Introduction

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—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.

Some formatting details remain. Very briefly:

Indentation

We use tabs for indentation and gofmt emits them by default. Use spaces only 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 require 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. 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. In any case, confusion is rare because the file name in the import defines which version is being used.

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. Similarly, the constructor for vector.Vector would normally be called NewVector but since Vector is the only type exported by the package, and since the package is called vector, it's called just New, which clients of the package see as vector.New. 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 use 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 be 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 required 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 _, value := range m {  // key is unused
    sum += value
}

For strings, the range 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

for pos, char := range "日本語" {
    fmt.Printf("character %c starts at byte position %d\n", char, pos)
}

prints

character 日 starts at byte position 0
character 本 starts at byte position 3
character 語 starts at byte position 6

Finally, since Go has no comma operator and ++ and -- are statements not expressions, if you want to run multiple variables in a for you should 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
}

Functions

Multiple return values

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 (-1 for EOF for example) and modifying an argument.

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

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

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

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.

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

You could use it to scan the numbers in an input array a like this:

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

Named result parameters

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 return statement with no arguments, the current values of the result parameters are used as the returned values.

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

func nextInt(b []byte, pos int) (value, nextPos int) {

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

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

Data

Allocation with new()

Go has two allocation primitives, new() and make(). They do different things and apply to different types, which can be confusing, but the rules are simple. Let's talk about new() 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 T and returns its address, a value of type *T. In Go terminology, it returns a pointer to a newly allocated zero value of type T.

Since the memory returned by new() 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 new() and get right to work. For example, the documentation for bytes.Buffer states that "the zero value for Buffer is an empty buffer ready to use." Similarly, 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.

The zero-value-is-useful property works transitively. Consider this type declaration:

type SyncedBuffer struct {
	lock	sync.Mutex;
	buffer	bytes.Buffer;
}

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

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

Constructors and composite literals

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

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

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

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

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:

	return &File{fd, name, nil, 0};

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

	return &File{fd: fd, name: name}

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:

new(File)
&File{}

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 EnoError, Eio, and Einval, as long as they are distinct:

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"};

Allocation with make()

Back to allocation. The built-in function make(T, args) serves a purpose different from new(T). It creates slices, maps, and channels only, and it returns an initialized (not zero) value of type T, not *T. 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 nil. For slices, maps, and channels, make initializes the internal data structure and prepares the value for use. For instance,

make([]int, 10, 100)

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, new([]int) returns a pointer to a newly allocated, zeroed slice structure, that is, a pointer to a nil slice value.

These examples illustrate the difference between new() and make():

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);

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

Arrays

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.

There are major differences between the ways arrays work in Go and C. In Go:

• Arrays are values. Assigning one array to another copies all the elements.
• In particular, if you pass an array to a function, it will receive a copy of the array, not a pointer to it.
• The size of an array is part of its type. The types [10]int and [20]int are distinct.

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:

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

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

But even this style isn't idiomatic Go. Slices are.

Slices

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.

Slices are reference types, 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 Read 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 Read method of the File type in package os:

func (file *File) Read(buf []byte) (n int, err os.Error)

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 b, slice (here used as a verb) the buffer:

	n, err := f.Read(buf[0:32]);

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:

	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;
	}

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 capacity of a slice, accessible by the built-in function cap, 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 len and cap are legal when applied to the nil slice, and return 0.

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;
}

We must return the slice afterwards because, although Append can modify the elements of slice, the slice itself (the run-time data structure holding the pointer, length, and capacity) is passed by value.

Maps

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.

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.

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,
}

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.

var seconds int;
var ok bool;
seconds, ok = timeZone[tz]

For obvious reasons this is called the “comma ok” idiom. In this example, if tz is present, seconds will be set appropriately and ok will be true; if not, seconds will be set to zero and ok will be false. Here's a function that puts it together:

func offset(tz string) int {
	if seconds, ok := timeZone[tz]; ok {
		return seconds
	}
	log.Stderr("unknown time zone", tz);
	return 0;
}

To test for presence in the map without worrying about the actual value, you can use the blank identifier, a simple underscore (_). 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.

_, present := timeZone[tz];

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.

timeZone["PDT"] = 0, false;  // Now on Standard Time

Printing

Formatted printing in Go uses a style similar to C's printf family but is richer and more general. The functions live in the fmt package and have capitalized names: fmt.Printf, fmt.Fprintf, fmt.Sprintf and so on. The string functions (Sprintf etc.) return a string rather than filling in a provided buffer.

