Short variable declaration

Short variable declaration is a very convenient manner of "declaring variable" in Go:

i := 10

It is shorthand of following (Please notice there is no type):

var i = 10

The Go compiler will infer the type according to the value of variable. It is a very handy feature, but on the other side of coin, it also brings some pitfalls which you should pay attention to:

(1) This format can only be used in functions:

package main

i := 10

func main() {
    fmt.Println(i)
}

The compiler will complain the following words:

syntax error: non-declaration statement outside function body

(2) You must declare at least 1 new variable:

package main

import "fmt"

func main() {
    var i = 1

    i, err := 2, true

    fmt.Println(i, err)
}

In i, err := 2, false statement, only err is a new declared variable, var is actually declared in var i = 1.

(3) The short variable declaration can shadow the global variable declaration, and it may not be what you want, and gives you a big surprise:

package main

import "fmt"

var i = 1

func main() {

    i, err := 2, true

    fmt.Println(i, err)
}

i, err := 2, true actually declares a new local i which makes the global i inaccessible in main function. To use the global variable but not introducing a new local one, one solution maybe like this:

package main

import "fmt"

var i int

func main() {

    var err bool

    i, err = 2, true

    fmt.Println(i, err)
}

Reference：
Short variable declarations.