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.
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.
Some formatting details remain. Very briefly,
gofmt emits them by default.
Use spaces only if you must.
if,
for, switch) do not require 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. 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.
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 function to make new instances of vector.Vector
—which is the definition of a constructor in Go—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.
Clients of the package see that 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.
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 use 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 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.
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.
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 _, 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]
}
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
}
A switch can also be used to discover the dynamic type of an interface
variable. Such a type switch uses the syntax of a type
assertion with the keyword type inside the parentheses.
If the switch declares a variable in the expression, the variable will
have the corresponding type in each clause.
switch t := interfaceValue.(type) {
default:
fmt.Printf("unexpected type");
case bool:
fmt.Printf("boolean %t\n", t);
case int:
fmt.Printf("integer %d\n", t);
case *bool:
fmt.Printf("pointer to boolean %t\n", *t);
case *int:
fmt.Printf("pointer to integer %d\n", *t);
}
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);
}
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;
}
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
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"};
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 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,
[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 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 underlying 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 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
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.
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 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);
We 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.
Interfaces in Go provide a way to specify the behavior of an
object: if something can do this, then it can be used
here. We've seen a couple of simple examples already;
custom printers can be implemented by a String method
while Fprintf can generate output to anything
with a Write method.
Interfaces with only one or two methods are common in Go code, and are
usually given a name derived from the method, such as io.Writer
for something that implements Write.
A type can implement multiple interfaces.
For instance, a collection can be sorted
by the routines in package sort if it implements
sort.Interface, which contains Len(),
Less(i, j int), and Swap(i, j int),
and it could also have a custom formatter.
In this contrived example Sequence satisfies both.
type Sequence []int
// Methods required by sort.Interface.
func (s Sequence) Len() int {
return len(s)
}
func (s Sequence) Less(i, j int) bool {
return s[i] < s[j]
}
func (s Sequence) Swap(i, j int) {
s[i], s[j] = s[j], s[i]
}
// Method for printing - sorts the elements before printing.
func (s Sequence) String() string {
sort.Sort(s);
str := "[";
for i, elem := range s {
if i > 0 {
str += " "
}
str += fmt.Sprint(elem);
}
return str + "]";
}
The String method of Sequence is recreating the
work that Sprint already does for slices. We can share the
effort if we convert the Sequence to a plain
[]int before calling Sprint.
func (s Sequence) String() string {
sort.Sort(s);
return fmt.Sprint([]int(s));
}
The conversion causes s to be treated as an ordinary slice
and therefore receive the default formatting.
Without the conversion, Sprint would find the
String method of Sequence and recur indefinitely.
Because the two types (Sequence and []int)
are the same if we ignore the type name, it's legal to convert between them.
The conversion doesn't create a new value, it just temporarily acts
as though the existing value has a new type.
(There are other legal conversions, such as from integer to float, that
do create a new value.)
It's an idiom of Go code to convert the
type of an expression to access a different
set of methods. As an example, we could use the existing
type sort.IntArray to reduce the entire example
to this:
type Sequence []int
// Method for printing - sorts the elements before printing
func (s Sequence) String() string {
sort.IntArray(s).Sort();
return fmt.Sprint([]int(s))
}
Now, instead of having Sequence implement multiple
interfaces (sorting and printing), we're using the ability of a data item to be
converted to multiple types (Sequence, sort.IntArray
and []int), each of which does some part of the job.
That's more unusual in practice but can be effective.
If a type exists only to implement an interface and has no exported methods beyond that interface, there is no need to export the type itself. Exporting just the interface makes it clear that it's the behavior that matters, not the implementation, and that other implementations with different properties can mirror the behavior of the original type. It also avoids the need to repeat the documentation on every instance of a common method.
In such cases, the constructor should return an interface value
rather than the implementing type.
As an example, in the hash libraries
both crc32.NewIEEE() and adler32.New()
return the interface 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.
A similar approach allows the streaming cipher algorithms
in the crypto/block package to be
separated from the block ciphers they chain together.
By analogy to the bufio package,
they wrap a Cipher interface
and they return hash.Hash,
io.Reader, or io.Writer
interface values, not specific implementations.
