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)

Module:Name

Give help on predicates with Name in Module and any arity. Used for loaded code only.

Module:Name/Arity

Give help on predicates with Name in Module and Arity. Used for loaded code only.

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.

help/1 shows documentation from the manual as well as from loaded user code if the code is documented using PlDoc. To show only the documentatoion of the loaded predicate we may prefix predicate indicator with the module in which it is defined.

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.

[nondet]help_apropos(+Query, -Obj, -Summary, -Score)

Find matching documented objects in the help database. Obj is the formal object identifier, Summary its summary description and Score is a number indicating the quality of the match.

[semidet]help_text(+Predicate:term, -HelpText:string)

When Predicate is a term of the form Name/Arity for which documentation exists, HelpText is the documentation in textual format (parsed from the HTML help).

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. Term can be any Prolog data object. Some terms have a specific meaning:

• A (partial) reference to a predicate gives the predicates, its main properties and references to the predicates. Partial references are:
  • Module:Name/Arity
  • Module:Head
  • Name/Arity
  • Name//Arity
  • Name
  • Module:Name

• Some predicate properties. This lists predicates as above the have this property. The specification can be of the shape Module:Property or just Property. The qualified version limits the result to predicates defined in Module. Supported properties are:
  • dynamic
  • thread_local
  • multifile
  • tabled

[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)).