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The wait operation pauses the current routine until the supplied value is finished. It has two behaviours, determined by the type of the single operand:

• Task operand If the value is a task object produced by async, wait blocks until that task completes. It always returns a two-item list. The first item is true if the task ended normally or false if the task ended with throw. The second item is either the task’s return value (on success) or the stringified error message (on failure). wait itself never throws when task fails.

• Numeric operand If the value is a number, it is interpreted as a duration in milliseconds. wait sleeps for that many milliseconds and then returns the Boolean true.

Any other operand type causes wait to raise an error.

The syntax of wait is:

[ $destination ] wait value

$destination is optional; if it is omitted, the result is assigned to the special slot _. Exactly one operand must be provided. If you write wait with no operand it is treated as wait _.

Examples

Pause for 200 ms:

wait 200
log "0.2 seconds elapsed"

Launch work in the background and wait for it safely:

$task async buildReport "today"
$result wait $task        # result is [flag value]

$flag  first $result
$val   second $result

$success compile "log report done: $val"
$failure compile "log report failed: $val"
if $flag $success $failure

Defaulting to _ when the current value is a task:

$job async crunchData
wait                    # same as wait $job

Errors

wait raises an error when the operand is neither a task object nor a number, or when no operand is available after the _ shorthand is expanded. The operation never mutates its input; it always returns a new value that follows single-assignment rules.