the MPTC Dilemma (please solve)

Simon Peyton-Jones simonpj at
Mon Feb 20 12:14:53 EST 2006

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:

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

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).


| -----Original Message-----
| From: haskell-prime-bounces at
[mailto:haskell-prime-bounces at] On Behalf Of
| isaac jones
| Sent: 11 February 2006 01:29
| To: haskell-prime at
| 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:
| Ticket:
| peace,
|   isaac
| --
| isaac jones <ijones at>
| _______________________________________________
| Haskell-prime mailing list
| Haskell-prime at

More information about the Haskell-prime mailing list