On Thu, Aug 20, 2009 at 16:54, Max Desyatov <explicitcall@googlemail.com> wrote:

Sean Leather <leather@cs.uu.nl> writes:

> I'm not sure the problem you're running into is strictly a generic
> programming (GP) one. Typically, GP takes code that is often written
> and generalizes it, so that it doesn't have to be written for multiple
> datatypes.

That seems to be GP problem, as your solution doesn't scale well when I
wan't to add/remove/change fields in the `Row` record. The perfect way
as I see it, would be just editing `Row` data declaration, nothing else.
Studying few papers about GP in Haskell, I reckon this could be
represented as generic traversal, using my `Row` declaration with
`Either`. I don't see really good way to write a generic producer from
`[String]` to version of `Row` without `Either`. But SYB doesn't
provide a way for passing type-class-parametric functions to gmapT, and
SYB-with-class has large overhead of its usage. I don't have enough
time to find out how this can be written in SYB-with-class, if it really can be
written. The restriction of EMGM was described in my initial message.

Indeed SYB doesn't work here because Typeable-based run-time type comparison only works for monomorphic types. Doing something like

readRow l = gmapT (mkT (\(Left (Just ri) :: E Name) -> Right $ l `atMay` ri >>= readMay))

would work, but this, of course, is not what you want. I'm guessing the polymorphic typeOf previously described by Oleg [1] could help here, were it integrated in SYB.

I don't think syb-with-class will help you here, since it only adds modularity to the type-based function extension. I think you would still have to write a case for every field in the Row record.

Multirec would possibly work, were it not for the fact that it doesn't support parametric datatypes yet...

Cheers,
Pedro

[1] http://osdir.com/ml/haskell-cafe@haskell.org/2009-03/msg00212.html