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)).