go/doc/effective_go.html

<!-- Effective Go -->

<!-- interfaces; cast,conversion, type assertion; embedding; errors; testing; initialization -->

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

<p>
Go is a new language.  Although it borrows ideas from
existing languages,
it has unusual properties that make effective Go programs
different in character from programs in its relatives.
A straightforward translation of a C++ or Java program into Go
is unlikely to produce a satisfactory result&mdash;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="http://s2/?dir=//depot2/go/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>.
</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 only 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 require 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.
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.
In any case, confusion is rare because the file name in the import
defines which version is being used.
</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>.
Similarly, the constructor for <code>vector.Vector</code>
would normally be called <code>NewVector</code> but since
<code>Vector</code> is the only type exported by the package, and since the
package is called <code>vector</code>, it's called just <code>New</code>,
which clients of the package see as <code>vector.New</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 use <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(dst, src chan Item) {
    go func() { for { dst &lt;- &lt;-src } }()
}
</pre>

<p>
In fact, semicolons can be 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 accepts 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 required
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="for">For</h3>

<p>
The Go <code>for</code> loop is similar to—but not the same as—C's.
It unifies <code>for</code>
and <code>while</code> and there is no <code>do-while</code>.
There are three forms, only one of which has semicolons:
</p>
<pre>
// Like a C for
for init; condition; post { }

// Like a C while
for condition { }

// Like a C for(;;)
for { }
</pre>

<p>
Short declarations make it easy to declare the index variable right in the loop:
</p>
<pre>
sum := 0;
for i := 0; i < 10; i++ {
    sum += i
}
</pre>

<p>
If you're looping over an array, slice, string, or map a <code>range</code> clause can set
it all up for you:
</p>
<pre>
var m map[string] int;
sum := 0;
for _, value := range m {  // key is unused
    sum += value
}
</pre>

<p>
For strings, the <code>range</code> does more of the work for you, breaking out individual
characters by parsing the UTF-8 (erroneous encodings consume one byte and produce the
replacement rune U+FFFD). The loop
</p>
<pre>
for pos, char := range "日本語" {
    fmt.Printf("character %c starts at byte position %d\n", char, pos)
}
</pre>
<p>
prints
</p>
<pre>
character 日 starts at byte position 0
character 本 starts at byte position 3
character 語 starts at byte position 6
</pre>

<p>
Finally, since Go has no comma operator and <code>++</code> and <code>--</code>
are statements not expressions, if you want to run multiple variables in a <code>for</code>
you should use parallel assignment:
</p>
<pre>
// Reverse a
for i, j := 0, len(a)-1; i < j; i, j = i+1, j-1 {
	a[i], a[j] = a[j], a[i]
}
</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="multiple-returns">Multiple return values</h3>

<p>
One of Go's unusual properties is that functions and methods
can return multiple values.  This feature can be used to
improve on a couple of clumsy idioms in C programs: in-band
error returns (<code>-1</code> for <code>EOF</code> for example)
and modifying an argument.
</p>

<p>
In C, a write error is signaled by a negative byte count with the
error code secreted away in a volatile location.
In Go, <code>Write</code>
can return a byte count <i>and</i> an error: "Yes, you wrote some
bytes but not all of them because you filled the device".
The signature of <code>*File.Write</code> in package <code>os</code> is:
</p>

<pre>
func (file *File) Write(b []byte) (n int, err Error)
</pre>

<p>
and as the documentation says, it returns the number of bytes
written and a non-nil <code>Error</code> when <code>n</code>
<code>!=</code> <code>len(b)</code>.
This is a common style; see the section on error handling for more examples.
</p>

<p>
A similar approach obviates the need to pass a pointer to a return
value to simulate a reference parameter.
Here's a simple-minded function to
grab a number from a position in a byte array, returning the number
and the next position.
</p>

<pre>
func nextInt(b []byte, i int) (int, int) {
	for ; i < len(b) &amp;&amp; !isDigit(b[i]); i++ {
	}
	x := 0;
	for ; i < len(b) &amp;&amp; isDigit(b[i]); i++ {
		x = x*10 + int(b[i])-'0'
	}
	return x, i;
}
</pre>

<p>
You could use it to scan the numbers in an input array <code>a</code> like this:
</p>

