Pattern-Match

Simple pattern-matching for Common Lisp

Usage

(pattern-case KEYFORM (PATTERN FORM*)*)

Examples

(defun factorial (n)
  (pattern-case n
    (0 1)
    (n (* n (factorial (1- n))))))

(defun kth (k list)
  (pattern-case (list k list)
    ((_ nil) (error "much k. little list. such fail. wow."))
    ((0 (x . _)) x)
    ((k (_ . xs)) (kth (1- k) xs))))

Patterns

• Literal values are typechecked; symbols aren’t.
• By convention _ is an ignored term and remains unbound; multiple _ may appear in PATTERN.

Pattern	Matches
t	generalized boolean (i.e. non-null)
nil	itself
:keyword	itself
_	anything
other symbol	anything; bind to expression
other atom	itself

Examples

Pattern	Expression	Match?	Binds
0	0	Y
0	0.0	N
(n . ns)	(1 2 3)	Y	n => 1 ns => (2 3)
(n . ns)	(1)	Y	n => 1 ns => ()
(a b)	(BATMAN ROBIN)	Y	a => BATMAN b => ROBIN
(a b)	(LARRY MOE CURLY)	N