prolog-debugging-workflowlisted

# Prolog Debugging & Tracing Workflow Use this skill when diagnosing test failures, unexpected predicate failures, or infinite loops in Prolog code. ## 1. Interactive Tracer Basics (4-Port Model) Prolog debuggers use the **4-Port Execution Model**: - **Call**: Initial invocation of a goal. - **Exit**: Successful unification/derivation of a goal. - **Redo**: Backtracking into a goal to find alternative solutions. - **Fail**: Complete failure of a goal to unify or satisfy constraints. --- ## 2. Setting Trace Points & Leashing ```prolog % Enable execution tracing :- trace. % Spy on specific predicate :- spy(my_predicate/2). % Turn off tracing :- nodebug. ``` --- ## 3. Engine-Specific Debugging Instructions ### SWI-Prolog - Interactive tracer: Call `trace, my_goal(X).` - Graphical tracer: `gtrace, my_goal(X).` - Term printing: `portray_clause(Term).` ### Scryer Prolog - Print tracing: Use `write/1`, `format/2`, or `portray_clause/1` from `library(format)` to inspect non-ground terms. - Use `library(debug)` directives when available. --- ## 4. Debugging Non-Ground Terms & Constraints When debugging constraint logic code (`CLP(Z)` / `CLP(FD)`), avoid plain `write/1` which may hide constraint attributes. Use `copy_term/3` or attribute inspection: ```prolog % Scryer Prolog attribute inspection :- use_module(library(clpz)). debug_vars(Vars) :- copy_term(Vars, Vars, Goals), format("Residual constraints: ~w~n", [Goals]). ``` --- ## 5. Compilation Failure & Hum