The interface to crypto/block includes:
type Cipher interface {
BlockSize() int;
Encrypt(src, dst []byte);
Decrypt(src, dst []byte);
}
// NewECBDecrypter returns a reader that reads data
// from r and decrypts it using c in electronic codebook (ECB) mode.
func NewECBDecrypter(c Cipher, r io.Reader) io.Reader
// NewCBCDecrypter returns a reader that reads data
// from r and decrypts it using c in cipher block chaining (CBC) mode
// with the initialization vector iv.
func NewCBCDecrypter(c Cipher, iv []byte, r io.Reader) io.Reader
NewECBDecrypter and NewCBCReader apply not
just to one specific encryption algorithm and data source but to any
implementation of the Cipher interface and any
io.Reader. Because they return io.Reader
interface values, replacing ECB
encryption with CBC encryption is a localized change. The constructor
calls must be edited, but because the code must treat the result only
as an io.Reader, it won't notice the difference.
Since almost anything can have methods attached, almost anything can
satisfy an interface. One illustrative example is in the http
package, which defines the Handler interface. Any object
that implements Handler can serve HTTP requests.
type Handler interface {
ServeHTTP(*Conn, *Request);
}
For brevity, let's ignore POSTs and assume HTTP requests are always GETs; that simplification does not affect the way the handlers are set up. Here's a trivial but complete implementation of a handler to count the number of times the page is visited.
// Simple counter server.
type Counter struct {
n int;
}
func (ctr *Counter) ServeHTTP(c *http.Conn, req *http.Request) {
ctr.n++;
fmt.Fprintf(c, "counter = %d\n", ctr.n);
}
(Keeping with our theme, note how Fprintf can print to an HTTP connection.)
For reference, here's how to attach such a server to a node on the URL tree.
import "http"
...
ctr := new(Counter);
http.Handle("/counter", ctr);
But why make Counter a struct? An integer is all that's needed.
(The receiver needs to be a pointer so the increment is visible to the caller.)
// Simpler counter server.
type Counter int
func (ctr *Counter) ServeHTTP(c *http.Conn, req *http.Request) {
*ctr++;
fmt.Fprintf(c, "counter = %d\n", *ctr);
}
What if your program has some internal state that needs to be notified that a page has been visited? Tie a channel to the web page.
// A channel that sends a notification on each visit.
// (Probably want the channel to be buffered.)
type Chan chan *http.Request
func (ch Chan) ServeHTTP(c *http.Conn, req *http.Request) {
ch <- req;
fmt.Fprint(c, "notification sent");
}
Finally, let's say we wanted to present on /args the arguments
used when invoking the server binary.
It's easy to write a function to print the arguments.
func ArgServer() {
for i, s := range os.Args {
fmt.Println(s);
}
}
How do we turn that into an HTTP server? We could make ArgServer
a method of some type whose value we ignore, but there's a cleaner way.
Since we can define a method for any type except pointers and interfaces,
we can write a method for a function.
The http package contains this code:
// The HandlerFunc type is an adapter to allow the use of
// ordinary functions as HTTP handlers. If f is a function
// with the appropriate signature, HandlerFunc(f) is a
// Handler object that calls f.
type HandlerFunc func(*Conn, *Request)
// ServeHTTP calls f(c, req).
func (f HandlerFunc) ServeHTTP(c *Conn, req *Request) {
f(c, req);
}
HandlerFunc is a type with a method, ServeHTTP,
so values of that type can serve HTTP requests. Look at the implementation
of the method: the receiver is a function, f, and the method
calls f. That may seem odd but it's no different from, say,
the receiver being a channel and the method sending on the channel.
To make ArgServer into an HTTP server, we first modify it
to have the right signature.
// Argument server.
func ArgServer(c *http.Conn, req *http.Request) {
for i, s := range os.Args {
fmt.Fprintln(c, s);
}
}
ArgServer now has same signature as HandlerFunc,
so it can be converted to that type to access its methods,
just as we converted Sequence to IntArray
to access IntArray.Sort.
The code to set it up is concise:
http.Handle("/args", http.HandlerFunc(ArgServer));
When someone visits the page /args,
the handler installed at that page has value ArgServer
and type HandlerFunc.
