## A.13 library(dcg/high_order): 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.

[nondet]sequence(:Element, ?List)`//`
  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] ;
      ...

[nondet]sequence(:Element, :Sep, ?List)`//`
  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.

[semidet]sequence(:Start, :Element, :Sep, :End, ?List)`//`
  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].

[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`.

[det]foreach(:Generator, :Element)`//`
[det]foreach(:Generator, :Element, :Sep)`//`
  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".