Short variable declaration

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

  1. i := 10

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

  1. 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: 

  1. package main
  3. i := 10
  5. func main() {
  6.     fmt.Println(i)
  7. }

The compiler will complain the following words: 

  1. syntax error: non-declaration statement outside function body

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

  1. package main
  3. import "fmt"
  5. func main() {
  6.     var i = 1
  8.     i, err := 2, true
  10.     fmt.Println(i, err)
  11. }

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: 

  1. package main
  3. import "fmt"
  5. var i = 1
  7. func main() {
  9.     i, err := 2, true
  11.     fmt.Println(i, err)
  12. }

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: 

  1. package main
  3. import "fmt"
  5. var i int
  7. func main() {
  9.     var err bool
  11.     i, err = 2, true
  13.     fmt.Println(i, err)
  14. }

Reference：
Short variable declarations.