Major Section: OTHER
By default, ACL2 checks input constraints on functions, known as
guards. When guards are violated, an informative message is printed;
but sometimes it is useful to investigate why the guard check fails. The
utility print-gv is provided for that purpose. (Alternatively, you may
prefer to avoid guards entirely with (set-guard-checking :none);
see set-guard-checking.)
Example Forms:
(print-gv)
(print-gv :evisc-tuple (evisc-tuple 4 4 nil nil))
(print-gv :evisc-tuple (evisc-tuple 4 ; print-level
5 ; print-length
(world-evisceration-alist state nil)
nil ; hiding-cars
))
General Form: (print-gv :evisc-tuple x)where the
:evisc-tuple argument is optional and defaults to one that
hides only the ACL2 logical world, by printing <world> instead of a
very large structure. See evisc-tuple for a discussion of evisc-tuples.
To see how one might use print-gv, consider the following definition.
(defun foo (x)
(declare (xargs :guard (and (my-first-predicate x)
(my-second-predicate x))))
(cdr x))
Now suppose we evaluate a call of foo and obtain a guard violation.
ACL2 !>(foo 3)We can obtain (in essence) the guard form that was false, as follows.ACL2 Error in TOP-LEVEL: The guard for the function call (FOO X), which is (AND (MY-FIRST-PREDICATE X) (MY-SECOND-PREDICATE X)), is violated by the arguments in the call (FOO 3). To debug: See :DOC print-gv, see :DOC trace, and see :DOC wet. See :DOC set-guard-checking for information about suppressing this check with (set-guard-checking :none), as recommended for new users.
ACL2 !>
ACL2 !>(print-gv)But which conjunct is to blame? With a bit of editing we can try each conjunct in turn.(LET ((X '3)) (DECLARE (IGNORABLE X)) (AND (MY-FIRST-PREDICATE X) (MY-SECOND-PREDICATE X)))
ACL2 !>
ACL2 !>(LET ((X '3))
(DECLARE (IGNORABLE X))
(MY-FIRST-PREDICATE X))
T
ACL2 !>(LET ((X '3))
(DECLARE (IGNORABLE X))
(MY-SECOND-PREDICATE X))
NIL
ACL2 !>
Aha! Now we can investigate why the second predicate fails for 3.
Finally, we mention a hack that will give you access in raw Lisp to the form
printed by (print-gv). After a guard violation, just submit this form to
raw Lisp:
(print-gv1 (wormhole-data (cdr (assoc 'ev-fncall-guard-er-wormhole
*wormhole-status-alist*)))
(w state))
And even more finally, we note that while print-gv is a utility for
debugging guard violations, in contrast, see guard-debug for a utility to
assist in debugging failed proofs arising from guard verification.