egisonではリストもmaybeも、matcherを適切に実装する事で、同じマッチ式でパターンマッチする事ができる。

gistfile1.txt

(define $maybe
  (lambda [$a]
    (matcher {
      ;; <primitive-pp> 
      [,$val [a] {[$tgt
        (match [val tgt] [(maybe a) (maybe a)] {
          [[<just $x> <just ,x>] {[x]}]
          [[<nothing> <nothing>] {[]}]
          [_ {}]})]}]
      ;; <primitive-dp> 
      [<just $> [a] {[<Just $x> {[x]}] [_ {}]}]
      [<nothing> [] {[<Nothing> {[]}] [_ {}]}]})))

(define $maybe-list
  (lambda [$a]
    (matcher {
      ;; <primitive-pp> 
      [,$val [a] {[$tgt
        (match [val tgt] [(maybe-list a) (maybe-list a)] {
          [[<just $x> <just ,x>] {[x]}]
          [[<nothing> <nothing>] {[]}]
          [_ {}]})]}]
      ;; <primitive-dp> 
      [<just $ ,$xs> [a] {[{$x @$xs} {[x]}] [_ {}]}]
      [<nothing> [] {[{} {[]}] [_ {}]}]})))

;;-----------------------------------------------------------------------

;; <Nothing>を<nothing>とパターンマッチ => <OK>
(test (match <Nothing> (maybe integer) {
  [<nothing> <OK>] 
  [_ <KO>]}))

;;Nilを<nothing>とパターンマッチ　=> <OK>
(test (match {} (maybe-list integer) {
  [<nothing> <OK>] 
  [_ <KO>]}))

;;-----------------------------------------------------------------------

;;<Just 999>を<just $x>とパターンマッチ => <OK 999>
(test (match <Just 999> (maybe integer) {
  [<just $x> <OK x>] 
  [_ <KO>]}))

;;リスト{999 1 2}を<just $x>とパターンマッチ => <OK 999>
(test (match {999 1 2} (maybe-list integer) {
  [<just $x> <OK x>] 
  [_ <KO>]}))