| I think the implementation is some 90% complete though, in GHC head. | Certainly you can write many associated types programs already -- the | missing part is finishing off associated type synonyms, iirc.
...and we have a working implementation of that too, thanks to Tom Schrijvers. It's not in the HEAD yet, but it will be in a few weeks.
so it will be a part of 6.8? great news! afaiu, ATS, rather than AT, is direct substitution for FD?
Functional dependencies desugar to indexed type families, an extension of the original associated, in GHC head already. For the full story, see the wiki page.
http://haskell.org/haskellwiki/GHC/Indexed_types
which includes an example of porting functional dependencies to associated types
http://haskell.org/haskellwiki/GHC/Indexed_types#A_associated_type_synonym_e...
It's really simple to replace a functional dependency with a type function. A class declaration class C a b | a -> b becomes: class FD a ~ b => C a b where type FD a and an instance instance C [x] x becomes instance C [x] x where type FD [x] = x That's it: only class and instance declarations have to be modified. Now you can start to drop dependent parameters, if you want. There are a few examples in my slides as well (if you don't mind the pptx): http://www.cs.kuleuven.be/~toms/Research/talks/Tyfuns.pptx Cheers, Tom -- Tom Schrijvers Department of Computer Science K.U. Leuven Celestijnenlaan 200A B-3001 Heverlee Belgium tel: +32 16 327544 e-mail: tom.schrijvers@cs.kuleuven.be