On Tue, Oct 31, 2006 at 11:02:03AM +0200, Yitzchak Gale wrote:
Consider the following sequence of functions that replace a single element in an n-dimensional list:
replace0 :: a -> a -> a replace1 :: Int -> a -> [a] -> [a] replace2 :: Int -> Int -> a -> [[a]] -> [[a]]
Generalize this using type classes.
It's quite easy if you allow the indices to be put in a single compound value. If you insist that each index should be given as a separate function argument, it may be possible to achieve it using the tricks that allow to write the variadic composition operator. Here's my version using MPTCs and fundeps: data Index t = I Int t class Replace i l a | i a -> l where replace :: i -> a -> l -> l instance Replace () a a where replace _ = const instance Replace i l a => Replace (Index i) [l] a where replace (I i0 is) x xs | null t = h | otherwise = h ++ (replace is x (head t) : tail t) where (h, t) = splitAt i0 xs Example use: *Nested> :t replace (I 0 $ I 2 $ I 3 $ ()) "qweqwe" replace (I 0 $ I 2 $ I 3 $ ()) "qweqwe" :: [[[[Char]]]] -> [[[[Char]]]] Best regards Tomasz