Just a follow up to my last post ... The HList paper also presents a way of removing overlapping instances from _any_ class. So infact support for overlapping instances is no longer required - and this removes all the messy problems with overlapping instances and functional dependancies. The current HList source distribution runs in hugs with "-98 +o" only because of "lazyness" on out part. All the occurances of overlapping instances can (will?) be removed from the source if it becomes an important issue (most of them are in auxilliary definitions that are not in the paper, like Show for HList. If you program in the "completely non overlapping instances" model, then compiler support for deriving TTypeable would be nice, or compiler support for a type level equality constraint (TypeEq could become a built-in). But just to make it clear - compiler support for this is not necessary, you just define instances of TTypeable for all your datatypes. There is a template-haskell library that can automatically derive TTypeable for any datatype as well. Keean. David Menendez wrote:
Keean Schupke writes:
Haskell already has static records (in H98)
Dynamic records are addressed by the HList library, which uses extensions already present in GHC and Hugs (namely Multi-parameter type-classes and function-dependancies).
Is this the case? Every implementation of HList that I've seen also uses overlapping and undecidable instances.