## 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.

### 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))`.