You don't need to provide a format string. For each of Printf, fmt.Fprintf and fmt.Sprintf there is another pair of functions, for instance Print and Println. These functions do not take a format string but instead generate a default format for each argument. The ln 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.

fmt.Printf("Hello %d\n", 23);
fmt.Fprint(os.Stdout, "Hello ", 23, "\n");
fmt.Println(fmt.Sprint("Hello ", 23));

Recall that fmt.Fprint and friends take as a first argument any object that implements the io.Writer interface; the variables os.Stdout and os.Stderr are familiar instances.

Here things start to diverge from C. First, the numeric formats such as %d do not take flags for signedness or size; instead, the printing routines use the type of the argument to decide these properties.

var x uint64 = 1<<64 - 1;
fmt.Printf("%d %x; %d %x\n", x, x, int64(x), int64(x));

prints

18446744073709551615 ffffffffffffffff; -1 -1

If you just want the default conversion, such as decimal for integers, you can use the catchall format %v (for “value”); the result is exactly what Print and Println would produce. Moreover, that format can print any value, even arrays, structs, and maps. Here is a print statement for the time zone map defined in the previous section.

fmt.Printf("%v\n", timeZone);  // or just fmt.Println(timeZone);

which gives output

map[CST:-21600 PST:-28800 EST:-18000 UTC:0 MST:-25200]

For maps the keys may be output in any order, of course. When printing a struct, the modified format %+v annotates the fields of the structure with their names, and for any value the alternate format %#v prints the value in full Go syntax.

type T struct {
	a int;
	b float;
	c string;
}
t := &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);

prints

&{7 -2.35 abc   def}
&{a:7 b:-2.35 c:abc     def}
&main.T{a:7, b:-2.35, c:"abc\tdef"}
map[string] int{"CST":-21600, "PST":-28800, "EST":-18000, "UTC":0, "MST":-25200}

(Note the ampersands.) That quoted string format is also available through %q when applied to a value of type string or []byte; the alternate format %#q will use backquotes instead if possible. Also, %x works on strings and arrays of bytes as well as on integers, generating a long hexadecimal string, and with a space in the format (% x) it puts spaces between the bytes.

Another handy format is %T, which prints the type of a value.

fmt.Printf("%T\n", timeZone);

prints

map[string] int

If you want to control the default format for a custom type, all that's required is to define a method String() string on the type. (Methods are the subject of the next section.) For our simple type T, that might look like this.

func (t *T) String() string {
	return fmt.Sprintf("%d/%g/%q", t.a, t.b, t.c);
}
fmt.Printf("%v\n", t);

to print in the format

7/-2.35/"abc\tdef"

Our String() method is able to call Sprintf 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 Printf uses the ... type for its final argument to specify that an arbitrary number of parameters can appear after the format.

func Printf(format string, v ...) (n int, errno os.Error) {

Within the function Printf, v is a variable that can be passed, for instance, to another print routine. Here is the implementation of the function log.Stderr we used above. It passes its arguments directly to fmt.Sprintln for the actual formatting.

// 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)
}

There's even more to printing than we've covered here. See the godoc documentation for package fmt for the details.

Methods

Pointers vs. Values

Methods can be defined for any named type except pointers and interfaces; the receiver does not have to be a struct.

In the discussion of slices above, we wrote an Append 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.

type ByteSlice []byte

func (slice ByteSlice) Append(data []byte) []slice {
	// Body exactly the same as above
}

This still requires the method to return the updated slice. We can eliminate that clumsiness by redefining the method to take a pointer to a ByteSlice as its receiver, so the method can overwrite the caller's slice.

func (p *ByteSlice) Append(data []byte) {
	slice := *p;
	// Body as above, without the return.
	*p = slice;
}

In fact, we can do even better. If we modify our function so it looks like a standard Write method, like this,

func (p *ByteSlice) Write(data []byte) (n int, err os.Error) {
	slice := *p;
	// Again as above.
	*p = slice;
	return len(data), nil)
}

then the type *ByteSlice satisfies the standard interface io.Writer, which is handy. For instance, we can print into one:

	var b ByteSlice;
	fmt.Fprintf(&b, "This hour has %d days\n", 7);

Notice that we must pass the address of a ByteSlice because only *ByteSlice satisfies io.Writer. 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.

By the way, the idea of using Write on a slice of bytes is implemented by bytes.Buffer.

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 idiosyncrasies 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 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.

TODO
verifying implementation
type Color uint32

// Check that Color implements image.Color and image.Image
var _ image.Color = Black
var _ image.Image = Black