[Haskell] Re: Fundep broken in GHC 6.6

Yitzchak Gale gale at sefer.org
Thu Nov 16 09:14:10 EST 2006

On 11/12/06, I wrote:
> The following class declaration worked in GHC 6.4.
> I recently upgraded to 6.6 (following Debian), and
> now it is broken.
> class Error e => Game b mv e | b -> mv e where
>     newBoard :: MonadState b m => m ()
> ...

Hmm, following up on Simon Peyton-Jones' recent
announcement of an overhaul of GHC's type
inference machinery, and reviewing the Haskell-Prime
ticket on fundeps, I see that this is not so simple.

I commend the work that is being done on this
important topic.

I hope it will be possible to get fundeps that will work
well with monad transformers and their various type
classes. For that, we will need fundeps that allow at
least some simple resolution back through class
context restraints on functions, instances, and classes.
(Or some suitable alternative.)

In the meantime, perhaps the -fallow-undecidable-instances
flag should have a similar effect for fundeps. Or maybe
there should be a separate flag for fundeps. Is this


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