<pre>
	for i := 0; i < len(a); {
		x, i = nextInt(a, i);
		fmt.Println(x);
	}
</pre>

<h3 id="named-results">Named result parameters</h3>

<p>
The return or result "parameters" of a Go function can be given names and
used as regular variables, just like the incoming parameters.
When named, they are initialized to the zero values for their types when
the function begins; if the function executes a <code>return</code> statement
with no arguments, the current values of the result parameters are
used as the returned values.
</p>

<p>
The names are not mandatory but they can make code shorter and clearer:
they're documentation.
If we name the results of <code>nextInt</code> it becomes
obvious which returned <code>int</code>
is which.
</p>

<pre>
func nextInt(b []byte, pos int) (value, nextPos int) {
</pre>

<p>
Because named results are initialized and tied to an unadorned return, they can simplify
as well as clarify.  Here's a version
of <code>io.ReadFull</code> that uses them well:
</p>

<pre>
func ReadFull(r Reader, buf []byte) (n int, err os.Error) {
	for len(buf) > 0 &amp;&amp; err != nil {
		var nr int;
		nr, err = r.Read(buf);
		n += nr;
		buf = buf[nr:len(buf)];
	}
	return;
}
</pre>

<h2 id="data">Data</h2>

<h3 id="allocation_new">Allocation with <code>new()</code></h3>

<p>
Go has two allocation primitives, <code>new()</code> and <code>make()</code>.
They do different things and apply to different types, which can be confusing,
but the rules are simple.
Let's talk about <code>new()</code> first.
It's a built-in function essentially the same as its namesakes
in other languages: it allocates zeroed storage for a new item of type
<code>T</code> and returns its address, a value of type <code>*T</code>.
In Go terminology, it returns a pointer to a newly allocated zero value of type
<code>T</code>.
</p>

<p>
Since the memory returned by <code>new()</code> is zeroed, it's helpful to arrange that the
zeroed object can be used without further initialization.  This means a user of
the data structure can create one with <code>new()</code> and get right to
work.
For example, the documentation for <code>bytes.Buffer</code> states that
"the zero value for <code>Buffer</code> is an empty buffer ready to use."
Similarly, <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>

<p>
The zero-value-is-useful property works transitively. Consider this type declaration:
</p>

<pre>
type SyncedBuffer struct {
	lock	sync.Mutex;
	buffer	bytes.Buffer;
}
</pre>

<p>
Values of type <code>SyncedBuffer</code> are also ready to use immediately upon allocation
or just declaration.  In this snippet, both <code>p</code> and <code>v</code> will work
correctly without further arrangement:
</p>

<pre>
p := new(SyncedBuffer);  // type *SyncedBuffer
var v SyncedBuffer;      // type  SyncedBuffer
</pre>

<h3 id="composite_literals">Constructors and composite literals</h3>

<p>
Sometimes the zero value isn't good enough and an initializing
constructor is necessary, as in this example derived from
package <code>os</code>:
</p>

<pre>
func NewFile(fd int, name string) *File {
	if fd &lt; 0 {
		return nil
	}
	f := new(File);
	f.fd = fd;
	f.name = name;
	f.error = nil;
	f.dirinfo = nil;
	f.nepipe = 0;
	return f;
}
</pre>

<p>
There's a lot of boilerplate in there.  We can simplify it
using a <i>composite literal</i>, which is
an expression that creates a
new instance each time it is evaluated.
</p>


<pre>
func NewFile(fd int, name string) *File {
	if file &lt; 0 {
		return nil
	}
	f := File{fd, name, nil, 0};
	return &amp;f;
}
</pre>

<p>
Note that it's perfectly OK to return the address of a local variable;
the storage associated with the variable survives after the function
returns.
In fact, taking the address of a composite literal
allocates a fresh instance each time it is evaluated,
so we can combine these last two lines:
</p>

<pre>
	return &amp;File{fd, name, nil, 0};
</pre>

<p>
The fields of a composite literal are laid out in order and must all be present.
However, by labeling the elements explicitly as <i>field</i><code>:</code><i>value</i>
pairs, the initializers can appear in any
order, with the missing ones left as their respective zero values.  Thus we could say
</p>

