[Hs-Generics] Re: compositional gMap in SYB1

[oleg at okmij.org]

Hello!


Alexey wrote:
> 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.)

Yes, this is correct. Although gmap2 could be given a more general
type, like (a -> b) -> f x -> f y
in which case the run-time errors are significant. Such errors will
arise when mapping id on [Int] and expect [Bool] in the result. The
mapping will succeed btw on empty lists.

Alexey wrote:
> I will mention this in the presentation though (and
> update the technical report, which admittedly is rather delayed).

Thank you! Perhaps the journal version of the comparison paper (which
several people suggested) could discuss the issue in some detail (at
least from the perspective of performance and static-dynamic
trade-off).

It seems there might be a possibility of a five-minute presentation
about gmap during the `Hot Topics' discussion at the Generic
Programming Workshop in Victoria in September -- provided the
organizers will schedule the hot topics session and deem gmap hot
enough. Should we inquire the workshop organizers?

	I will be back on Aug 18. I was in Japan and Taiwan in July,
and will be in Germany (Hamburg) in August. I guess I get a chance to
compare German trains with Japanese ones.

Claus wrote:
> if I had followed my own choice of metaphor to the end, I would have
> known that X marks the spot only on a map, not on the real thing (or
> else everyone could find the treasure..).

I really like that metaphor!

	Cheers,
	Oleg