AmbiguousType.hs

{-# LANGUAGE FlexibleInstances #-}
{-# LANGUAGE MultiParamTypeClasses #-}
{-# LANGUAGE DeriveFunctor #-}
{-# LANGUAGE RankNTypes #-}
{-# LANGUAGE TypeApplications #-}
{-# LANGUAGE AllowAmbiguousTypes #-}

import qualified Data.List as L

class Uncons l a where
  uncons :: l -> Maybe (a, l)

instance Uncons [a] a where uncons = L.uncons

newtype Parser l a = Parser {runParser :: l -> Maybe (a, l)} deriving (Functor)

end :: forall l a. Uncons l a => Parser l ()
end = Parser $ \input -> case uncons input of
  Just _ -> Nothing
  Nothing -> Just ((), input)

main = putStrLn "Hello, World!"