<pre>
	return &amp;File{fd: fd, name: name}
</pre>

<p>
As a limiting case, if a composite literal contains no fields at all, it creates
a zero value for the type.  These two expressions are equivalent:
</p>

<pre>
new(File)
&amp;File{}
</pre>

<p>
Composite literals can also be created for arrays, slices, and maps,
with the field labels being indices or map keys as appropriate.
In these examples, the initializations work regardless of the values of <code>EnoError</code>,
<code>Eio</code>, and <code>Einval</code>, as long as they are distinct:
</p>

<pre>
a := [...]string   {Enone: "no error", Eio: "Eio", Einval: "invalid argument"};
s := []string      {Enone: "no error", Eio: "Eio", Einval: "invalid argument"};
m := map[int]string{Enone: "no error", Eio: "Eio", Einval: "invalid argument"};
</pre>

<h3 id="allocation_make">Allocation with <code>make()</code></h3>

<p>
Back to allocation.
The built-in function <code>make(T, </code><i>args</i><code>)</code> serves
a purpose different from <code>new(T)</code>.
It creates slices, maps, and channels only, and it returns an initialized (not zero)
value of type <code>T</code>, not <code>*T</code>.
The reason for the distinction
is that these three types are, under the covers, references to data structures that
must be initialized before use.
A slice, for example, is a three-item descriptor
containing a pointer to the data (inside an array), the length, and the
capacity; until those items are initialized, the slice is <code>nil</code>.
For slices, maps, and channels,
<code>make</code> initializes the internal data structure and prepares
the value for use.
For instance,
</p>

<pre>
make([]int, 10, 100)
</pre>

<p>
allocates an array of 100 ints and then creates a slice
structure with length 10 and a capacity of 100 pointing at the first
10 elements of the array.
(When making a slice, the capacity can be omitted; see the section on slices
for more information.)
In contrast, <code>new([]int)</code> returns a pointer to a newly allocated, zeroed slice
structure, that is, a pointer to a <code>nil</code> slice value.

<p>
These examples illustrate the difference between <code>new()</code> and
<code>make()</code>:
</p>

<pre>
var p *[]int = new([]int);       // allocates slice structure; *p == nil; rarely useful
var v  []int = make([]int, 100); // v now refers to a new array of 100 ints

// Unnecessarily complex:
var p *[]int = new([]int);
*p = make([]int, 100, 100);

// Idiomatic:
v := make([]int, 100);
</pre>

<p>
Remember that <code>make()</code> applies only to maps, slices and channels.
To obtain an explicit pointer allocate with <code>new()</code>.
</p>

<h3 id="arrays">Arrays</h3>

<p>
Arrays are useful when planning the detailed layout of memory and sometimes
can help avoid allocation but primarily
they are a building block for slices, the subject of the next section.
To lay the foundation for that topic, here are a few words about arrays.
</p>

<p>
There are major differences between the ways arrays work in Go and C.
In Go:
</p>
<ul>
<li>
Arrays are values. Assigning one array to another copies all the elements.
</li>
<li>
In particular, if you pass an array to a function, it
will receive a <i>copy</i> of the array, not a pointer to it.
<li>
The size of an array is part of its type.  The types <code>[10]int</code>
and <code>[20]int</code> are distinct.
</li>
</ul>

<p>
The value property can be useful but also expensive; if you want C-like behavior and efficiency,
you can pass a pointer to the array:
</p>

<pre>
func Sum(a *[]float) (sum float) {
	for _, v := range a {
		sum += v
	}
	return
}

array := [...]float{7.0, 8.5, 9.1};
x := sum(&amp;array);  // Note the explicit address-of operator
</pre>

