What are closures in Go and what is the loop variable capture gotcha?

A closure is a function that references and closes over variables from its surrounding scope. The closure captures the variable itself — not a copy of its value at the moment of creation. This distinction is the source of one of the most common Go bugs.

// Closure capturing outer variable
func makeCounter() func() int {
    count := 0
    return func() int {
        count++         // captures 'count' — reads its current value each call
        return count
    }
}

c := makeCounter()
fmt.Println(c(), c(), c())  // 1 2 3

c2 := makeCounter()         // independent counter
fmt.Println(c2())           // 1 (fresh)

// CLASSIC GOTCHA: loop variable capture
funcs := make([]func(), 3)
for i := 0; i < 3; i++ {
    funcs[i] = func() { fmt.Println(i) }  // captures &i, not a copy!
}
funcs[0]()  // prints 3 (not 0!)
funcs[1]()  // prints 3
funcs[2]()  // prints 3
// By the time any func runs, the loop ended and i == 3

// FIX 1: pass i as an argument (creates a per-iteration copy)
for i := 0; i < 3; i++ {
    go func(n int) { fmt.Println(n) }(i)  // n is a copy of i
}

// FIX 2: shadow i with a new variable per iteration
for i := 0; i < 3; i++ {
    i := i  // new 'i' that belongs to this iteration
    funcs[i] = func() { fmt.Println(i) }
}

// Go 1.22+: loop variables are per-iteration by default — bug gone!

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