/usr/share/acl2-7.1/books/tools/safe-case.lisp is in acl2-books-source 7.1-1.
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; Original author: Jared Davis <jared@centtech.com>
(in-package "ACL2")
(include-book "xdoc/top" :dir :system)
(defsection safe-case
:parents (case)
:short "Error-checking alternative to @(see case)."
:long "<p>@('Safe-case') is a drop-in replacement for @(see case) and is
logically identical to @('case'). The only difference is that @('safe-case')
adds some extra error-checking during execution.</p>
<p>In particular, when @('case') is used and none of the cases match, the
answer is @('nil'):</p>
@({
ACL2 !> (case 3
(1 'one)
(2 'two))
NIL
})
<p>But when @('safe-case') is used and none of the cases match, the result is
an error:</p>
@({
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.
})
<p>To use @('safe-case') you need to include it, e.g.,:</p>
@({
(include-book \"tools/safe-case\" :dir :system)
})"
(defmacro safe-case (&rest l)
(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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