<p>
But even this style isn't idiomatic Go.  Slices are.
</p>

<h3 id="slices">Slices</h3>

<p>
Slices wrap arrays to give a more general, powerful, and convenient
interface to sequences of data.  Except for items with explicit
dimension such as transformation matrices, most array programming in
Go is done with slices rather than simple arrays.
</p>
<p>
Slices are <i>reference types</i>, which means that if you assign one
slice to another, both refer to the same underlying array.  For
instance, if a function takes a slice argument, changes it makes to
the elements of the slice will be visible to the caller, analogous to
passing a pointer to the underlying array.  A <code>Read</code>
function can therefore accept a slice argument rather than a (pointer
to an) array and a count; the length within the slice sets an upper
limit of how much data to read.  Here is the signature of the
<code>Read</code> method of the <code>File</code> type in package
<code>os</code>:
</p>
<pre>
func (file *File) Read(buf []byte) (n int, err os.Error)
</pre>
<p>
The method returns the number of bytes read and an error value, if
any.  To read into the first 32 bytes of a larger buffer
<code>b</code>, <i>slice</i> (here used as a verb) the buffer:
</p>
<pre>
	n, err := f.Read(buf[0:32]);
</pre>
<p>
Such slicing is common and efficient.  In fact, leaving efficiency aside for
the moment, this snippet would also read the first 32 bytes of the buffer:
</p>
<pre>
	var n int;
	var err os.Error;
	for i := 0; i < 32; i++ {
		nbytes, e := f.Read(buf[i:i+1]);
		if nbytes == 0 || e != nil {
			err = e;
			break;
		}
		n += nbytes;
	}
</pre>
<p>
The length of a slice may be changed as long as it still fits within
the limits of the underyling array; just assign it to a slice of
itself.  The <i>capacity</i> of a slice, accessible by the built-in
function <code>cap</code>, reports the maximum length the slice may
assume.  Here is a function to append data to a slice.  If the data
exceeds the capacity, the slice is reallocated.  The
resulting slice is returned.  The function uses the fact that
<code>len</code> and <code>cap</code> are legal when applied to the
<code>nil</code> slice, and return 0.
</p>
<pre>
func Append(slice, data[]byte) []byte {
	l := len(slice);
	if l + len(data) > cap(slice) {	// reallocate
		// Allocate double what's needed, for future growth.
		newSlice := make([]byte, (l+len(data))*2);
		// Copy data (could use bytes.Copy()).
		for i, c := range slice {
			newSlice[i] = c
		}
		slice = newSlice;
	}
	slice = slice[0:l+len(data)];
	for i, c := range data {
		slice[l+i] = c
	}
	return slice;
}
</pre>
<p>
We must return the slice afterwards because, although <code>Append</code>
can modify the elements of <code>slice</code>, the slice itself (the run-time data
structure holding the pointer, length, and capacity) is passed by value.
</p>


<h3 id="maps">Maps</h3>

<p>
Maps are a convenient and powerful built-in data structure to associate
values of different types.
The key can be of type that implements equality, such as integers,
floats, strings, pointers, and interfaces  (as long as the dynamic type
supports equality), but not structs, arrays or slices
because those types do not have equality defined upon them.
Like slices, maps are a reference type. If you pass a map to a function
that changes the contents of the map, the changes will be visible
in the caller.
</p>
<p>
Maps can be constructed using the usual composite literal syntax
with colon-separated key-value pairs,
so it's easy to build them during initialization.
</p>
<pre>
var timeZone = map[string] int {
	"UTC":  0*60*60,
	"EST": -5*60*60,
	"CST": -6*60*60,
	"MST": -7*60*60,
	"PST": -8*60*60,
}
</pre>
<p>
Assigning and fetching map values looks syntactically just like
doing the same for arrays except that the index doesn't need to
be an integer.  An attempt to fetch a map value with a key that
is not present in the map will cause the program to crash, but
there is a way to do so safely using a multiple assignment.
</p>
<pre>
var seconds int;
var ok bool;
seconds, ok = timeZone[tz]
</pre>
<p>
For obvious reasons this is called the &ldquo;comma ok&rdquo; idiom.
In this example, if <code>tz</code> is present, <code>seconds</code>
will be set appropriately and <code>ok</code> will be true; if not,
<code>seconds</code> will be set to zero and <code>ok</code> will
be false.
Here's a function that puts it together:
</p>
<pre>
func offset(tz string) int {
	if seconds, ok := timeZone[tz]; ok {
		return seconds
	}
	log.Stderr("unknown time zone", tz);
	return 0;
}
</pre>
<p>
To test for presence in the map without worrying about the actual value,
you can use the <em>blank identifier</em>, a simple underscore (<code>_</code>).
The blank identifier can be assigned or declared with any value of any type, with the
value discarded harmlessly.  For testing presence in a map, use the blank
identifier in place of the usual variable for the value.
</p>
<pre>
_, present := timeZone[tz];
</pre>
<p>
To delete a map entry, turn the multiple assignment around by placing
an extra boolean on the right; if the boolean is false, the entry
is deleted. It's safe to do this even if the key is already absent
from the map.
</p>
<pre>
timeZone["PDT"] = 0, false;  // Now on Standard Time
</pre>
<h3 id="printing">Printing</h3>

