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.
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 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:
gofmt emits them by default.
Use spaces if you must.
if,
for, switch) do not have parentheses in
their syntax.
Also, the operator precedence hierarchy is shorter and clearer, so
x<<8 + y<<16means what the spacing implies.
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 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.
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.
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.
Finally, the convention in Go is to used MixedCaps
or mixedCaps rather than underscores to write
multiword names.
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.
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.
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);
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
}
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]
}
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
}
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 program: 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 overwrite an argument. 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);
}
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 for their type 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;
}
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.
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.