optional//2

Availability::- use_module(library(dcg/high_order)).

[det]optional(:Match, :Default)//

Perform an optional match, executing Default if Match is not matched. This is comparable to ignore/1. Both Match and Default are DCG body terms. Default is typically used to instantiate the output variables of Match, but may also be used to match a default representation. Using [] for Default succeeds without any additional actions if Match fails. For example:

?- phrase(optional(number(X), {X=0}), `23`, Tail).
X = 23,
Tail = [].
?- phrase(optional(number(X), {X=0}), `aap`, Tail).
X = 0,
Tail = `aap`.