[Go]2.Declaration

Variable Declarations

Syntax

• VarDecl = "var" ( VarSpec | "(" { VarSpec ";" } ")" ) .
• VarSpec = IdentifierList ( Type [ "=" ExpressionList ] | "=" ExpressionList ) .

Example

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"

1. If no type is specified in the declaration, the default type of the literal or the type of the right hand side is used as the type of the variable.
2. If no value is provided, it will use the zero value of the corresponding type to initalize.

Short Variable Declarations

Syntax

• ShortVarDecl = IdentifierList ":=" ExpressionList .

Example

• It can be used for multiple declaration at the same time: i, j := 0, 10
• Declare a function: f := func() int { return 7 }
• Declare a potiner: ch := make(chan int)
• Omit some values: r, w, _ := os.Pipe() // os.Pipe() returns a connected pair of Files and an error, if any

Attention

• Short variable declaration can't be used for global variable declaration, since it can only appears within blocks.
• It can be used as long as one variable on the left hand side is not initialized, the already declared vairables will be downgraded to assignments.
• field1, offset := nextField(str, 0)
• field2, offset := nextField(str, offset) // still OK, redeclares offset
• We can't do double declaration like: a, a := 1, 2

Reference

1. The Go Programming Language Specification