The Go Programming Language Specification Program initialization and execution Program execution

A complete program is created by linking a single, unimported package called the main package with all the packages it imports, transitively. The main package must have package name main and declare a function main that takes no arguments and returns no value.

func main() { … }

The Go Programming Language Specification Program initialization and execution Package initialization

Multiple variables on the left-hand side of a variable declaration initialized by single (multi-valued) expression on the right-hand side are initialized together: If any of the variables on the left-hand side is initialized, all those variables are initialized in the same step.

var x = a
var a, b = f() // a and b are initialized together, before x is initialized

The Go Programming Language Specification Program initialization and execution Package initialization

For example, given the declarations

var (
	a = c + b  // == 9
	b = f()    // == 4
	c = f()    // == 5
	d = 3      // == 5 after initialization has finished
)

func f() int {
	d++
	return d
}

The Go Programming Language Specification Program initialization and execution Package initialization

For instance, given the declarations

var x = I(T{}).ab()   // x has an undetected, hidden dependency on a and b
var _ = sideEffect()  // unrelated to x, a, or b
var a = b
var b = 42

type I interface      { ab() []int }
type T struct{}
func (T) ab() []int   { return []int{a, b} }

The Go Programming Language Specification Program initialization and execution Package initialization

Variables may also be initialized using functions named init declared in the package block, with no arguments and no result parameters.

func init() { … }

The Go Programming Language Specification Program initialization and execution The zero value

the following holds:

t.i == 0
t.f == 0.0
t.next == nil

The Go Programming Language Specification Program initialization and execution The zero value

The same would also be true after

var t T

The Go Programming Language Specification Program initialization and execution The zero value

These two simple declarations are equivalent:

var i int
var i int = 0

The Go Programming Language Specification Program initialization and execution The zero value

After

type T struct { i int; f float64; next *T }
t := new(T)

The Go Programming Language Specification Packages Import declarations

An import declaration states that the source file containing the declaration depends on functionality of the imported package (§Program initialization and execution) and enables access to exported identifiers of that package. The import names an identifier (PackageName) to be used for access and an ImportPath that specifies the package to be imported.

ImportDecl       = "import" ( ImportSpec | "(" { ImportSpec ";" } ")" ) .
ImportSpec       = [ "." | PackageName ] ImportPath .
ImportPath       = string_lit .

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