temp.hs

{-# LANGUAGE ScopedTypeVariables,Rank2Types #-}
module Temp where
newtype Mu a = Mu (Mu a -> a)

replaceMthNth :: Int -> Int -> a -> [[a]] -> [[a]]
replaceMthNth = ( \(replaceNth :: forall b.Int -> b -> [b] -> [b]) ->
                    -- definition of replaceMthNth in terms of some replaceNth
                    \m n v arg -> replaceNth m (replaceNth n v (arg !! m)) arg 
                )$  
                -- y combinator
                ((\f -> (\h -> h $ Mu h) $ \x -> f $ (\(Mu g) -> g) x $ x) :: (a -> a) -> a) $
                (\(replaceNth :: forall b.Int -> b -> [b] -> [b]) ->
                  -- definition of replaceNth given a recursive definition
                  (\(n::Int) newVal xs -> case xs of
                      [] -> []
                      (x:xs) -> if n == 0 then newVal:xs else x:replaceNth (n-1) newVal xs
                  )   
                ) :: (forall b.Int -> b -> [b] -> [b]) -> (forall b.Int -> b -> [b] -> [b])