<p>
Formatted printing in Go uses a style similar to C's <code>printf</code>
family but is richer and more general. The functions live in the <code>fmt</code>
package and have capitalized names: <code>fmt.Printf</code>, <code>fmt.Fprintf</code>,
<code>fmt.Sprintf</code> and so on.  The string functions (<code>Sprintf</code> etc.)
return a string rather than filling in a provided buffer.
</p>
<p>
You don't need to provide a format string.  For each of <code>Printf</code>,
<code>fmt.Fprintf</code> and <code>fmt.Sprintf</code> there is another pair
of functions, for instance <code>Print</code> and <code>Println</code>.
These functions do not take a format string but instead generate a default
format for each argument. The <code>ln</code> version also inserts a blank
between arguments if neither is a string and appends a newline to the output.
In this example each line produces the same output.
</p>
<pre>
fmt.Printf("Hello %d\n", 23);
fmt.Fprint(os.Stdout, "Hello ", 23, "\n");
fmt.Println(fmt.Sprint("Hello ", 23));
</pre>
<p>
Recall that <code>fmt.Fprint</code> and friends take as a first argument any object
that implements the <code>io.Writer</code> interface; the variables <code>os.Stdout</code>
and <code>os.Stderr</code> are familiar instances.
</p>
<p>
Here things start to diverge from C.  First, the numeric formats such as <code>%d</code>
do not take flags for signedness or size; instead, the printing routines use the
type of the argument to decide these properties.
</p>
<pre>
var x uint64 = 1<<64 - 1;
fmt.Printf("%d %x; %d %x\n", x, x, int64(x), int64(x));
</pre>
<p>
prints
</p>
<pre>
18446744073709551615 ffffffffffffffff; -1 -1
</pre>
<p>
If you just want the default conversion, such as decimal for integers, you can use
the catchall format <code>%v</code> (for &ldquo;value&rdquo;); the result is exactly
what <code>Print</code> and <code>Println</code> would produce.
Moreover, that format can print <em>any</em> value, even arrays, structs, and
maps.  Here is a print statement for the time zone map defined in the previous section.
</p>
<pre>
fmt.Printf("%v\n", timeZone);  // or just fmt.Println(timeZone);
</pre>
<p>
which gives output
</p>
<pre>
map[CST:-21600 PST:-28800 EST:-18000 UTC:0 MST:-25200]
</pre>
<p>
For maps the keys may be output in any order, of course.
When printing a struct, the modified format <code>%+v</code> annotates the
fields of the structure with their names, and for any value the alternate
format <code>%#v</code> prints the value in full Go syntax.
</p>
<pre>
type T struct {
	a int;
	b float;
	c string;
}
t := &amp;T{ 7, -2.35, "abc\tdef" };
fmt.Printf("%v\n", t);
fmt.Printf("%+v\n", t);
fmt.Printf("%#v\n", t);
fmt.Printf("%#v\n", timeZone);
</pre>
<p>
prints
</p>
<pre>
&amp;{7 -2.35 abc   def}
&amp;{a:7 b:-2.35 c:abc     def}
&amp;main.T{a:7, b:-2.35, c:"abc\tdef"}
map[string] int{"CST":-21600, "PST":-28800, "EST":-18000, "UTC":0, "MST":-25200}
</pre>
<p>
(Note the ampersands.)
That quoted string format is also available through <code>%q</code> when
applied to a value of type <code>string</code> or <code>[]byte</code>;
the alternate format <code>%#q</code> will use backquotes instead if possible.
Also, <code>%x</code> works on strings and arrays of bytes as well as on integers,
generating a long hexadecimal string, and with
a space in the format (<code>%&nbsp;x</code>) it puts spaces between the bytes.
</p>
<p>
Another handy format is <code>%T</code>, which prints the <em>type</em> of a value.
<pre>
fmt.Printf(&quot;%T\n&quot;, timeZone);
</pre>
<p>
prints
</p>
<pre>
map[string] int
</pre>
<p>
If you want to control the default format for a custom type, all that's required is to define
a method <code>String() string</code> on the type.  (Methods are the subject of the next
section.) For our simple type <code>T</code>, that might look like this.
</p>
<pre>
func (t *T) String() string {
	return fmt.Sprintf("%d/%g/%q", t.a, t.b, t.c);
}
fmt.Printf("%v\n", t);
</pre>
<p>
to print in the format
</p>
<pre>
7/-2.35/"abc\tdef"
</pre>
<p>
Our <code>String()</code> method is able to call <code>Sprintf</code> because the
print routines are fully reentrant and can be used recursively.
We can even go one step further and pass a print routine's arguments directly to another such routine.
The signature of <code>Printf</code> uses the <code>...</code>
type for its final argument to specify that an arbitrary number of parameters can appear
after the format.
</p>
<pre>
func Printf(format string, v ...) (n int, errno os.Error) {
</pre>
<p>
Within the function <code>Printf</code>, <code>v</code> is a variable that can be passed,
for instance, to another print routine.  Here is the implementation of the
function <code>log.Stderr</code> we used above. It passes its arguments directly to
<code>fmt.Sprintln</code> for the actual formatting.
</p>
<pre>
// Stderr is a helper function for easy logging to stderr. It is analogous to Fprint(os.Stderr).
func Stderr(v ...) {
	stderr.Output(2, fmt.Sprintln(v));  // Output takes parameters (int, string)
}
</pre>
<p>
There's even more to printing than we've covered here.  See the <code>godoc</code> documentation
for package <code>fmt</code> for the details.
</p>

