## 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.

## Semicolons

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 `funcs` and the like:

    func CopyInBackground(dst, src chan Item) {
        go func() { for { dst <- <-src } }()
    }

In fact, semicolons can omitted at the end of any "StatementList" in the grammar, which includes things like cases in `switch` statements:

    switch {
    case a < b:
        return -1
    case a == b:
        return 0
    case a > b:
        return 1
    }

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 `struct` declarations, or of `funcs` for that matter) and often left off one-liners. But within functions, place them as you see fit.

## Control structures

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

### If

In Go a simple `if` looks like this:

    if x > 0 {
        return y
    }

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

Since `if` and `switch` accept an initialization statement, it's common to see one used to set up a local variable:

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

In the Go libraries, you'll find that when an `if` statement doesn't flow into the next statement—that is, the body ends in `break`, `continue`, `goto`, or `return`—the unnecessary `else` is omitted.

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

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 `return` statements, the resulting code needs no `else` statements:

    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);

### For

The Go `for` loop is similar to—but not the same as—C's. It unifies `for` and `while` and there is no `do-while`. There are three forms, only one of which has semicolons:

    // Like a C for:
    for init; condition; post { }

    // Like a C while:
    for condition { }

    // Like a C for(;;)
    for { }

Short declarations make it easy to declare the index variable right in the loop:

    sum := 0;
    for i := 0; i < 10; i++ {
        sum += i
    }

If you're looping over an array, slice, string, or map a `range` clause can set it all up for you:

    var m map[string] int;
    sum := 0;
    for key, value := range m {  // key is unused; could call it '_'
        sum += value
    }

Finally, since Go has no comma operator and `++` and `--` are statements not expressions, if you want to run multiple variables in a `for` you can use parallel assignment:

    // 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]
    }

### Switch

Go's `switch` 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 `switch` has no expression it switches on `true`. It's therefore possible—and idiomatic—to write an `if`-`else`-`if`-`else` chain 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
    }

There is no automatic fall through, but cases can be presented in comma-separated lists:

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

Here's a comparison routine for byte arrays that uses two `switch` statements:

    // 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
    }

## More to come

### 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.

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