On Sun, May 29, 2011 at 7:59 PM, David Mazieres
One of the more disgusting but also powerful implications of FunctionalDependencies is that, in conjunction with OverlappingInstances and UndecidableInstances, you can do recursive programming at the type level. This has been (ab)used to do some cool things (e.g., HList, HaskellDB, OOHaskell).
It would seem very strange to me if haskell-prime made the choice of fundeps/type families based on the behaviour with OverlappingInstances. I'm under the impression that Overlapping is generally considered one of the more controversial extensions, and on the LanguageQualities wiki page [1] it's explicitly given as an example of something which violates them. So not only is Overlapping not in the language, but I imagine there are many people (myself included) who would like to ensure it stays out. My personal opinion is that if Haskell wants a more complete facility for type-level programming, that should be addressed directly, instead of via creative abuse of the class system and related machinery. Ben [1]: http://hackage.haskell.org/trac/haskell-prime/wiki/LanguageQualities