<h2>Methods</h2>

<h3 id="pointers_vs_values">Pointers vs. Values</h3>
<p>
Methods can be defined for any named type except pointers and interfaces;
the receiver does not have to be a struct.
<p>
In the discussion of slices above, we wrote an <code>Append</code>
function.  We can define it as a method on slices instead.  To do
this, we first declare a named type to which we can bind the method, and
then make the receiver for the method a value of that type.
</p>
<pre>
type ByteSlice []byte

func (slice ByteSlice) Append(data []byte) []slice {
	// Body exactly the same as above
}
</pre>
<p>
This still requires the method to return the updated slice.  We can
eliminate that clumsiness by redefining the method to take a
<i>pointer</i> to a <code>ByteSlice</code> as its receiver, so the
method can overwrite the caller's slice.
</p>
<pre>
func (p *ByteSlice) Append(data []byte) {
	slice := *p;
	// Body as above, without the return.
	*p = slice;
}
</pre>
<p>
In fact, we can do even better.  If we modify our function so it looks
like a standard <code>Write</code> method, like this,
</p>
<pre>
func (p *ByteSlice) Write(data []byte) (n int, err os.Error) {
	slice := *p;
	// Again as above.
	*p = slice;
	return len(data), nil)
}
</pre>
<p>
then the type <code>*ByteSlice</code> satisfies the standard interface
<code>io.Writer</code>, which is handy.  For instance, we can
print into one:
</p>
<pre>
	var b ByteSlice;
	fmt.Fprintf(&amp;b, "This hour has %d days\n", 7);
</pre>
<p>
Notice that we must pass the address of a <code>ByteSlice</code>
because only <code>*ByteSlice</code> satisfies <code>io.Writer</code>.
The rule about pointers vs. values for receivers is that value methods
can be invoked on pointers and values, but pointer methods can only be
invoked on pointers.  This is because pointer methods can modify the
receiver; invoking them on a copy of the value would cause those
modifications to be discarded.
</p>
<p>
By the way, the idea of using <code>Write</code> on a slice of bytes
is implemented by <code>bytes.Buffer</code>.
</p>

<h2>More to come</h2>

<!---

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


<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="http://go/godoc/src/pkg/os/file.go">http://go/godoc/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>

<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>


<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 marshaling.
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 idiosyncrasies 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 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.
</p>

<pre>
TODO
verifying implementation
type Color uint32

// Check that Color implements image.Color and image.Image
var _ image.Color = Black
var _ image.Image = Black
</pre>
-->