2012-10-14 14:34, Gábor Lehel skrev:
So apologies for constantly suggesting new things, but if we have,
- All Typeable instances for type constructors are generated by the compiler, and - All Typeable instances for composite types (if that's the word?) are via instance (Typeable f, Typeable a) => Typeable (f a), and - User-written instances, and therefore overlap, are disallowed,
how difficult would it be to add:
foo :: Typeable (f a) => Dict (Typeable f, Typeable a) -- data Dict c where Dict :: c => Dict c
i.e. to make it possible to go in the other direction, and deduce that if a composite type is Typeable, its components must also be?
(alternate encoding: foo :: Typeable (f a) => ((Typeable f, Typeable a) => r) -> r)
Use case: nothing very serious, if it would take significant work, it's not worth it.
I have a use case: http://hackage.haskell.org/packages/archive/syntactic/1.3/doc/html/Data-Dyna... This is a reimplementation of Data.Dynamic to support casting type `a` to `Dynamic` given a constraint `Typeable (a -> b)`: toDyn :: Typeable (a -> b) => P (a -> b) -> a -> Dynamic With your suggestion, it seems I should be able to use the ordinary Data.Dynamic instead. / Emil