/usr/share/acl2-6.5/books/tools/safe-case.lisp is in acl2-books-source 6.5-2.
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; Original author: Jared Davis <jared@centtech.com>
(in-package "ACL2")
(defmacro safe-case (&rest l)
":Doc-Section Case
Error-checking alternative to `case'.~/
~c[;; (include-book \"tools/safe-case\" :dir :system)] ~/
~c[Safe-case] is a drop-in replacement for ~il[case], and is logically identical
to ~c[case]. The only difference is during execution. When ~c[case] is used and
none of the cases match, the answer is ~c[nil]:
~bv[]
ACL2 !> (case 3
(1 'one)
(2 'two))
NIL
~ev[]
But when ~c[safe-case] is used and none of the cases match, the result is an
error:
~bv[]
ACL2 !> (safe-case (+ 0 3)
(1 'one)
(2 'two))
HARD ACL2 ERROR in SAFE-CASE: No case matched:
(SAFE-CASE (+ 0 3) (1 'ONE) (2 'TWO)). Test was 3.
~ev[]"
(declare (xargs :guard (and (consp l)
(legal-case-clausesp (cdr l)))))
(let* ((clauses (cdr l))
(tests (strip-cars clauses)))
(if (or (member t tests)
(member 'otherwise tests))
`(case ,@l)
`(case ,@l
(otherwise
(er hard? 'safe-case "No case matched: ~x0. Test was ~x1.~%"
'(safe-case ,@l) ,(car l)))))))
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