Re: [Haskell-cafe] Trying to make a Typeable instance

Benja Fallenstein wrote:

Hi Adrian,

2007/7/8, Adrian Hey :

...

So it seems ghc doesn't like kinds (* -> *) either :-(

Actually, AFAICT the problem seems to be with Data.Typeable itself rather than ghc. There is no proper TypeRep for (ListGT map k a) because map is not a type.

Have you tried using (Typeable1 map) as the constraint?

Yes, see my earlier post. This seems to "work" in that the code compiles and showing the resulting TypeRep at least terminates in the tests I did so far. But I don't see how it can be regarded correct as AFAICT types like this just aren't properly representable as a TypeRep (as provided by Data.Typeable). But I might be wrong. As is often the case, the documentation for Data.Typeable is somewhat terse and inadequate IMO, so it's not at all clear to me how it should be used or what other users can reasonably expect from "sane" instances. Typeable1 etc.. seem problematic in that although the typeOf methods apparently return TypeReps, I don't see how they can possibly be correct. Surely they must be returning a representation of a type constructor that's masquerading as a TypeRep? In fact that's exactly what the macros in Typeable.h seem to do so I guess this is generally accepted as correct, but it seems wrong to me :-( AFAICT it's unsafe to assume that a TypeRep represents an actual proper type (I.E. the type that a monomorphic expression may have). Maybe that's intentional, I dunno. It'd be nice if the Data.Typeable Haddock made it clear exactly what a "TypeRep" is supposed to be a representation of :-) Regards -- Adrian Hey