On Tue, Dec 26, 2006 at 07:01:52PM -0500, lists@qseep.net wrote:
I'm working on a program that I've been dabbling with for years. For the first time, I tried to compile it with GHC 6.6, and got an error, explaining that I was violating the Coverage Condition in my instance declaration. The instance declaration looks like this:
instance MonadReader r m => MonadReader r (CPST o m) where ...
1. Does 'CPS' stand for Continuation Passing Style? If so, have a look at Control.Monad.Cont.ContT and eliminate your problem entirely (free pre-debugged code) 2. Your problem, is that *looking only at the instance head*, r could be anything. GHC doesn't look at the context when deciding wether an instance is compatible with functional dependencies. 3. http://darcs.haskell.org/packages/mtl/Control/Monad/Cont.hs Looking at the standard CPS monad, there is a {-# OPTIONS_GHC -fallow- undecidable-instances #-} pragma and a comment to the effect of it being required for the MonadReader instance.