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 awhere dflat = id
else fails
Happy new year!
-Iavor
On Thu, Dec 30, 2010 at 3:52 AM, o...@okmij.org 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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