My current take, FWIW. * MPTCs are very useful. They came along very rapidly (well before H98). I think we must put them in H' * But MPTCs are hamstrung without FDs or ATs * FDs and ATs are of the same order of technical difficulty, as Martin says * ATs are (I believe) a bit weaker from the expressiveness point of view (zip example), but are (I believe) nicer to program with. * BUT we have way more experience with actually programming with FDs. ATs fail the "well-established" test by a mile. * Largely due to Martin's work, we now have a much better handle on just what restrictions on FDs make type inference tractable. So I believe there is a solid MPTC+FD story that we could embody in H'. * Medium term, I think ATs may *at the programming-language level* displace FDs, because they are nicer to program with. But that's just my opinion, and we don't have enough experience to know one way or the other. Tentative conclusion: H' should have MPTC + FDs, but not ATs. Simon | -----Original Message----- | From: haskell-prime-bounces@haskell.org [mailto:haskell-prime-bounces@haskell.org] On Behalf Of | Ross Paterson | Sent: 02 February 2006 11:25 | To: haskell-prime@haskell.org | Subject: MPTCs and functional dependencies | | On Thu, Feb 02, 2006 at 11:38:07AM +0100, John Hughes wrote: | > The problem with Haskell 98 is that it *lacks* features which | > have become absolutely essential to Haskell programmers today. Those | > features are what really *need* discussion and energy spent on them. | > | > [...] | > | > Multi-parameter classes with functional dependencies | > - used everywhere... for example in monad transformers... so | > *must* be included this time | > - omitted from Haskell 98 because "the right design" wasn't clear | > - it's still unclear! Functional dependencies *in some form* | > are essential, but associated types and datatypes look nicer | > in many ways! | > - is it too late, in practice, to replace fundeps by something | > else? How will we know? If we are to standardize on associated | > types instead, we need a major effort to *make sure* all | > important applications of fundeps can be represented. How will | > we organize that? | | I agree that MPTCs are much less useful (though not completely useless) | without something like FDs or associated types. But the specification | of FDs is far from clear: the system described in Mark's paper is quite | a bit weaker than what is implemented by GHC and (more shakily) by Hugs. | It seems that associated types aren't ready yet, but I don't think FDs | are either, accustomed as people are to them. | | I have another worry about MPTCs. They require require relaxations on | the form of instances (FlexibleInstances on the wiki), which in turn | require relaxations on contexts and thus deferred context reduction (see | FlexibleContexts). The result is that missing instances get reported | later than they do now. MPTCs are very useful and probably necessary, | but there is a cost. | | _______________________________________________ | Haskell-prime mailing list | Haskell-prime@haskell.org | http://haskell.org/mailman/listinfo/haskell-prime