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.194The
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.
- 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.