Major Section: PROGRAMMING
See hard-error, see illegal, and see cw for examples of functions
to call in the first argument of
(Prog2$ x y) equals
y; the value of
x is ignored.
x is first evaluated for side effect. Since the ACL2
programming language is applicative, there can be no logical impact
x may involve a call of a function such
illegal, which can cause so-called ``hard errors'',
or a call of
cw to perform output.
Here is a simple, contrived example using
hard-error. The intention
is to check at run-time that the input is appropriate before calling
(defun foo-a (x) (declare (xargs :guard (consp x))) (prog2$ (or (good-car-p (car x)) (hard-error 'foo-a "Bad value for x: ~p0" (list (cons #\0 x)))) (bar x)))The following similar function uses
illegalhas a guard of
nil, guard verification would guarantee that the call of
illegalbelow will never be made (at least when guard checking is on; see set-guard-checking).
(defun foo-b (x) (declare (xargs :guard (and (consp x) (good-car-p (car x))))) (prog2$ (or (good-car-p (car x)) (illegal 'foo-b "Bad value for x: ~p0" (list (cons #\0 x)))) (bar x)))
We conclude with a simple example using
cw from the ACL2 sources.
(defun print-terms (terms iff-flg wrld)
; Print untranslations of the given terms with respect to iff-flg, following ; each with a newline.
; We use cw instead of the fmt functions because we want to be able to use this ; function in print-type-alist-segments (used in brkpt1), which does not return ; state.
(if (endp terms) terms (prog2$ (cw "~q0" (untranslate (car terms) iff-flg wrld)) (print-terms (cdr terms) iff-flg wrld))))