explain.pl -- 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.

 explain(@Term) is det

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

 explain(@Term, -Explanation) is nondet

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

 known_predicate(:Head)[private]

Succeeds if we know anything about this predicate. Undefined predicates are considered `known' for this purpose, so we can provide referenced messages on them.

 explain_property(+Property, ?Module, -Explanation) is nondet[private]

Explain predicates that have some property. Only does user predicates.

 explain_predicate(:Head, -Explanation) is det[private]