SWI-Prolog -- prolog_frame

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

Availability:built-in

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.