Major Section: SWITCHES-PARAMETERS-AND-MODES
Forms (see below for explanations and GCL exceptions):(set-debugger-enable t) ; enable breaks into the raw Lisp debugger (set-debugger-enable :break) ; same as above :set-debugger-enable t ; same as above (set-debugger-enable :break-bt) ; as above, but print a backtrace first (set-debugger-enable :bt-break) ; as above, but print a backtrace first (set-debugger-enable :bt) ; print a backtrace but do not enter debugger (set-debugger-enable :never) ; disable all breaks into the debugger (set-debugger-enable nil) ; disable debugger except when calling break$
Introduction. Suppose we define foo in :program mode to
take the car of its argument. This can cause a raw Lisp error. ACL2
will then return control to its top-level loop unless you enable the Lisp
debugger, as shown below (except: the error message can be a little
different in GCL).
ACL2 !>(defun foo (x) (declare (xargs :mode :program)) (car x))Summary Form: ( DEFUN FOO ...) Rules: NIL Warnings: None Time: 0.00 seconds (prove: 0.00, print: 0.00, other: 0.00) FOO ACL2 !>(foo 3) *********************************************** ************ ABORTING from raw Lisp *********** Error: Attempt to take the car of 3 which is not listp. ***********************************************
If you didn't cause an explicit interrupt (Control-C), then the root cause may be call of a :program mode function that has the wrong guard specified, or even no guard specified (i.e., an implicit guard of t). See :DOC guards.
To enable breaks into the debugger (also see :DOC acl2-customization): (SET-DEBUGGER-ENABLE T) ACL2 !>(SET-DEBUGGER-ENABLE T) <state> ACL2 !>(foo 3) Error: Attempt to take the car of 3 which is not listp. [condition type: TYPE-ERROR]
Restart actions (select using :continue): 0: Abort entirely from this (lisp) process. [Current process: Initial Lisp Listener] [1] ACL2(1): [RAW LISP]
Details. ACL2 usage is intended to take place inside the ACL2 read-eval-print loop (see lp). Indeed, in most Lisp implementations ACL2 comes up inside that loop, as evidenced by the prompt:
ACL2 !>However, one can occasionally hit a raw Lisp error. Here is the above example again, this time for a GCL implementation, which unfortunately gives a slightly less aesthetic report.
ACL2 !>(foo 3)Error: 3 is not of type LIST. Fast links are on: do (si::use-fast-links nil) for debugging Error signalled by CAR. Backtrace: funcall > system:top-level > lisp:lambda-closure > lp > acl2_*1*_acl2::foo > foo > car > system:universal-error-handler > system::break-level-for-acl2 > let* > UNLESS ACL2 !>
Here, the user has defined foo in :program mode, with an
implicit guard of t. The ACL2 evaluator therefore called the Lisp
evaluator, which expected nil or a consp argument to car.
By default, ACL2 will return to its top-level loop (at the same level of
LD) when there is a raw Lisp error, as though a call of ER with
flag HARD has been evaluated. If instead you want to enter the raw Lisp
debugger in such cases, evaluate the following form.
(set-debugger-enable t)You can subsequently return to the default behavior with:
(set-debugger-enable nil)Either way, you can enter the Lisp debugger from within the ACL2 loop by evaluating
(break$). If you want break$ disabled, then
evaluate the following, which disables entry to the Lisp debugger not only
for Lisp errors but also when executing (break$).
(set-debugger-enable :never)
The discussion above also applies to interrupts (from Control-C) in some,
but not all, host Common Lisps.
It remains to discuss options :break, :bt, :break-bt, and
:bt-break. Option :break is synonymous with option t, while
option :bt prints a backtrace (except in GCL, where a backtrace is
already printed for :break). Options :break-bt and :bt-break are
equivalent, and each has the combined effect of :bt and :break: a
backtrace is printed and then the debugger is entered.
Note that set-debugger-enable applies not only to raw Lisp errors, but
also to ACL2 errors: those affected by break-on-error. However, for
ACL2 errors, entering the debugger is controlled only by break-on-error,
not by set-debugger-enable; so for ACL2 errors, set-debugger-enable
values of :bt, :break-bt, and :bt-break have the same effect
(namely, of causing a backtrace to be printed, other than for GCL).
Remark for Common Lisp hackers (except for GCL). You can customize the form
of the backtrace printed by entering raw Lisp (with :q) and then
redefining function print-call-history, whose definition immediately
precedes that of break-on-error in ACL2 source file ld.lisp. Of
course, all bets are off when defining any function in raw Lisp, but as a
practical matter you are probably fine as long as your books are ultimately
certified with an unmodified copy of ACL2. If you come up with improvements
to print-call-history, please pass them along to the ACL2 implementors.
