Introduction

Go is a new language. Although it's in the C family of languages it has some unusual properties that make effective Go programs different in character from programs in C, C++, or Java. To write Go well, it's important to understand its properties and idioms.

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

Read good code

The first step in learning to write good code is to read good code. The Go package sources are intended to serve not only as the core library but also as examples of how to use the language. Read them and follow their example.

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.

Formatting

Formatting issues are the most contentious but the least consequential. People adapt to different formatting styles, but they shouldn't be asked to. Everyone should use the same formatting; as in English, consistent punctuation and spacing make the text easier to read. Most of the local formatting style can be picked up by reading existing Go programs, but to make them explicit here are some common points.

Use tabs

Use tabs, not spaces, for indentation.

Don't worry about columnation

Let tools such as gofmt take care of lining things up.

Trim trailing white space

There should be no trailing white space at the end of lines.

Don't wrap lines

Go has no 80-character limit. Don't bother with fancy line wrapping just because a line is wider than a punched card. If a line is too long, indent with an extra tab.

Omit parentheses in control structures

Go does not require parentheses around the expression following the for, if, range, switch, and return keywords.

Use line comments

Go provides C-style /* */ block comments and C++-style // line comments. Use line comments by default, reserving block comments for top-level package comments and commenting out large swaths of code.

Write package comments

Every package should have a package comment, a block comment preceding the package clause. It should introduce the package and provide information relevant to the package as a whole.

/*
	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

Consider how the package comment contributes to the appearance of the godoc page for the package. Don't just echo the doc comments for the components. The package comment can be brief.

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

Write doc comments

If a comment immediately precedes a top-level declaration, the Go documentation server (TODO: that's not a public URL.) uses that comment as the documentation for the constant, function, method, package, type or variable being declared. These are called doc comments. To detach a comment from a declaration, insert a blank line between them.

Every exported (capitalized) name in a program should have a doc comment, as should the package declaration itself. If a name appears multiple times due to forward declarations or appearance in multiple source files within a package, only one instance requires a doc comment, and any one will do.

Doc comments consist of complete English sentences. The first sentence should be a one-sentence summary that starts with the name being declared:

// Quote returns a double-quoted Go string literal
// representing s.  The returned string s uses Go escape
// sequences (\t, \n, \xFF, \u0100) for control characters
// and non-ASCII characters.
func Quote(s string) string {

Use of complete English sentences admits a wider variety of automated presentations.

Avoid ASCII Art

Go programs are meant to read equally well using fixed-width and variable-width fonts. Don't use fancy formattings that depend on fixed-width fonts. In particular, don't assume that a single space is the same width as every other character. If you need to make a columnated table, use tabs to separate the columns and the pretty printer will make sure the columns are lined up properly in the output.

If you need comments to separate sections in a file, use a simple block comment:

/*
 * Helper routines for simplifying the fetching of optional
 * fields of basic type. If the field is missing, they return
 * the zero for the type.
 */

/*
    Helper routines for simplifying the fetching of optional
    fields of basic type. If the field is missing, they return
    the zero for the type.
 */

Comments are text, not HTML; they contain no markup. Refrain from ASCII embellishment like *this* or /this/.

Use grouping to organize declarations

Go's declaration syntax allows grouping of declarations. A comment can introduce a group of related constants or variables.

// Flags to Open, wrapping those of the underlying system.
// Not all flags may be implemented on a given system.
const (
	O_RDONLY = syscall.O_RDONLY;	// Open file read-only.
	O_WRONLY = syscall.O_WRONLY;	// Open file write-only.
	...
)

A grouping can also indicate relationships between items, such as the fact that a set of variables is controlled by a mutex.

// Variables protected by countLock.
var (
	countLock	sync.Mutex;
	inputCount	uint32;
	outputCount	uint32;
	errorCount	uint32;
)

Names

Use MixedCaps

Go uses the case of the first letter in a name to decide whether the name is visible in other packages. Multiword names use MixedCaps or mixedCaps rather than underscores.

Use short package names

Package names are lower case single-word names: there should be no need for underscore or mixedCaps. The package name is conventionally the base name of the 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 package name is only the default name used when importing the package; it need not be unique across all source code.

Avoid long names

A name's length should not exceed its information content. For a function-local variable in scope only for a few lines, the name i conveys just as much information as index or idx and is easier to read. Letters are easier to distinguish than numbers; use i and j not i1 and i2.

Exported names must convey more information because they appear far from their origin. Even so, longer names are not always better, and the package name can help convey information: the buffered Reader is bufio.Reader, not bufio.BufReader. Similarly, once.Do is as precise and evocative as once.DoOrWaitUntilDone, and once.Do(f) reads better than once.DoOrWaitUntilDone(f). Encoding small essays into function names is not Go style; clear names with good documentation is.

Use the -er convention for interface names

One-method interfaces are conventionally named by the method name plus the -er suffix: Reader, Writer, Formatter.

Use canonical names

A few method names—Read, Write, Close, Flush, String—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.

Some function-local variables have canonical names too. Just as i is idiomatic in Go for an index variable, n is idiomatic for a count, b for a []byte, s for a string, r for a Reader, err for an os.Error and so on. Don't mix shorthands: it is especially confusing to have two different variables i and idx, or n and cnt.

Idioms

Address literals to allocate and initialize

Taking the address of a struct or array literal evaluates to a new instance each time it is evaluated. Use these expressions to avoid the repetition of filling out a data structure.

// 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 jumps, 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();
}

open /etc/passwx: no such file or directory

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.

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 sometimes 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 2, 3, 5, and 7 (or for 2, 4, 8, and 16) than to report that isPrime gives the wrong answer for 2 and therefore no more tests were run.

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.