[Hs-Generics] make type witnesses 'phi ix' persistent?

[Sebastian Menge]

Am Sun, 19 Sep 2010 19:57:25 +0200
schrieb Dominique Devriese <dominique.devriese at cs.kuleuven.be>:

> > serialize :: forall phi ix . (Fam phi, El phi ix)
> >          => phi ix -> String
> > serialize MyWitness1 = "MyWitness1"
> > serialize MyWitness2 = "MyWitness2"
> >
> > But deserialize wont work:
> >
> > deserialize :: String -> phi ix -- <--- impossible
> > deserialize "MyWitness1" = MyWitness1
> > deserialize "MyWitness2" = MyWitness2
> >
> > Is there _any_ way to get this going ? Any dirty hacks are also
> > appreciated ;)
> 
> I didn't test this, but maybe it helps to dualize the deserialize
> function:
> 
> deserialize :: String -> (forall ix. phi ix -> r) -> r
> deserialize "MyWitness1" f = f MyWitness1
> deserialize "MyWitness2" f = f MyWitness2
> 
> I expect there's no way to do this with a function f whose return type
> depends on the type ix.

Why not? I guess, there is a good formal reason for this? 

Sebastian.