With help from Martin Sulzmann and Ross Paterson, GHC (HEAD) now implements a richer form of functional dependencies than Mark Jones's version, but still decidable etc. The rules for what must appear in the context of an instance declaration are also relaxed.
The specification is here: http://www.haskell.org/ghc/dist/current/docs/users_guide/type-extensions .html#instance-decls I think this is a step forward, and a serious candidate for Haskell'. I think that if you stick to these rules, everything is nailed down as Martin so rightly says it should be. And I am not sure we can go much further. Of course -fallow-undecidable-instances still lifts all restrictions, and then all bets are off. Many thanks to Ross and Martin. You can try it out by downloading a GHC snapshot (or by building from source). Simon | -----Original Message----- | From: [EMAIL PROTECTED] [mailto:[EMAIL PROTECTED] On Behalf Of | isaac jones | Sent: 11 February 2006 01:29 | To: haskell-prime@haskell.org | Subject: the MPTC Dilemma (please solve) | | I've created a wiki page and a ticket to record solutions to what I'm | calling the "Multi Parameter Type Class Dilemma". It's summarized | thusly: | | MultiParamTypeClasses are very useful, but mostly in the context of | FunctionalDependencies. They are particularly used in the monad | transformer library found in fptools. The dilemma is that functional | dependencies are "very, very tricky" (spj). AssociatedTypes are | promising but unproven. Without a solution, Haskell' will be somewhat | obsolete before it gets off the ground. | | I've proposed a few solutions. Please help to discover more solutions | and/or put them on the ticket/wiki. | | Wiki page: | http://hackage.haskell.org/trac/haskell-prime/ticket/90 | | Ticket: | http://hackage.haskell.org/trac/haskell-prime/wiki/MultiParamTypeClasses Dilemma | | | peace, | | isaac | | | | -- | isaac jones <[EMAIL PROTECTED]> | | _______________________________________________ | Haskell-prime mailing list | Haskell-prime@haskell.org | http://haskell.org/mailman/listinfo/haskell-prime _______________________________________________ Haskell-prime mailing list Haskell-prime@haskell.org http://haskell.org/mailman/listinfo/haskell-prime
