Why does Golang handle closures differently in goroutines?

Question

Consider the following Golang code (also on the Go Playground):

package main

import \"fmt\"
import \"time\"

func main() {
    for _, s := range []string{\"         


        

Answer 1

Closures in Go are lexically scoped. This means that any variables referenced within the closure from the "outer" scope are not a copy but are in fact a reference. A for loop actually reuses the same variable multiple times, so you're introducing a race condition between the read/write of the s variable.

But x is allocating a new variable (with the :=) and copying s, which results in that being the correct result every time.

In general, it is a best practice to pass in any arguments you want so that you don't have references. Example:

for _, s := range []string{"foo", "bar"} {
    x := s
    go func(s string) {
        fmt.Printf("s: %s\n", s)
        fmt.Printf("x: %s\n", x)
    }(s)
}