> If there's one thing that I sometimes wish Lisp had, it's types.

Let's write some very silly code to turn an integer into a list of digits in Common Lisp:

  (deftype Digit ()
    "A non-negative integer smaller than 10."
    '(Mod 10))

  (defun integer->digits (integer)
    "Turns a given INTEGER into a list of digits."
    (declare (type Integer integer))
    (labels ((digit-loop (integer digits)
                (declare (type Integer integer)
                         (type List digits))
                (if (< integer 10)
                    (list* integer digits)
                    (multiple-value-bind (quotient remainder)
                        (truncate integer 10)
                      (declare (type Integer quotient)
                               (type Digit remainder))
                      (digit-loop quotient
                                  (list* remainder digits))))))
      (declare (ftype (Function (Integer List) List) digit-loop))
      (digit-loop (abs integer)
                  nil)))

  (digit-loop 2026) ; => (2 0 2 6)

  (digit-loop "2026")
  ; The value
  ;     "2026"
  ; is not of type
  ;     INTEGER
  ; when binding INTEGER
  ;
  ; Type HELP for debugger help, or (SB-EXIT:EXIT) to exit from SBCL.
  ;
  ; Restarts:
  ;   0: [ABORT] Exit debugger, returning to top level.