trap/1

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- Reference manual
  - The SWI-Prolog library
    - library(prolog_debug): User level debugging tools
- Packages

Availability::- use_module(library(prolog_debug)).(can be autoloaded)

[det]trap(+Formal)

[det]notrap(+Formal)

Install a trap on error(Formal, Context) exceptions that unify. The tracer is started when a matching exception is raised. This predicate enables debug mode using debug/0 to get more context about the exception. Even with debug mode disabled exceptions are still trapped and thus one may call nodebug/0 to run in normal mode after installing a trap. Exceptions are trapped in any thread. Debug mode is only enabled in the calling thread. To enable debug mode in all threads use tdebug/0.

Calling debugging/0 lists the enabled traps. The predicate notrap/1 removes matching (unifying) traps.

In many cases debugging an exception that is caught is as simple as below (assuming run/0 starts your program).

?- trap(_).
?- run.

The multifile hook trap_alias/2 allow for defining short hands for commonly used traps. Currently this defines

det: Trap determinism exceptions raised as a result of the det/1 directive.
=>: Trap rule existence error exceptions.

See also: - gtrap/1 to trap using the graphical debugger.
- Edit exceptions menu in PceEmacs and the graphical debugger that provide a graphical frontend to trap exceptions.