

<h2 id="interfaces">Interfaces</h2>

<h3 id="accept-interface-values">Accept interface values</h3>

buffered i/o takes a Reader, not an os.File.  XXX

<h3 id="return-interface-values">Return interface values</h3>

<p>
If a type exists only to implement an interface
and has no exported methods beyond that interface,
there is no need to publish the type itself.
Instead, write a constructor that returns an interface value.
</p>

<p>
For example, both <code>crc32.NewIEEE()</code> and <code>adler32.New()</code>
return type <code>hash.Hash32</code>.
Substituting the CRC-32 algorithm for Adler-32 in a Go program
requires only changing the constructor call:
the rest of the code is unaffected by the change of algorithm.
</p>

<h3 id="asdf">Use interface adapters to expand an implementation</h3>

XXX

<h3 id="fdsa">Use anonymous fields to incorporate an implementation</h3>

XXX

<h2>Data-Driven Programming</h2>

<p>
tables
</p>

<p>
XXX struct tags for marshalling.
template
eventually datafmt
</p>

<h2>Concurrency</h2>

<h3 id="share-memory">Share memory by communicating</h3>

<p>
Do not communicate by sharing memory;
instead, share memory by communicating.
</p>

<p>
XXX, more here.
</p>


<h2>Testing</h2>

<h3 id="no-abort">Run tests to completion</h3>

<p>
Tests should not stop early just because one case has misbehaved.
If at all possible, let tests continue, in order to characterize the
problem in more detail.
For example, it is more useful for a test to report that <code>isPrime</code>
gives the wrong answer for 4, 8, 16 and 32 than to report
that <code>isPrime</code> gives the wrong answer for 4 and therefore
no more tests were run.
XXX
test bottom up
test runs top to bottom
how to use gotest
XXX
</p>

<h3 id="good-errors">Print useful errors when tests fail</h3>

<p>
If a test fails, print a concise message explaining the context,
what happened, and what was expected.
Many testing environments encourage causing the
program to crash, but stack traces and core dumps
have low signal to noise ratios and require reconstructing
the situation from scratch.
The programmer who triggers the test failure may be someone
editing the code months later or even someone editing a different
package on which the code depends.
Time invested writing a good error message now pays off when
the test breaks later.
</p>

<h3 id="data-driven-tests">Use data-driven tests</h3>

<p>
Many tests reduce to running the same code multiple times,
with different input and expected output.
Instead of using cut and paste to write this code,
create a table of test cases and write a single test that
iterates over the table.
Once the table is written, you might find that it
serves well as input to multiple tests.  For example,
a single table of encoded/decoded pairs can be
used by both <code>TestEncoder</code> and <code>TestDecoder</code>.
</p>

<p>
This data-driven style dominates in the Go package tests.
<br>
<!-- search for for.*range here -->
</p>

<h3 id="reflect.DeepEqual">Use reflect.DeepEqual to compare complex values</h3>

<p>
The <code>reflect.DeepEqual</code> function tests
whether two complex data structures have equal values.
If a function returns a complex data structure, 
<code>reflect.DeepEqual</code> combined with table-driven testing
makes it easy to check that the return value is 
exactly as expected.
</p>

<h2 id="be-consistent">Be consistent</h2>

<p>
Programmers often want their style to be distinctive,
writing loops backwards or using custom spacing and
naming conventions. Such idiosyncracies come at a
price, however: by making the code look different,
they make it harder to understand.
Consistency trumps personal
expression in programming.
</p>

<p>
If a program does the same thing twice,
it should do it the same way both times.
Conversely, if two different sections of a
program look different, the reader will
expect them to do different things.
</p>

<p>
Consider <code>for</code> loops.
Traditionally, a loop over <code>n</code>
elements begins:
</p>

<pre>
for i := 0; i &lt; n; i++ {
</pre>

<p>
Much of the time, the loop could run in the opposite order
and still be correct:
</p>

<pre>
for i := n-1; i &gt;= 0; i-- {
</pre>

<p>
The convention
is to count up unless to do so would be incorrect.
A loop that counts down implicitly says &ldquo;something
special is happening here.&rdquo;
A reader who finds a program in which some
loops count up and the rest count down
will spend time trying to understand why.
</p>

<p>
Loop direction is just one
programming decision that must be made
consistently; others include
formatting, naming variables and methods,
whether a type
has a constructor, what tests look like, and so on.
Why is this variable called <code>n</code> here and <code>cnt</code> there?
Why is the <code>Log</code> constructor <code>CreateLog</code> when
the <code>List</code> constructor is <code>NewList</code>?
Why is this data structure initialized using
a structure literal when that one
is initialized using individual assignments?
These questions distract from the important one:
what does the code do?
Moreover, internal consistency is important not only within a single file,
but also within the the surrounding source files.
When editing code, read the surrounding context
and try to mimic it as much as possible, even if it
disagrees with the rules here.
It should not be possible to tell which lines
you wrote or edited based on style alone.
Consistency about little things
lets readers concentrate on big ones.
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