Short variable declaration rules

This is a spin-off post from my main post of Visual how to guide to Go variables, go check it out.

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👉 You can’t use it to declare package-level variables.

illegal := 42
func foo() {
  legal := 42
}

👉 You can’t use it twice:

legal := 42
legal := 42 // <-- error

Because, := introduces "a new variable", hence using it twice does not redeclare a second variable, so it's illegal.

👉 You can use them twice in “multi-variable” declarations, if one of the variables are new:

foo, bar  := someFunc()
foo, jazz := someFunc()  // <-- jazz is new
baz, foo  := someFunc()  // <-- baz is new

This is legal, because, you’re not redeclaring variables, you’re just reassigning new values to the existing variables, with some new variables.

👉 You can use them if a variable already declared with the same name before:

var foo int = 34
func some() {
  // because foo here is scoped to some func
  foo := 42  // <-- legal
  foo = 314  // <-- legal
}

Here, foo := 42 is legal, because, it redeclares foo in some() func's scope. foo = 314 is legal, because, it just reassigns a new value to foo.

👉 You can use them for multi-variable declarations and assignments:

foo, bar   := 42, 314
jazz, bazz := 22, 7

👉 You can reuse them in scoped statement contexts like if, for, switch:

foo := 42

if foo := someFunc(); foo == 314 {
  // foo is scoped to 314 here
  // ...
}

// foo is still 42 here

Because, if foo := ... assignment, only accessible to that if clause.

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Originally published on Go Short Variable Declaration Rules in Learn Go Programming.