Common Lisp 代碼例子：Factorial

factorial.lisp

(defun factorial (n)
  (declare (optimize speed))
  (labels ((fact (n acc)
    (declare (type fixnum n)
             (type bignum acc))
    (if (<= n 0)
        acc
        (fact (the fixnum (1- n))
              (the bignum (* n acc))))))
    (fact n 1)))

When acc is not a bignum, will encouter another error

CL-USER> (defun factorial (n)
  (declare (optimize speed))
  (labels ((fact (n acc)
    (declare (type fixnum n)
             (type bignum acc))
    (if (<= n 0)
        acc
        (fact (the fixnum (1- n))
              (the bignum (* n acc))))))
    (fact n 1)))
; in: DEFUN FACTORIAL
;     (FACT N 1)
; 
; caught WARNING:
;   Constant 1 conflicts with its asserted type BIGNUM.
;   See also:
;     The SBCL Manual, Node "Handling of Types"
; 
; compilation unit finished
;   caught 1 WARNING condition
FACTORIAL