B.1 Examining the Environment Stack

prolog_current_frame(-Frame)

Unify Frame with an integer providing a reference to the parent of the current local stack frame. A pointer to the current local frame cannot be provided as the predicate succeeds deterministically and therefore its frame is destroyed immediately after succeeding.

prolog_frame_attribute(+Frame, +Key, -Value)

Obtain information about the local stack frame Frame. Frame is a frame reference as obtained through prolog_current_frame/1, prolog_trace_interception/4 or this predicate. The key values are described below.

alternative

Value is unified with an integer reference to the local stack frame in which execution is resumed if the goal associated with Frame fails. Fails if the frame has no alternative frame.

has_alternatives

Value is unified with true if Frame still is a candidate for backtracking. false otherwise.

goal

Value is unified with the goal associated with Frame. If the definition module of the active predicate is not user the goal is represented as <module>:<goal>. Do not instantiate variables in this goal unless you know what you are doing! Note that the returned term may contain references to the frame and should be discarded before the frame terminates.^{81The returned term is actually an illegal Prolog term that may hold references from the global- to the local stack to preserve the variable names.}

parent_goal

If Value is instantiated to a callable term, find a frame executing the predicate described by Value and unify the arguments of Value to the goal arguments associated with the frame. This is intended to check the current execution context. The user must ensure the checked parent goal is not removed from the stack due to last-call optimisation and be aware of the slow operation on deeply nested calls.

clause

Value is unified with a reference to the currently running clause. Fails if the current goal is associated with a foreign (C) defined predicate. See also nth_clause/3 and clause_property/2.

level

Value is unified with the recursion level of Frame. The top level frame is at level `0'.

parent

Value is unified with an integer reference to the parent local stack frame of Frame. Fails if Frame is the top frame.

context_module

Value is unified with the name of the context module of the environment.

top

Value is unified with true if Frame is the top Prolog goal from a recursive call back from the foreign language. false otherwise.

hidden

Value is unified with true if the frame is hidden from the user, either because a parent has the hide-childs attribute (all system predicates), or the system has no trace-me attribute.

pc

Value is unified with the program-pointer saved on behalf of the parent-goal if the parent-goal is not owned by a foreign predicate.

argument(N)

Value is unified with the N-th slot of the frame. Argument 1 is the first argument of the goal. Arguments above the arity refer to local variables. Fails silently if N is out of range.

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 3-th argument. 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.

deterministic(-Boolean)

Unifies its argument with true if the clause in which is appears has not created any choice-points since it was started. There are few realistic situations for using this predicate. It is used by the prolog/0 toplevel to check whether Prolog should prompt the user for alternatives.