parser.hs

type Parser a = String -> [(a, String)]

return_ :: a -> Parser a
return_ v = \inp -> [(v, inp)]

failure :: Parser a
failure = \inp -> []

item = \inp -> case inp of
  []      ->  []
  (x:xs)  ->  [(x,xs)]

parse :: Parser a -> String -> [(a, String)]
parse p inp = p inp

example code of "programming haskell".

executing in ghci

parse (return_ 1) "abc"

parse failure "abc"

parse item "abc"