On Tue, Jul 29, 2008 at 9:51 AM, <oleg@okmij.org> wrote:

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.

That's a clever combination of both techniques. You are passing around an explicit type representation at run-time, to ensure that the transforming function applies only to X-positions. Also you don't pretend that there are X-values in there (as Claus' version did) so there is no need for unsafeCoerce.

One question: the runtime checks in traverse are for internal consistency only, right?. I could not think of runtime errors arising from gmap2-calls. Furthermore, I think the issue of unfold errors is solved, right? (Although these properties are not known by the type system.)

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?

Yes, that's a pity. I will mention this in the presentation though (and update the technical report, which admittedly is rather delayed).

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,

Alexey