Availability:built-in
call_residue_vars(:Goal,
-Vars)Find residual attributed variables left by Goal. This
predicate is intended for reasoning about and debugging programs that
use coroutining or constraints. To see why this predicate is necessary,
consider a predicate that poses contradicting constraints on a variable,
and where that variable does not appear in any argument of the predicate
and hence does not yield any residual goals on the toplevel when the
predicate is invoked. Such programs should fail, but sometimes succeed
because the constraint solver is too weak to detect the contradiction.
Ideally, delayed goals and constraints are all executed at the end of
the computation. The meta predicate
call_residue_vars/2
finds variables that are given attributes or whose attributes are
modified by Goal, regardless of whether or not these
variables are reachable from the arguments of
Goal.191The
implementation of call_residue_vars/2
is completely redone in version 7.3.2 (7.2.1) after discussion with Bart
Demoen. The current implementation no longer performs full scans of the
stacks. The overhead is proportional to the number of attributed
variables on the stack, dead or alive..