MPTCs and functional dependencies

John Meacham john at
Thu Feb 2 09:33:39 EST 2006

On Thu, Feb 02, 2006 at 11:36:34AM -0000, Simon Peyton-Jones wrote:
> I'm confident that it is premature to standardise functional
> dependencies at this stage, very useful though they are.  If you doubt
> me, read the JFP journal submission that Martin Sulzmann and Peter
> Stuckey and I have been working on.
> Fundeps are very, very tricky.
> My own view is that associated types are a more promising way forward at
> the programming language level.  They are a little less expressive than
> fundeps, I think, but a whole lot less tricky.  But we don't have an
> implementation of them yet, so we can hardly standardise them!
> Multi-parameter type classes, yes.  Functional dependencies, no.

Yeah, I have been coming to the same conclusion myself. it pains me a
lot. (monad transformers! I need thee!) but its not like fundeps will go
away, they will just still be experimental so it isn't the end of the
world. My main reasons for feeling this way are that assosiated type
synonyms seem to solve the problems fundeps were meant to solve, and the
other uses seem to be various tricks in order to finagle the class
system into giving a sort of  type-level programming. which is cool, but
it makes me think there is room for an actual type-level programming
extension there somewhere. (user defined kinds + GADTs?)


John Meacham - ⑆⑆john⑈ 

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