SWI-Prolog -- prolog_choice

Documentation
- Reference manual
  - Hackers corner
    - Examining the Environment Stack
- Packages

Availability:built-in

prolog_choice_attribute(+ChoicePoint, +Key, -Value)

Extract attributes of a choice point. ChoicePoint is a reference to a choice point as passed to prolog_trace_interception/4 on the 3rd argument or obtained using prolog_current_choice/1. Key specifies the requested information:

parent: Requests a reference to the first older choice point.
frame: Requests a reference to the frame to which the choice point refers.
type: Requests the type. Defined values are clause (the goal has alternative clauses), foreign (non-deterministic foreign predicate), jump (clause internal choice point), top (first dummy choice point), catch (catch/3 to allow for undo), debug (help the debugger), or none (has been deleted).

This predicate is used for the graphical debugger to show the choice point stack.