Dear all, Simon PJ wrote:
Multi-parameter type classes, yes. Functional dependencies, no.
My experience is that even with very simple applications of MPTCs, I often end up needing functional dependencies to make things work. Thus, if my hunch is right, and other people have a similar experience, my fear would be that omitting FDs or something similar would result in a Haskell' standard that ends up being out of date even before it is finalised, in that a large number of real world applications and libraries would fail to be standard compliant and could not easily be adapted to be. I think that would be very unfortunate, even though I do agree that the Haskell' effort should strive to be as conservative as possible. Might it be possible to at least allow some simple forms of FDs that would cover the most common cases, e.g. like what's described in Mark's original paper? I personally would not be too worried about adopting a possibly non "future-proof" solution in this respect, as I would expect that the amount of code that would have to be change would be fairly small if some scheme like associated types later are adopted as a replacement. What would worry me is the lack of ANY functionality like FDs, as having to work without them means that code often would have to be too type specific, which in the end might result in lots of code duplication. Refactoring such code later would seem to be much more work (even if it might not be strictly necessary to do so, as the code would continue to work). But maybe other people have a somewhat difference experience, and feel that this is less of an issue than I believe it is? All the best, /Henrik -- Henrik Nilsson School of Computer Science and Information Technology The University of Nottingham nhn@cs.nott.ac.uk This message has been checked for viruses but the contents of an attachment may still contain software viruses, which could damage your computer system: you are advised to perform your own checks. Email communications with the University of Nottingham may be monitored as permitted by UK legislation.