Bug hunting toolbox

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Note: For now this is just a random collection of items as a list. They still need to be properly formatted and have a more detailed explanation with examples, but for now having them collected in one place is better than not having them at all. As noted, anyone can edit this and even add more details and example code to this. :smiley:

Prolog Unit Tests

Debugging and Tracing Programs






Control Predicates

Failure slice





List the program, predicates or clauses

Verify Type of a Term


library(debug) - Manage debug messages and check assertions. Example of using debug/1

Obtaining Runtime Statistics

Execution profiling

error.pl – Error generating support

  • is_of_type(+Type, @Term) is semidet
    True if Term satisfies Type.

Runtime determinacy checker (rdet) - can be used to trap predicates that should never fail.

library(error): Error generating support

  • must_be (+Type, @Term)
    True if Term satisfies the type constraints for Type. Defined types are atom , atomic , between , boolean , callable , chars , codes , text , compound , constant , float , integer , nonneg , positive_integer , negative_integer , nonvar , number , oneof , list , list_or_partial_list , symbol , var , rational , encoding , dict and string .

library(check): Consistency checking - This library provides some consistency checks for the loaded Prolog program.

Cross-referencer - A cross-referencer is a tool that examines the caller-callee relation between predicates, and, using this information to explicate dependency relations between source files, finds calls to non-existing predicates and predicates for which no callers can be found.

  • gxref - Run cross-referencer on all currently loaded files and present a graphical overview of the result.

Exception handling

Draw diagrams - Graph drawing software (Wikipedia) or Pen and paper.
GraphViz is a popular console application which has several domain specific languages for describing graphs on which DOT seems to be the most popular. Example here.

Consider Cognitive dimensions of notations

If the data has a sequential pattern to it check Online Encyclopedia of Integer Sequences(OEIS) for references and ideas.Catalan numbers and Hamilton Paths are common

When writing formatted text to a string:
format/3 with with_output_to/2

Environment Control (Prolog flags)

  • generate_debug_info(bool, changeable)
    If true (default) generate code that can be debugged using trace/0, spy/1, etc. Can be set to false using the -nodebug . This flag is scoped within a source file. Many of the libraries have :- set_prolog_flag(generate_debug_info, false) to hide their details from a normal trace.
    See: Append/3 isn’t deterministic if first arg is var?

listing/2 The option source(true) is quite helpful to find the actual definition of multifile predicates and where the clauses come from.

Add this to your code at the place where you want a conditional break and run make/0 .

    (some_condition -> gtrace ; true),

Using the Logtalk linter to check Prolog modules code: See: Linter

Descriptions of the icons/glyphs in the graphical debugger?

For a Finite State Automata of the states using gtrace/0 see: Gtrace - Example finite-state automaton
The FSA is a PDF which needs to be downloaded: SWI-Prolog Debugger FSA.pdf (682.7 KB)

prolog_clause.pl – Get detailed source-information about a clause. 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.

Examining the program - Noted in topic: Line number (and file name) of fact read, specifically this post.

Edit Exceptions in the prolog editor. Who knew that existed and had error defined to trace “if not caught”? - Trace_on_error

SWI-Prolog web-enabled monitor (GitHub) (post)
Usage: ?- prolog_ide(debug_monitor).

?- debug(concurrent). See: Some notes on writing a concurrent programing Howto. Comments and corrections welcome