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, wrote:

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"]]]

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