high_order.pl -- High order grammar operations

This library provides facilities comparable maplist/3, ignore/1 and foreach/2 for DCGs.

STATUS: This library is experimental. The interface and implementation may change based on feedback. Please send feedback to the mailinglist or the issue page of the swipl-devel.git repository.

 sequence(:Element, ?List)// is nondet

Match or generate a sequence of Element. This predicate is deterministic if List is fully instantiated and Element is deterministic. When parsing, this predicate is gready and does not prune choice points. For example:

?- phrase(sequence(digit, Digits), `123a`, L).
Digits = "123",
L = [97] ;
Digits = [49, 50],
L = [51, 97] ;
...

 sequence(:Element, :Sep, ?List)// is nondet

Match or generate a sequence of Element where each pair of elements is separated by Sep. When parsing, a matched Sep commits. The final element is not committed. More formally, it matches the following sequence:

(Element, (Sep,Element)*)?

See also sequence//5.

 sequence(:Start, :Element, :Sep, :End, ?List)// is semidet

Match or generate a sequence of Element enclosed by Start end End, where each pair of elements is separated by Sep. More formally, it matches the following sequence:

Start, (Element, (Sep,Element)*)?, End

The example below matches a Prolog list of integers:

?- phrase(sequence(("[",blanks),
                   number, (",",blanks),
                   (blanks,"]"), L),
                   `[1, 2, 3 ] a`, Tail).
L = [1, 2, 3],
Tail = [32, 97].

 optional(:Match, :Default)// is det

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`.

 foreach(:Generator, :Element)// is det

 foreach(:Generator, :Element, :Sep)// is det

Generate a list from the solutions of Generator. This predicate collects all solutions of Generator, applies Element for each solution and Sep between each pair of solutions. For example:

?- phrase(foreach(between(1,5,X), number(X), ", "), L).
L = "1, 2, 3, 4, 5".