[Hs-Generics] compositional gMap in SYB1

[oleg at okmij.org]

Hello!

Claus wrote:
> Perhaps we can combine our versions to get the best of both?

I have done so. I have committed a new version of 
	/comparison/SYB1_2/GMap.lhs
with gmap2 of the type

> gmap2 :: forall a b c . (Data a, Data b, 
>		           Data (c a), Data (c b), Data (c X)) => 
>          (a -> b) -> c a -> c b

The compositionality tests seem to pass. There are no unsafe
operations or any unsafe extensions. Only standard Data.Typeable and
Generics.Data operations are being used. The code also contains an
optimization: if it is determined that a structured value has no
components to map, no traversal is performed and the value is returned
as it is. That should speed things up a little. The trick is producing
something out of nothing.

	Too bad that the deadline to amend a Haskell workshop paper
has passed, and so has the deadline for the generics workshop. Perhaps
we should write a separate paper? 

	I'm afraid I would be out of town again for two weeks and so
would not be able to follow the discussion closely.


	Cheers,
	Oleg