This post is about declaration scopes and shadowing in Go.
package main
import "fmt"
func f(x int) {
for x := 0; x < 10; x++ {
fmt.Println(x)
}
}
var x int
func main() {
var x = 200
f(x)
}
This program declares x four times. All four are different variables because they exist in different scopes.
package main
import "fmt"
func f() {
x := 200
fmt.Println("inside f: x =", x)
}
func main() {
x := 100
fmt.Println("inside main: x =", x)
f()
fmt.Println("inside main: x =", x)
}
In Go the scope of a declaration is bound to the closest pair of curly braces, { and }. In this example, we declare x to be 100 inside main, and 200 inside f.
What do you expect this program will print?
package main
import "fmt"
func main() {
x := 100
for i := 0; i < 5; i++ {
x := i
fmt.Println(x)
}
fmt.Println(x)
}
There are several scopes in a Go program; block scope, function scope, file scope, package scope, and universe scope. Each scope encompasses the previous. What you are seeing is called shadowing.
var x = 100
func main() {
var x = 200
fmt.Println(x)
}
Most developers are comfortable with a function scoped variable shadowing a package scoped variable.
func f() {
var x = 99
if x > 90 {
x := 60
fmt.Println(x)
}
}
But a block scoped variable shadowing a function scoped variable may be surprising.
The justification for a declaration in one scope shadowing another is consistency, prohibiting just block scoped declarations from shadowing another scope, would be inconsistent.
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