- Documentation
- Reference manual
- The SWI-Prolog library
- library(aggregate): Aggregation operators on backtrackable predicates
- library(ansi_term): Print decorated text to ANSI consoles
- library(apply): Apply predicates on a list
- library(assoc): Association lists
- library(broadcast): Broadcast and receive event notifications
- library(charsio): I/O on Lists of Character Codes
- library(check): Consistency checking
- library(clpb): CLP(B): Constraint Logic Programming over Boolean Variables
- library(clpfd): CLP(FD): Constraint Logic Programming over Finite Domains
- library(clpqr): Constraint Logic Programming over Rationals and Reals
- library(csv): Process CSV (Comma-Separated Values) data
- library(dcg/basics): Various general DCG utilities
- library(dcg/high_order): High order grammar operations
- library(debug): Print debug messages and test assertions
- library(dicts): Dict utilities
- library(error): Error generating support
- library(gensym): Generate unique identifiers
- library(intercept): Intercept and signal interface
- library(iostream): Utilities to deal with streams
- library(listing): List programs and pretty print clauses
- library(lists): List Manipulation
- library(main): Provide entry point for scripts
- library(nb_set): Non-backtrackable set
- library(www_browser): Activating your Web-browser
- library(occurs): Finding and counting sub-terms
- library(option): Option list processing
- library(optparse): command line parsing
- library(ordsets): Ordered set manipulation
- library(pairs): Operations on key-value lists
- library(persistency): Provide persistent dynamic predicates
- library(pio): Pure I/O
- library(predicate_options): Declare option-processing of predicates
- library(prolog_jiti): Just In Time Indexing (JITI) utilities
- library(prolog_pack): A package manager for Prolog
- library(prolog_xref): Prolog cross-referencer data collection
- library(quasi_quotations): Define Quasi Quotation syntax
- library(random): Random numbers
- library(readutil): Read utilities
- library(record): Access named fields in a term
- library(registry): Manipulating the Windows registry
- library(settings): Setting management
- library(strings): String utilities
- library(simplex): Solve linear programming problems
- library(solution_sequences): Modify solution sequences
- library(tables): XSB interface to tables
- library(thread): High level thread primitives
- library(thread_pool): Resource bounded thread management
- library(ugraphs): Unweighted Graphs
- library(url): Analysing and constructing URL
- library(varnumbers): Utilities for numbered terms
- library(yall): Lambda expressions

- The SWI-Prolog library
- Packages

- Reference manual

## 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".