Introduction

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.

This document gives tips for writing clear, idiomatic Go code. It augments the language specification and the tutorial, both of which you should read first.

Examples

The Go package sources 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.

Formatting

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.

With Go we take an unusual approach and let the machine take care of most formatting issues. A program, gofmt, 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 gofmt; if the answer doesn't seem right, fix the program (or file a bug), don't work around it.

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

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

gofmt will make the columns line up:

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

All code in the libraries has been formatted with gofmt. TODO

Some formatting details remain. Very briefly:

Indentation

We use tabs for indentation and gofmt emits them by default. Use spaces if you must.

Line length

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.

Parentheses

Go needs fewer parentheses: control structures (if, for, switch) do not have parentheses in their syntax. Also, the operator precedence hierarchy is shorter and clearer, so

x<<8 + y<<16

means what the spacing implies.

Commentary

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

The program—and web server—godoc 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 godoc produces.

Every package should have a package comment, 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 godoc page and should set up the detailed documentation that follows.

/*
	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

If the package is simple, the package comment can be brief.

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

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—godoc, like gofmt, takes care of that. Finally, the comments are uninterpreted plain text, so HTML and other annotations such as _this_ will reproduce verbatim and should not be used.

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

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:

// 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) {

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.

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

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.

var (
	countLock	sync.Mutex;
	inputCount	uint32;
	outputCount	uint32;
	errorCount	uint32;
)

Names

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.

Package names

When a package is imported, the package name becomes an accessor for the contents. After

import "bytes"

the importing package can talk about bytes.Buffer. 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 a priori. 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.

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

The importer of a package will use the name to refer to its contents (the import . 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 bufio package is called Reader, not BufReader, because users see it as bufio.Reader, which is a clear, concise name. Moreover, because imported entities are always addressed with their package name, bufio.Reader does not conflict with io.Reader. Use the package structure to help you choose good names.

Another short example is once.Do; once.Do(setup) reads well and would not be improved by writing once.DoOrWaitUntilDone(setup). 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.

Interface names

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

There are a number of such names and it's productive to honor them and the function names they capture. Read, Write, Close, Flush, String 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 String not ToString.

MixedCaps

Finally, the convention in Go is to used MixedCaps or mixedCaps rather than underscores to write multiword names.

Idioms

Allocate using literals

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.

length := Point{x, y}.Abs();

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

Use parallel assignment to slice a buffer

header, body, checksum := buf[0:20], buf[20:n-4], buf[n-4:n];

Control Flow

Omit needless else bodies

When an if statement doesn't flow into the next statement—that is, the body ends in break, continue, goto, or return—omit the else.

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

Switch

Go's switch is more general than C's. When an if-else-if-else chain has three or more bodies, or an if condition has a long list of alternatives, it will be clearer if rewritten as a switch.

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

func shouldEscape(c byte) bool {
    switch c {
    case ' ', '?', '&', '=', '#', '+', '%':
        return true
    }
    return false
}

// Compare returns an integer comparing the two byte arrays
// lexicographically.
// The result will be 0 if a==b, -1 if a < b, and +1 if a > b
func Compare(a, b []byte) int {
    for i := 0; i < len(a) && i < len(b); i++ {
        switch {
        case a[i] > b[i]:
            return 1
        case a[i] < b[i]:
            return -1
        }
    }
    switch {
    case len(a) < len(b):
        return -1
    case len(a) > len(b):
        return 1
    }
    return 0
}

Functions

Omit needless wrappers

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.

Return multiple values

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

Errors

Return os.Error, not bool

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

Handle errors first

Error cases tend to be simpler than non-error cases, and it helps readability when the non-error flow of control is always down the page. Also, error cases tend to end in return statements, so that there is no need for an explicit else.

if len(name) == 0 {
	return os.EINVAL;
}
if IsDir(name) {
	return os.EISDIR;
}
f, err := os.Open(name, os.O_RDONLY, 0);
if err != nil {
	return err;
}
codeUsing(f);

Return structured errors

// 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 + " " + e.Path + ": " + e.Error.String();
}

PathError's String formats the error nicely, including the operation and file name tha failed; just printing the error generates a message, such as

open /etc/passwx: no such file or directory

that is useful even if printed far from the call that triggered it.

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 PathErrors this might include examining the internal Error to see if it is os.EPERM or os.ENOENT, for instance.

Programmer-defined types

Use NewTypeName for constructors

The constructor for the type pkg.MyType should be named pkg.NewMyType and should return *pkg.MyType. The implementation of NewTypeName often uses the struct allocation idiom.

func NewFile(fd int, name string) *File {
	if file < 0 {
		return nil
	}
	return &File{fd, name, nil, 0}
}

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

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

func New(len int) *Vector {
	return new(Vector).Init(len)
}

Make the zero value meaningful

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.

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

Interfaces

Accept interface values

Return interface values

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.

For example, both crc32.NewIEEE() and adler32.New() return type hash.Hash32. 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.

Use interface adapters to expand an implementation

Use anonymous fields to incorporate an implementation

Data-Driven Programming

tables

XXX struct tags for marshalling. template eventually datafmt

Concurrency

Share memory by communicating

Do not communicate by sharing memory; instead, share memory by communicating.

XXX, more here.

Testing

Run tests to completion

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 isPrime gives the wrong answer for 4, 8, 16 and 32 than to report that isPrime 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

Print useful errors when tests fail

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.

Use data-driven tests

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 TestEncoder and TestDecoder.

This data-driven style dominates in the Go package tests.

Use reflect.DeepEqual to compare complex values

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

Be consistent

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.

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.

Consider for loops. Traditionally, a loop over n elements begins:

for i := 0; i < n; i++ {

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

for i := n-1; i >= 0; i-- {

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

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 n here and cnt there? Why is the Log constructor CreateLog when the List constructor is NewList? 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.