1_4_3_satisfyE.hs

{-
GHCi> runPrsE (charE 'A') "ABC"
Right ('A',"BC")
GHCi> runPrsE (charE 'A') "BCD"
Left "unexpected B"
GHCi> runPrsE (charE 'A') ""
Left "unexpected end of input"
-}
newtype PrsE a = PrsE { runPrsE :: String -> Either String (a, String) }

satisfyE :: (Char -> Bool) -> PrsE Char
satisfyE pr = PrsE f where
  f "" = Left "unexpected end of input"
  f (c:cs) | pr c = Right (c, cs)
           | otherwise = Left $ "unexpected " ++ [c]

charE :: Char -> PrsE Char
charE c = satisfyE (== c)