subseqのサンプル

subseq.hs

subseq :: (Eq a) => [a] -> [a] -> Bool
subseq seq sseq = subseq' seq sseq == seq

subseq' :: (Eq a) => [a] -> [a] -> [a]
subseq' seq sseq = foldr unifier [] sseq
  where
    unifier x [] | elem x seq = [x]
                 | otherwise  = []
    unifier x sseq'@(s:_)
      | elem x seq && s /= x = (x:sseq')
      | otherwise  = sseq'