I have done so. I have committed a new version of /comparison/SYB1_2/GMap.lhs with gmap2 of the type
Interesting.
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?
We're thinking about extending the Haskel Symposium paper to a journal version, in which we would also take efficiency into account (i.e., also try to set up something like a benchmark for generic programming). Would something like that be suitable you think? -- Johan