The HTTP server will invoke the method ServeHTTP
of that type, with ArgServer as the receiver, which will in turn call
ArgServer (via the invocation f(c, req)
inside HandlerFunc.ServeHTTP).
The arguments will then be displayed.
In this section we have made an HTTP server from a struct, an integer, a channel, and a function, all because interfaces are just sets of methods, which can be defined for (almost) any type.
Go does not provide the typical, type-driven notion of subclassing, but it does have the ability to “borrow” pieces of an implementation by embedding types within a struct or interface.
Interface embedding is very simple.
We've mentioned the io.Reader and io.Writer interfaces before;
here are their definitions.
type Reader interface {
Read(p []byte) (n int, err os.Error);
}
type Writer interface {
Write(p []byte) (n int, err os.Error);
}
The io package also exports several other interfaces
that specify objects that can implement several such methods.
For instance, there is io.ReadWriter, an interface
containing both Read and Write.
We could specify io.ReadWriter by listing the
two methods explicitly, but it's easier and more evocative
to embed the two interfaces to form the new one, like this:
// ReadWrite is the interface that groups the basic Read and Write methods.
type ReadWriter interface {
Reader;
Writer;
}
This says just what it looks like: A ReadWriter can do
what a Reader does and what a Writer
does; it is a union of the embedded interfaces (which must be disjoint
sets of methods).
Only interfaces can be embedded within interfaces.
The same basic idea applies to structs, but with more far-reaching
implications. The bufio package has two struct types,
bufio.Reader and bufio.Writer, each of
which of course implements the analogous interfaces from package
io.
And bufio also implements a buffered reader/writer,
which it does by combining a reader and a writer into one struct
using embedding: it lists the types within the struct
but does not give them field names.
// ReadWriter stores pointers to a Reader and a Writer.
// It implements io.ReadWriter.
type ReadWriter struct {
*Reader;
*Writer;
}
This struct could be written as
type ReadWriter struct {
reader *Reader;
writer *Writer;
}
but then to promote the methods of the fields and to
satisfy the io interfaces, we would also need
to provide forwarding methods, like this:
func (rw *ReadWriter) Read(p []byte) (n int, err os.Error) {
return rw.reader.Read(p)
}
By embedding the structs directly, we avoid this bookkeeping.
The methods of embedded types come along for free, which means that bufio.ReadWriter
not only has the methods of bufio.Reader and bufio.Writer,
it also satisfies all three interfaces:
io.Reader,
io.Writer, and
io.ReadWriter.
There's one important way in which embedding differs from subclassing. When we embed a type,
the methods of that type become methods of the outer type,
but when they are invoked the receiver of the method is the inner type, not the outer one.
In our example, when the Read method of a bufio.ReadWriter is
invoked, it has the exactly the same effect as the forwarding method written out above;
the receiver is the reader field of the ReadWriter, not the
ReadWriter itself.
Library routines must often return some sort of error indication to
the caller. As mentioned earlier, Go's multivalue return makes it
easy to return a detailed error description alongside the normal
return value. By convention, errors have type os.Error,
a simple interface.
type Error interface {
String() string;
}
A library writer is free to implement this interface with a
richer model under the covers, making it possible not only
to see the error but also to provide some context.
For example, os.Open returns an os.PathError.
// PathError records an error and the operation and
// file path that caused it.
type PathError struct {
Op string; // "open", "unlink", etc.
Path string; // The associated file.
Error Error; // Returned by the system call.
}
func (e *PathError) String() string {
return e.Op + " " + e.Path + ": " + e.Error.String();
}
PathError's String generates
a string like this:
open /etc/passwx: no such file or directory
Such an error, which includes the problematic file name, the operation, and the operating system error it triggered, is useful even if printed far from the call that caused it; it is much more informative than the plain "no such file or directory".
Callers that care about the precise error details can
use a type switch or a type assertion to look for specific
errors and extract details. For PathErrors
this might include examining the internal Error
field for recoverable failures.
for try := 0; try < 2; try++ {
file, err = os.Open(filename, os.O_RDONLY, 0);
if err == nil {
return
}
if e, ok := err.(*os.PathError); ok && e.Error == os.ENOSPC {
deleteTempFiles(); // Recover some space.
continue
}
return
}