2.7 Online Help

2.7.1 library(help): Text based manual

This module provides help/1 and apropos/1 that give help on a topic or searches the manual for relevant topics.

By default the result of help/1 is sent through a pager such as less. This behaviour is controlled by the following:

• The Prolog flag help_pager, which can be set to one of the following values:

  false

  Never use a pager.

  default

  Use default behaviour. This tries to determine whether Prolog is running interactively in an environment that allows for a pager. If so it examines the environment variable PAGER or otherwise tries to find the less program.

  Callable

  A Callable term is interpreted as program_name(Arg, ...). For example, less('-r') would be the default. Note that the program name can be an absolute path if single quotes are used.

[det]help

[det]help(+What)

Show help for What. What is a term that describes the topics(s) to give help for. Notations for What are:

Atom

This ambiguous form is most commonly used and shows all matching documents. For example:

?- help(append).

Name / Arity

Give help on predicates with matching Name/Arity. Arity may be unbound.

Name // Arity

Give help on the matching DCG rule (non-terminal)

f(Name/Arity)

Give help on the matching Prolog arithmetic functions.

c(Name)

Give help on the matching C interface function

section(Label)

Show the section from the manual with matching Label.

If an exact match fails this predicates attempts fuzzy matching and, when successful, display the results headed by a warning that the matches are based on fuzzy matching.

If possible, the results are sent through a pager such as the less program. This behaviour is controlled by the Prolog flag help_pager. See section level documentation.

See also

apropos/1 for searching the manual names and summaries.

[semidet,multifile]show_html_hook(+HTML:string)

Hook called to display the extracted HTML document. If this hook fails the HTML is rendered to the console as plain text using html_text/2.

[det]apropos(+Query)

Print objects from the manual whose name or summary match with Query. Query takes one of the following forms:

Type:Text

Find objects matching Text and filter the results by Type. Type matching is a case intensitive prefix match. Defined types are section, cfunction, function, iso_predicate, swi_builtin_predicate, library_predicate, dcg and aliases chapter, arithmetic, c_function, predicate, nonterminal and non_terminal. For example:

?- apropos(c:close).
?- apropos(f:min).

Text

Text is broken into tokens. A topic matches if all tokens appear in the name or summary of the topic. Matching is case insensitive. Results are ordered depending on the quality of the match.

2.7.2 library(explain): Describe Prolog Terms

The library(explain) describes prolog-terms. The most useful functionality is its cross-referencing function.

?- explain(subset(_,_)).
"subset(_, _)" is a compound term
    from 2-th clause of lists:subset/2
    Referenced from 46-th clause of prolog_xref:imported/3
    Referenced from 68-th clause of prolog_xref:imported/3
lists:subset/2 is a predicate defined in
    /staff/jan/lib/pl-5.6.17/library/lists.pl:307
    Referenced from 2-th clause of lists:subset/2
    Possibly referenced from 2-th clause of lists:subset/2

Note that PceEmacs can jump to definitions and gxref/0 can be used for an overview of dependencies.

[det]explain(@Term)

Give an explanation on Term. The argument may be any Prolog data object. If the argument is an atom, a term of the form Name/Arity or a term of the form Module:Name/Arity, explain/1 describes the predicate as well as possible references to it. See also gxref/0.

[nondet]explain(@Term, -Explanation)

True when Explanation is an explanation of Term. The explaination is a list of elements that is printed using print_message(information, explain(Explanation)).