### GUARDS-FOR-SPECIFICATION

guards as a specification device
Major Section: GUARD

A use of guard verification that has nothing to do with efficiency
is as a way to gain confidence in specifications. This use has the
feel of ``types'' in many traditional programming languages, though
guards allow much greater expressiveness than most systems of types
(and unfortunately, as a result they are not syntactically
checkable).

For more discussion of guards in general, see guard.

Suppose you have written a collection of function definitions that
are intended to specify the behavior of some system. Perhaps
certain functions are only intended to be called on certain sorts of
inputs, so you attach guards to those functions in order to
``enforce'' that requirement. And then, you verify the guards for
all those functions.

Then what have you gained, other than somewhat increased efficiency
of execution (as explained above), which quite possibly isn't your
main concern? You have gained the confidence that when evaluating
any call of a (specification) function whose arguments satisfy that
function's guard, all subsequent function calls during the course of
evaluation will have this same property, that the arguments satisfy
the guard of the calling function. In logical terms, we can say
that the equality of the original call with the returned value is
provable from weakened versions of the definitions, where each
definitional axiom is replaced by an implication whose antecedent is
the requirement that the arguments satisfy the guard and whose
consequent is the original axiom. For example,

(defun foo (x)
(declare (xargs :guard (consp x)))
(cons 1 (cdr x)))

originally generates the axiom
(equal (foo x)
(cons 1 (cdr x)))

but in fact, when evaluation involves no guard violation then the
following weaker axiom suffices in the justification of the
evaluation.
(implies (consp x)
(equal (foo x)
(cons 1 (cdr x))))

If you are following links to read this documentation as a hypertext
style document, then please see guard-miscellany. This
concludes our discussion of guards with miscellaneous remarks, and
also contains pointers to related topics.