This module walks over the loaded program, searching for callable predicates. It started as part of library(prolog_autoload) and has been turned into a seperate module to facilitate operations that require the same reachability analysis, such as finding references to a predicate, finding unreachable code, etc.

For example, the following determins the call graph of the loaded program. By using source(true), The exact location of the call in the source file is passed into _Where.

:- dynamic
        calls/2.

assert_call_graph :-
        retractall(calls(_, _)),
        prolog_walk_code([ trace_reference(_),
                           on_trace(assert_edge),
                           source(false)
                         ]),
        predicate_property(calls(_,_), number_of_clauses(N)),
        format('Got ~D edges~n', [N]).

assert_edge(Callee, Caller, _Where) :-
        calls(Caller, Callee), !.
assert_edge(Callee, Caller, _Where) :-
        assertz(calls(Caller, Callee)).

 pdt_prolog_walk_code(+Options) is det

Walk over all loaded (user) Prolog code. The following code is processed:

1. The bodies of all clauses in all user and library modules. This steps collects, but does not scan multifile predicates to avoid duplicate work.
2. All multi-file predicates collected.
3. All goals registered with initialization/1

Options processed:

undefined(+Action)

Action defines what happens if the analysis finds a definitely undefined predicate. One of ignore or error.

autoload(+Boolean)

Try to autoload code while walking. This is enabled by default to obtain as much as possible information about goals and find references from autoloaded libraries.

module(+Module)

Only process the given module

clauses(+ListOfClauseReferences)

Only process the given clauses. Can be used to find clauses quickly using source(false) and then process only interesting clauses with source information.

module_class(+ModuleClass)

Limit processing to modules of this class. See module_property/2 for details on module classes. Default is to scan the classes user and library.

infer_meta_predicates(+BooleanOrAll)

Use infer_meta_predicate/2 on predicates with clauses that call known meta-predicates. The analysis is restarted until a fixed point is reached. If true (default), analysis is only restarted if the inferred meta-predicate contains a callable argument. If all, it will be restarted until no more new meta-predicates can be found.

trace_reference(Callable)

Print all calls to goals that subsume Callable. Goals are represented as Module:Callable (i.e., they are always qualified). See also subsumes_term/2.

on_trace(:OnTrace)

If a reference to trace_reference is found, call call(OnTrace, Callee, Caller, Location), where Location is one of these:

• clause_term_position(+ClauseRef, +TermPos)
• clause(+ClauseRef)
• file_term_position(+Path, +TermPos)
• file(+File, +Line, -1, _)
• a variable (unknown)

Caller is the qualified head of the calling clause or the atom '<initialization>'.

source(+Boolean)

If false (default true), to not try to obtain detailed source information for printed messages.

@compat OnTrace was called using Caller-Location in older versions.