[Haskell-cafe] Are functional dependencies around to stay?

[Iavor Diatchki]

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
<Eduard.Sergeev at gmail.com> 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..
>
>
> --
> View this message in context: http://old.nabble.com/Are-functional-dependencies-around-to-stay--tp26873777p26891353.html
> Sent from the Haskell - Haskell-Cafe mailing list archive at Nabble.com.
>
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