Simon Peyton-Jones:
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
Both FDs and ATs come in various versions of different levels of expressiveness. They may be of the same level of difficulty with all bells and whistles, but that's a bit of a red herring. The real question is how do the levels of difficulty compare at the level of expressiveness that we want. Or phrased differently, for a version that is not too difficult, how does the expressiveness compare.
* ATs are (I believe) a bit weaker from the expressiveness point of view (zip example), but are (I believe) nicer to program with.
The zip example can be done with ATs - it's actually in one of Martin's papers. I currently don't know of any FD example that you can't do with ATs. It's a matter of what forms of AT definitions you want to allow.
* BUT we have way more experience with actually programming with FDs. ATs fail the "well-established" test by a mile.
Indeed!
* 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.
Maybe not only at the programming-language level. Given our latest paper, http://www.cse.unsw.edu.au/~chak/papers/SCP06.html for example, the translation of ATs is simpler than FDs if we also have existential types (but admittedly that became clear to us only after your email message).
Tentative conclusion: H' should have MPTC + FDs, but not ATs.
My conclusion is that we should not include FDs or ATs into the standard at the moment. Standardising FDs as a stopgap measure may easily put us into the same situation that we are having with records at the moment. Nobody is really happy with it, but we don't dare to change it either. Manuel