| I'm trying to understand what fundeps do and don't let me do. One
| particular source of confusion is why the following program doesn't
| typecheck:
|
| {-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies #-}
| module Fundeps where
|
| class Dep a b | a -> b, b -> a
|
| conv :: (Dep a b1,Dep a b2) => b1 -> b2
| conv = id
| {- end of program -}
GHC implements "improvement" by *identifying* the two equal types. Here they
cannot be identified because they are separate type variables. A good way to
think of this is to imagine translating the program into System F: you can't do
it. Functional dependencies guide type inference by adding extra unifications,
resolving types that would otherwise be under-constrained and ambiguous -- but
fundeps (or at least GHC's implementation thereof) does not deal with the case
where the types become *over*-constrained.
GHC's new intermediate language, System FC, is specifically designed to do
this. Currently we're in transition: equality constraints are starting to
work, but fundeps are implemented as they always were. I hope we can
eventually switch over to implementing fundeps using equality constraints, and
then the above program will work.
Meanwhile, in the HEAD you can write
conv :: (a~b) => a -> b
conv = id
Which, IHMO, is a much clearer way to say it!
You may also like to try the paper that Martin and I and others wrote about
fundeps:
http://research.microsoft.com/%7Esimonpj/papers/fd-chr
Simon
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