Thu Feb 9 04:13:34 PST 2006 [EMAIL PROTECTED]
* Add -fallow-undecidable-instances to some Control.Monad modules
I have recently tightened up GHC's implementation of the coverage
condition. As a result some of the Control.Monad modules are rejected.
Example:
class (Monad m) => MonadReader r m | m -> r where
instance (Monoid w, MonadReader r m) => MonadReader r (WriterT w m)
Here, fv(Writer w m) is not a superset of fv(r).
The flag allows it. I wonder if it's possible to use these modules
to send the type cheker into a loop.
M ./Control/Monad/Cont.hs +5
M ./Control/Monad/Error.hs +3
M ./Control/Monad/List.hs +3
M ./Control/Monad/State.hs +7
M ./Control/Monad/Writer.hs +7
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