Re: [Haskell-cafe] Conceptualizing Functor vs Performance

My version of GHC (7.6) didn't have closed type families, but after upgrading to 7.8.2 it seems with your suggestion I can indeed get halfway to what I want (having such functionality only be available to pre-selected types is totally acceptable). I can now write import qualified Data.Vector.Storable as SV type family F a where F Double = SV.Vector F a = [] data T a = T ((F a) a) instance Functor T where ??? But I still have no idea how to implement Functor. Once again, conceptually I know that if (F a) == SV.Vector, fmap = SV.map, otherwise fmap = map, but how can I tell this to Haskell? Thanks. On Sun, Jun 29, 2014 at 4:08 AM, John Lato wrote:

First of all, in your use case are you sure that using Data.Vector is much less efficient than one of the specialized types? It's possible that all the allocations are fused away entirely after all.

Secondly, the module Data.Vector.Generic allows you to write functions that will work on all types of vectors, including unboxed and Storable-based vectors. Would it be possible to use this module to write the desired code? You'd then need to include a type annotation only once at the top level.

Finally, I supposed you could get something like this by creating a class similar to Data.Vector.Generic but using closed type families. I haven't worked out the details but I think it would work. The vector would only be available for a pre-defined list of available types though.

On Sat, Jun 28, 2014 at 5:37 PM, Yuri Lensky wrote:

...

I am having trouble consolidating a performance-based data representation decision I would like to make against the concept of a "functor". The question can be reduced to the following:

Conceptually I think that Data.Vector /should/ be a functor. More specifically, I see no CONCEPTUAL reason that it shouldn't be possible to define some "efficient" container-like object (for example for fast Matrix operations for unboxable numbers, but something that still works for a symbolic variable type) that chooses to represent its data as a Data.Vector.Unboxed if possible, but chooses some more generic structure (perhaps a strict list or a generic Data.Vector) if that is not the case. I haven't found such a functor, and can't seem to implement it on my own. The point is that in theory there is no true constraint on the type of the object being contained so it SHOULD be a Functor/Traversable/etc., the representation simply changes to something more efficient only if possible, and decays to a more generic type gracefully if necessary.

Perhaps there is some type hackery that can be done with TypeFamilies? I.E. define some container type "data C a = (ListType a) a", but I haven't found a generic way to do this.

Finally, I understand that "efficient" data structure can be problem-dependent, but I have no problem defining many different such "generic" types for different applications. The simplest example would be something like (invalid Haskell):

(Data.Vector.Unboxed.Vector v a) => data instance (ListType a) = v (Data.Vector.Generic.Vector v a) => data instance (ListType a) = v data instance (ListType a) = ([])

Of course implementing fmap is a whole separate issue. The point is I do not think this is the right way/possible, but am wondering what the "true" solution is.

Thanks.

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