type families not advertised for 6.8

Remi Turk rturk at science.uva.nl
Sat Oct 20 16:25:20 EDT 2007

On Fri, Oct 19, 2007 at 08:25:22AM +0100, Simon Peyton-Jones wrote:
> | What does this imply for 6.8 support for FD's, as they now use
> | the same type-coercions?
> Actually FDs do not use type coercions, in GHC at least.  As Mark

Excuse me, it turns out I didn't look carefully enough: It's not
functional dependencies, it's classes-with-only-one-method:

module Bar where

bar = fmap id []

Compiles to the following Core with

Bar.bar :: forall a_a5M. [a_a5M]
Bar.bar =
  \ (@ a_a5M) ->
     `cast` ((GHC.Base.:Co:TFunctor) []
             :: (GHC.Base.:TFunctor) []
                forall a_a5G b_a5H. (a_a5G -> b_a5H) -> [a_a5G] -> [b_a5H]))
      @ a_a5M @ a_a5M (GHC.Base.id @ a_a5M) (GHC.Base.[] @ a_a5M)

Or does this simply mean that only type-functions (the type/axiom
stuff) is not supported in 6.8, but type coercions (~ and cast) are supported
(although perhaps not at the source level)?

Cheers, Remi

> originally described them, FDs guide inference; and in
> particular, they give rise to some unifications that would not
> otherwise occur.  In terms of the intermediate language, that
> means there is no "evidence" associated with a FD; it's just the
> type checker's business. That means that various
> potentially-useful things can't be expressed, notably when FDs
> are combined with existentials or GADTs, that involve *local*
> equalities, which were beyond the scope of Marks's original
> paper.
> As the recent thread about FDs shows, FDs are quite tricky, at
> least if one goes beyond the well-behaved definition that Mark
> originally gave.  (And FDs are much more useful if you go
> beyond.)
> Our current plan is to regard FDs as syntactic sugar for indexed
> type families.  We think this can be done -- see our IFL workshop
> paper http://research.microsoft.com/%7Esimonpj/papers/assoc-types
> No plans to remove them, however.  After all, we do not have much
> practical experience with indexed type families yet, so it's too
> early to draw many judgements about type families vs FDs.
> I recommend Iavor's thesis incidentally, which has an interesting
> chapter about FDs, including some elegant (but I think
> unpublished) syntactic sugar that makes a FD look more like a
> function.  I don't think it's online, but I'm sure he can rectify
> that.
> Simon
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