Hello, I just noticed that the instances for this example look more readable when written with two recently proposed Haskell extensions. Perhaps we should consider implementing these in GHC? Using chain instances: (http://web.cecs.pdx.edu/~mpj/pubs/instancechains.pdf )
instance DeepFlat a b => DeepFlat [a] b where dflat = concatMap dflat else DeepFlat a a where dflat = id else fails
And with the fun. deps. in functional notation: ( http://web.cecs.pdx.edu/~mpj/pubs/fundeps-design.pdf)
instance DeepFlat [a] (DeepFlat a) where dflat = concatMap dflat else DeepFlat a a where dflat = id else fails
Happy new year!
-Iavor
On Thu, Dec 30, 2010 at 3:52 AM,
William Murphy wrote:
I've spent a lot of time trying to write a version of concat, which concatenates lists of any "depth":
It is a little bit more involved, but quite possible. The code is not much longer than the one you wrote (essentially, three lines: one class and two instance declarations). Here is the complete code:
{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies #-} {-# LANGUAGE TypeFamilies, FlexibleInstances #-} {-# LANGUAGE UndecidableInstances #-} {-# LANGUAGE OverlappingInstances #-}
module DeepFlat where
class DeepFlat a b | a -> b where dflat :: [a] -> [b]
-- If we flatten a list of lists instance DeepFlat a b => DeepFlat [a] b where dflat = concatMap dflat
-- If we are given a list of non-lists instance a ~ b => DeepFlat a b where dflat = id
test1 = dflat "abracadabra" -- "abracadabra"
test2 = dflat ["abra","cadabra"]
test3 = dflat [["ab","ra"],["cad","abra"]] test4 = dflat [[["a","b"],["ra"]],[["cad","abra"]]]
_______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe