[lispm]

SBCL is a strange Lisp implementation, since it does some compile-time type checks, too.

  * (defun id (f) f)
  ID
  * (declaim (ftype (function (fixnum) fixnum) id))
  (ID)

Our new identity function is declared to be of type fixnum -> fixnum.

  * (declaim (optimize (safety 0)))
  NIL

Now we've set safety to 0.

Let's define a function which calls ID with a string

  * (defun use-id (s) (declare (type string s)) (id s))
  ; in: DEFUN USE-ID
  ;     (ID S)
  ; 
  ; caught WARNING:
  ;   Derived type of S is
  ;     (VALUES STRING &OPTIONAL),
  ;   conflicting with its asserted type
  ;     FIXNUM.
  ;   See also:
  ;     The SBCL Manual, Node "Handling of Types"
  ; 
  ; compilation unit finished
  ;   caught 1 WARNING condition
  USE-ID

Even though safety is 0, the compiler warned us about a compile-time type problem.