Closures in Go

A function value captures the variables it uses, not their values.

func counter(start int) func() int {
    count := start
    return func() int {
        count++
        return count
    }
}

func adders() []int {
    var funcs []func() int
    for i := 1; i <= 3; i++ {
        funcs = append(funcs, func() int { return i * 10 })
    }

    var out []int
    for _, f := range funcs {
        out = append(out, f())
    }
    return out
}

How it works

1. Each call to the outer function gets its own captured state.
2. Returning a closure is how Go writes a counter or a generator.
3. Since Go 1.22 each loop iteration has its own variable.

Keywords and builtins used here

• append
• for
• func
• int
• range
• return
• var

The run, in numbers

Lines

20

Characters to type

301

Tokens

99

Three-star pace

90 tpm

At the three-star pace of 90 tokens a minute, this run takes about 66 seconds.

Type this snippet

Step 3 of 6 in Functions, step 12 of 30 in Language basics.

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Closures in other languages

• Rust