Golang in sixty seconds — scope

Where you define a variable is very important depending on where you want to use it. The range of places that you are allowed to use the variable is called thescope of the variable.

According to the language specification “Go is lexically scoped using blocks”. For us mere mortals, this means that you can use a variable from within the nearest curly braces ({}), called a “block”. Let's look at a few examples:

func main() {
  x:= "Hi"
  fmt.Println(x) // Hi
}

This works because x is defined within the same block (curly braces) as it is being used. If we were to add another function to our program below main it would not have access to x:

func f() {
  fmt.Println(x) // this would error
}

However, we could define the variable in the global scope of the program and it would be available to both functions:

var x = "Hi"
func main() {
  fmt.Println(x) // Hi
}
func f() {
  fmt.Println(x) // Hi
}

Global variables should be used with caution since variables are mutable in Go. This means that you could define a variable in the global scope, and have a function change this variable, making your program more difficult to understand. You will more often see constants (const) rather than variables defined in global scope for this reason.

Local scope will take preference over the global variables, so you could redefine x within one of your functions to affect only that local scope:

var x = "Hi"
func main() {
  x:= "Hello"
  fmt.Println(x) // Hello
}
func f() {
  fmt.Println(x) // Hi
}

Again, note the difference between what you could do and what you should do. Think about the readability of your code and make it as easy to understand as possible.

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