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Concurrency · 3 MIN READ

default makes select try, not wait

Receive from a channel if a value is ready, and return if it is not.

The Tour's "Default Selection" page: a default case runs when no other case is ready, so the select returns instead of blocking. That is how you poll a channel, and how you try a send without parking the goroutine.

Without default, select waits. With it, "nothing ready" is a value you handle, not a hang. A busy loop of default-selects is a spin; use it for a single try, not as a substitute for a timer.

SEE THE IDEA

Default means continue immediately when nothing is ready

  1. 01

    Ready case

    go
    case value := <-ch:

    Receive without waiting

  2. 02

    No ready case

    go
    default:

    Take the fallback immediately

  3. 03

    Blocking alternative

    go
    select without default

    Wait until a case can proceed

Default changes a blocking select into a try operation.

In practice

Use a default case when a skipped operation is part of the contract, such as a best-effort status update. A loop with default can spin continuously when no work is ready. Prefer a blocking select with cancellation or an explicit timing source for a waiting loop.

Watch out

Default does not run after a timeout and does not wait briefly. It is selected immediately when no communication is ready.

Official reference ↗

Work through the exercise

go
package main

// TryRecv returns the next value and true when one is ready,
// or 0 and false without waiting.
func TryRecv(ch <-chan int) (int, bool) {
	return <-ch, true
}

func main() {}
Try it yourself

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