类型声明包括：常量、自定义类型、变量、函数、标签、还有包声明。
程序中的标识符都必须声明。块、包、文件中的标识符不能重复。

Declaration   = ConstDecl | TypeDecl | VarDecl .
TopLevelDecl  = Declaration | FunctionDecl | MethodDecl .

标签作用域

标签是在标签语句中使用，常用在break、continue、goto语句中。定义了标签但不使用是非法的。与其他标识符相比，标签是非块作用域，不会与其他非标签标识符有冲突。

预声明标识符

如下的标识符在golang中已保留

Types:
    bool byte complex64 complex128 error float32 float64
    int int8 int16 int32 int64 rune string
    uint uint8 uint16 uint32 uint64 uintptr

Constants:
    true false iota

Zero value:
    nil

Functions:
    append cap close complex copy delete imag len
    make new panic print println real recover

可导出方法或字段

一个标识符可导出让另一个包访问，只需要满足

• 首字母大写
• 在包中申明的字段或方法

常量声明

用const 关键字

ConstDecl = "const" ( ConstSpec | "(" { ConstSpec ";" } ")" ) 
ConstSpec = IdentifierList [ [ Type ] "=" ExpressionList ] 
IdentifierList = identifier { "," identifier } .
ExpressionList = Expression { "," Expression } .

const Pi float64 = 3.14159265358979323846
const zero = 0.0         // untyped floating-point constant
const (
    size int64 = 1024
    eof        = -1  // untyped integer constant
)
const a,b,c = 3, 4,"foo"  // a = 3,b = 4,c = "foo",untyped integer and string constants
const u,v float32 = 0, 3    // u = 0.0,v = 3.0

Itoa(枚举)

itoa表示连续的无类型的整形常量，从0开始计算，当又有itoa出现时，又重新置为0.

const (  // iota is reset to 0
    c0 = iota  // c0 == 0
    c1 = iota  // c1 == 1
    c2 = iota  // c2 == 2
)

const (
    a = 1 << iota  // a == 1 (iota has been reset)
    b = 1 << iota  // b == 2
    c = 1 << iota  // c == 4
)

const (
    u         = iota * 42  // u == 0 (untyped integer constant)
    v float64 = iota * 42  // v == 42.0 (float64 constant)
    w         = iota * 42  // w == 84 (untyped integer constant)
)

const x = iota  // x == 0 (iota has been reset)
const y = iota  // y == 0 (iota has been reset)

类型声明

用type 来定义一个新的类型

TypeDecl     = "type" ( TypeSpec | "(" { TypeSpec ";" } ")" ) .
TypeSpec     = identifier Type .
type IntArray [16]int

type ( Point struct{ x,y float64 } Polar Point )

type TreeNode struct { left,right *TreeNode value *Comparable }

type Block interface { BlockSize() int Encrypt(src,dst []byte) Decrypt(src,dst []byte) }

自定义类型不会继承原有类型的方法，但接口方法或组合类型的元素则保留原有的方法。

// A Mutex is a data type with two methods,Lock and Unlock.
type Mutex struct { /* Mutex fields */ }
func (m *Mutex) Lock()    { /* Lock implementation */ }
func (m *Mutex) Unlock()  { /* Unlock implementation */ }

// NewMutex has the same composition as Mutex but its method set is empty.
type NewMutex Mutex

// The method set of the base type of PtrMutex remains unchanged,
// but the method set of PtrMutex is empty.
type PtrMutex *Mutex

// The method set of *PrintableMutex contains the methods
// Lock and Unlock bound to its anonymous field Mutex.
type PrintableMutex struct { Mutex }

// MyBlock is an interface type that has the same method set as Block.
type MyBlock Block

自定义类型可用于布尔、数值、或字符串类型，还可为其附加方法

type TimeZone int

const (
    EST TimeZone = -(5 + iota)
    CST
    MST
    PST
)

func (tz TimeZone) String() string {
    return fmt.Sprintf("GMT%+dh",tz)
}

变量声明表示创建一个或多个变量，然后为之绑定相关类型，并赋之于初始值。

VarDecl     = "var" ( VarSpec | "(" { VarSpec ";" } ")" ) .
VarSpec     = IdentifierList ( Type [ "=" ExpressionList ] | "=" ExpressionList ) .
var i int
var U,V,W float64
var k = 0
var x,y float32 = -1, -2
var (
    i       int
    u,v,s = 2.0, 3.0,"bar"
)
var re,im = complexSqrt(-1)
var _,found = entries[name]  // map lookup; only interested in "found"

如果变量在声明但未使用，编译器会报错

短变量声明

短变量声明语法

ShortVarDecl = IdentifierList ":=" ExpressionList .

i,j := 0, 10
f := func() int { return 7 }
ch := make(chan int)
r,w := os.Pipe(fd)  // os.Pipe() returns two values
_,y,_ := coord(p)  // coord() returns three values; only interested in y coordinate

但，短变量声明只能在函数内部，但在if、for、switch语句中可声明为临时变量。

函数声明

语法如下

FunctionDecl = "func" FunctionName ( Function | Signature ) 
FunctionName = identifier 
Function     = Signature FunctionBody 
FunctionBody = Block

方法声明

方法是带接受者的函数。语法如下

MethodDecl   = "func" Receiver MethodName ( Function | Signature ) 
Receiver     = Parameters

接收者可以示T或*T类型

func (p *Point) Length() float64 {
    return math.Sqrt(p.x * p.x + p.y * p.y)
}

func (p *Point) Scale(factor float64) {
    p.x *= factor
    p.y *= factor
}