Subseq

Subseq.hs

subseq :: Eq a => [a] -> [a] -> [a]
subseq xs ys = let res = go xs ys
               in if res /= xs then [] else res
  where
    go _         []        = []
    go []        _         = []
    go xs@(x:xt) ys@(y:yt)
      | x  ==  y  = x : go xt yt
      | otherwise = go xs yt

subseq123 = subseq [1,2,3]

testsEq = map subseq123 $
  [ [1,2,3]
  , [1,1,1,2,2,3]
  , [1,2,7,3]
  , [5,6,1,9,9,2,7,3,8]
  , [5,6,1,9,9,2,7,3]
  ]

testsNotEq = map subseq123 $
  [ [1,2]
  , [1,3]
  , [5,6,2,1,7,3,8]
  ]

tests = all (==[1,2,3]) testsEq
     && all (==[])      testsNotEq