B.1 Examining the Environment Stack

[det]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.

[semidet]prolog_current_choice(-Choice)

Unify Choice with an integer provided a reference to the last choice point. Fails if the current environment has no choice points. See also prolog_choice_attribute/3.

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 the calling context, 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.^{248The 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

parent_goal(-Parent)

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.

The variant parent_goal(-Parent) unifies the frame reference of the parent of the found frame with Parent. That allows for finding frames higher up in the stack running the same goal.

predicate_indicator

Similar to goal, but only returning the [<module>:]<name>/<arity> term describing the term, not the actual arguments. It avoids creating an illegal term as goal and is used by the library library(prolog_stack).

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.

skipped

Value is true if this frame was skipped in the debugger.

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 or belongs to a compound meta-call (e.g., call((a,b))).

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 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.

deterministic(-Boolean)

Unifies its argument with true if no choice point exists that is more recent than the entry of the clause in which it appears. There are few realistic situations for using this predicate. It is used by the prolog/0 top level to check whether Prolog should prompt the user for alternatives. Similar results can be achieved in a more portable fashion using call_cleanup/2.