Common Lisp added type declarations in 1984. A usual type declaration looks like this: (defun month-name (month) (declare (type (integer 1 12) month

Common Lisp added type declarations in 1984. A usual type declaration looks like this: (defun month-name (month) (declare (type (integer 1 12) month)) (the string (ecase month (1 "Jan") (2 "Feb") (3 "Mar") (4 "Apr") (5 "May") (6 "Jun") (7 "Jul") (8 "Aug") (9 "Sep") (10 "Oct") (11 "Nov") (12 "Dez")))) You can also declare the type of the function, outside of the function: (declaim (ftype (function ((integer 1 12)) ; the parameter list string) ; the return value month-name)) ; the function name CLOS (the Common Lisp Object System) was added a few years later. CL-USER 1 > (defmethod add ((a integer) (s string)) (+ a (parse-integer s))) #<STANDARD-METHOD ADD NIL (INTEGER STRING) 402005B1C3> CL-USER 2 > (add 12 "23") 35 What do you think the words 'integer' and 'string" are about?