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).
pc: Requests the program counter to which the choice point refers. Only applicable for in-clause choice points.
clause: Request the clause that will be tried if this choice point is activated. Only applicable for choice points of type clause.

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