> > A.hs:3:23: parse error on input: "("
>
> I should have said that I've implemented the choices given
> on the Standard Haskell web discussion page. In particular:
>
> ===================
> Choice 7a
> ~~~~~~~~~
>
> The context in a class declaration (which introduces superclasses)
> must constrain only type variables. For example, this is legal:
>
> class (Foo a b, Baz b b) => Flob a b where ...
> but not
> class (Foo [a] b, Baz (a,b) b) => Flob a b where ...
>
> It might be possible to relax this restriction (which is the same
> as in current Haskell) without losing decideability, but we're not
> sure. Choice 7a is conservative, and we don't know of any examples
> that motivate relaxing the restriction.
> ===================
>
> I'm frankly unsure of the consequences of lifting the
> restriction. Can you give a compact summary of why you want
> to? Our multi-parameter type-class paper gives none, and if
> you've got one I'd like to add it.
>
> In the short term, you're stuck. Damn! First customer too!
I ran into *exactly* the same problem with my own monad transformer
code, but haven't reported it yet because there's a lot of other stuff
I need to do to massage it into GHC-friendly form.
The problem is, the above parens appear in the standard definition of
a monad transformer, the motivating example for MPCs in the first
place!
Consider the state monad transformer:
> type StateT s m v = s -> m (v,s)
>
> instance Monad m => Monad (StateT s m) where
> return v = \s -> return (v,s)
> m >>= f = \s -> m s >>= \(v,s') ->
> (f v) s
>
> instance (Monad m, Monad (StateT s m)) -- here is the problem
> => MonadT (StateT s) m where
> lift m = \s -> m >>= \v ->
> return (v,s)
>
> class Monad m => StateMonad s m where
> getS :: m s
> setS :: s -> m ()
>
> instance Monad m => StateMonad s (StateT s m) where
> getS = \s -> return (s ,s)
> setS s = \_ -> return ((),s)
>
> instance (StateMonad s m, MonadT t m)
> => StateMonad s (t m) where
> getS = lift getS
> setS s = lift (setS s)
Note we assume here the definition of MonadT:
> class (Monad m, Monad (t m)) => MonadT t m where -- here again
> liftM :: m a -> t m a
You can see the two lines that violate 7a.
> Simon
--KW 8-)
--
: Keith Wansbrough, MSc, BSc(Hons) (Auckland) -------------------------:
: PhD Student, Computing Science, University of Glasgow, Scotland. :
: Native of Antipodean Auckland, New Zealand: 174d47' E, 36d55' S. :
: http://www.dcs.gla.ac.uk/~keithw/ mailto:[EMAIL PROTECTED] :
:----------------------------------------------------------------------:
