Semi Off Topic: If the ultimate nature of reality is mathematical, as many physicist say, then everything is mathematical. Then the question must be rephrased to ¿is this or that isomorphic with a mathematical structure powerful enough (general enough, simple enough, but not more) or is out there another better structure?. How much of mathematical discovery, rather than engineering, are in programming languages design ? Merry christmas! 2009/12/22 Stephen Tetley

2009/12/22 Eduard Sergeev :

...

As was previously noted they are supposed to be replaced by type families,

Hi Eduard

Currently this seems a more like a rumour than a fact - from [1] Type Families and Fun Deps are equivalently expressive which seems a worthwhile point to restate. From [2] the Haskell prime committee want one or the other but not both, and will resolve matters sometime in the future.

Best wishes

Stephen

[1] http://www.haskell.org/pipermail/haskell-cafe/2009-February/055890.html [2] http://www.haskell.org/pipermail/haskell-prime/2008-April/002434.html _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe

Hi, Not everyone in the community is keen on replacing functional dependencies with type families. My advice would be to use whichever language construct seems more suitable to your problem and disregard the occasional posts by people claiming that functional dependencies are obsolete or deprecated. -Iavor On Tue, Dec 22, 2009 at 9:18 AM, Eduard Sergeev wrote:

Hi Stephen,

Stephen Tetley-2 wrote:

...

Currently this seems a more like a rumour than a fact - from [1] Type Families and Fun Deps are equivalently expressive which seems a worthwhile point to restate.

I've got the same impresion initially and was keen to use TF in favor to FD. And I'm probably missing something here... but here is wiki example which, I think, gives an example of the 'difference' I was refering to: http://www.haskell.org/haskellwiki/GHC/AdvancedOverlap (see '2 Notes and variations', last part).

As an additional example I can point to Oleg Kiselyov's TypeCast implementation (http://okmij.org/ftp/Haskell/deepest-functor.lhs), here is its slightly modified version:

{-# OPTIONS -fglasgow-exts #-} {-# OPTIONS -fallow-undecidable-instances #-} {-# OPTIONS -fallow-overlapping-instances #-}

module FMAP where

data Atom

-- Check if a type is a collection type. This is the only typeclass that -- needs overlapping instances class IsCollection  t coll | t -> coll instance IsCollection (m a) (m ()) instance Atom ~ coll => IsCollection t coll

-- The desired deep functor. Needs no overlapping instances class Funct a b c1 c2 | c1 -> a, c1 b -> c2 where    f_map :: (a -> b) -> c1 -> c2

instance (IsCollection c1 coll, Funct' coll a b c1 c2)    => Funct a b c1 c2 where    f_map = f_map' (undefined::coll)

class Funct' coll a b c1 c2 | coll c1 -> a, coll c1 b -> c2 where    f_map' :: coll -> (a -> b) -> c1 -> c2

instance Funct' Atom a b a b where    f_map' _ = id

instance (Functor m, Funct a b c d) => Funct' (m ()) a b (m c) (m d) where    f_map' _ = fmap . f_map

test1 = f_map (+1) [[[1::Int,2,3]]] test2 = f_map not [[True], [False]] test3 = f_map not (Just [Just True, Nothing]) test4 = f_map not (print "here" >>                   return (Just (Just [Just [True], Nothing])))        >>= print

Still I am not sure how to rewrite this example using Type Families..

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