The two main libraries providing pattern matching in Common Lisp are Optima and Trivia. Both provide a similar matching API and syntax. However trivia provides a unified interface to extend matching, defpattern.
Because a clack request is represented as a plist, we can use pattern matching as the entry point to the clack app as a way to route request to their appropriate controllers
(defvar *app*
(lambda (env)
(match env
((plist :request-method :get
:request-uri uri)
(...
Using pattern matching one can intertwine function definition and pattern matching, similar to SML.
(trivia:defun-match fib (index)
"Return the corresponding term for INDEX."
(0 1)
(1 1)
(index (+ (fib (1- index)) (fib (- index 2)))))
(fib 5)
;; => 8
Cons-cells, structures, vectors, lists and such can be matched with constructor patterns.
(loop for i from 1 to 30
do (format t "~5<~a~;~>"
(match (cons (mod i 3)
(mod i 5))
((cons 0 0) "Fizzbuzz")...
Guard patterns can be used to check that a value satisfies an arbitrary test-form.
(dotimes (i 5)
(format t "~d: ~a~%"
i (match i
((guard x (oddp x)) "Odd!")
(_ "Even!"))))
; 0: Even!
; 1: Odd!
; 2: Even!
; 3: Odd!
; 4: ...
