defer, precisely in Go

Deferred calls run last-in-first-out, with arguments evaluated immediately.

func order() []string {
    var log []string
    record := func(s string) { log = append(log, s) }

    value := "first"
    defer record("deferred with " + value) // captured now
    defer func() { record("closure sees " + value) }()
    value = "second"

    record("body")
    return log // note: defers run after this is evaluated
}

func cleanup() (steps []string) {
    defer func() { steps = append(steps, "outer") }()
    defer func() { steps = append(steps, "inner") }()
    return []string{"body"}
}

How it works

1. The argument is captured at the defer statement, not at the call.
2. A closure sees the variable's final value instead.
3. Defers run on every path out, including a panic.

Keywords and builtins used here

• append
• defer
• func
• return
• string
• var

The run, in numbers

Lines

18

Characters to type

465

Tokens

117

Three-star pace

105 tpm

At the three-star pace of 105 tokens a minute, this run takes about 67 seconds.

Type this snippet

Step 2 of 4 in defer, panic & recover, step 11 of 15 in Errors.

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