This module started life as part of the GUI tracer. As it is generally useful for debugging purposes it has moved to the general Prolog library.

The tracer library library(trace/clause) adds caching and dealing with dynamic predicates using listing to XPCE objects to this. Note that clause_info/4 as below can be slow.

 clause_info(+ClauseRef, -File, -TermPos, -VarOffsets) is semidet

 clause_info(+ClauseRef, -File, -TermPos, -VarOffsets, +Options) is semidet

Fetches source information for the given clause. File is the file from which the clause was loaded. TermPos describes the source layout in a format compatible to the subterm_positions option of read_term/2. VarOffsets provides access to the variable allocation in a stack-frame. See make_varnames/5 for details.

Note that positions are character positions, i.e., not bytes. Line endings count as a single character, regardless of whether the actual ending is \n or =|\r\n|_.

Defined options are:

variable_names(-Names)

Unify Names with the variable names list (Name=Var) as returned by read_term/3. This argument is intended for reporting source locations and refactoring based on analysis of the compiled code.

 unify_term(+T1, +T2)

Unify the two terms, where T2 is created by writing the term and reading it back in, but be aware that rounding problems may cause floating point numbers not to unify. Also, if the initial term has a string object, it is written as "..." and read as a code-list. We compensate for that.

NOTE: Called directly from library(trace/clause) for the GUI tracer.

 open_source(+File, -Stream) is semidet[multifile]

Hook into clause_info/5 that opens the stream holding the source for a specific clause. Thus, the query must succeed. The default implementation calls open/3 on the File property.

clause_property(ClauseRef, file(File)),
prolog_clause:open_source(File, Stream)

 make_varnames(+ReadClause, +DecompiledClause, +Offsets, +Names, -Term) is det

Create a Term varnames(...) where each argument contains the name of the variable at that offset. If the read Clause is a DCG rule, name the two last arguments <DCG_list> and <DCG_tail>

This predicate calles the multifile predicate make_varnames_hook/5 with the same arguments to allow for user extensions. Extending this predicate is needed if a compiler adds additional arguments to the clause head that must be made visible in the GUI tracer.

Arguments:

Offsets	- List of Offset=Var
Names	- List of Name=Var

 unify_goal(+Read, +Decompiled, +Module, +TermPosRead, -TermPosDecompiled) is semidet[multifile]

This hook is called to fix up source code manipulations that result from goal expansions.

 initialization_layout(+SourceLocation, ?InitGoal, -ReadGoal, -TermPos) is semidet

Find term-layout of :- initialization directives.

 predicate_name(:Head, -PredName:string) is det

Describe a predicate as [Module:]Name/Arity.

 clause_name(+Ref, -Name)

Provide a suitable description of the indicated clause.

Undocumented predicates

The following predicates are exported, but not or incorrectly documented.

 clause_info(Arg1, Arg2, Arg3, Arg4, Arg5)