Examples from 'Defunctionalization at Work' (paper written by Olivier Danvy and Lasse R. Nielsen). Matching the context-free language L := {0^n1^n | n \in N} where N = {0, 1, 2, ...}

match0n1n-1.hs

-- L = {0^n 1^n | n \in N}

match :: String -> Bool
match s = case trace s of
            Just [] -> True
            _       -> False
          where trace :: String -> Maybe String
                trace ('0':xs) = do
                  remainString <- trace xs
                  (case remainString of
                     '1':xs' -> return xs'
                     _       -> Nothing)
                trace xs = return xs