fundeps help

[Ganesh Sittampalam]

Hi,

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 -}

It seems as if inferring that b1 = b2 is precisely what "improvement" is 
about, but I'm not really sure when GHC actually applies that. Is there 
any documentation of that? I've read Mark Jones' paper, the haskell-prime 
wiki entry about Fundeps and the GHC manual but am still rather lost.

Cheers,

Ganesh