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 separate 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)).

 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 (default is ignore).

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.

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(+Module)

Only process the given module

module_class(+ModuleClassList)

Limit processing to modules of the given classes. 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.

walk_meta_predicates(Boolean)

When false (default true), do not analyse the arguments of meta predicates. Standard Prolog control structures are always analysed.

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.

trace_condition(:Cond)

Additional filter condition applied after trace_reference. Called as call(Cond, Callee, Context), where Context is a dict containing the following keys:

caller:Context

Qualified term representing the caller or the atom '<initialization>'.

module:Context

Module being processed

clause:Context

If we are processing a normal clause, the clause reference to this clause.

initialization:Context

If we are processing an initialization/1 directive, a term File:Line representing the location of the declaration.

on_edge(:OnEdge)

If a reference to trace_reference is found, call call(OnEdge, Callee, Caller, Location), where Location is a dict containing a subset of the keys clause, file, character_count, line_count and line_position. If full position information is available all keys are present. If the clause layout is unknown the only the clause, file and line_count are available and the line is the start line of the clause. For a dynamic clause, only the clause is present. If the position is associated to a directive, the clause is missing. If nothing is known the Location is an empty dict.

on_trace(:OnTrace)

As on_edge, but location is not translated and 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.

verbose(+Boolean)

If true (default false), report derived meta-predicates and iterations.

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

 subterm_pos(+SubTerm, +Term, :Cmp, +TermPosition, -SubTermPos) is nondet

True when SubTerm is a sub term of Term, compared using Cmp, TermPosition describes the term layout of Term and SubTermPos describes the term layout of SubTerm. Cmp is typically one of same_term, ==, =@= or subsumes_term

 prolog_program_clause(-ClauseRef, +Options) is nondet

True when ClauseRef is a reference for clause in the program. Options is a subset of the options processed by prolog_walk_code/1. The logic for deciding on which clauses to enumerate is shared with prolog_walk_code/1.

• module(?Module)
• module_class(+list(Classes))