buzzard(118)% cat Bug5.hs
import Control.Monad.State
data S = S Int
newtype M a = M (StateT S IO a)
deriving (Monad)
main = return ()
buzzard(119)% ghc -c Bug5.hs
Bug5.hs:3:
Can't make a derived instance of `Monad M'
(too hard for cunning newtype deriving)
When deriving instances for type `M'
buzzard(120)% ghc --version
The Glorious Glasgow Haskell Compilation System, version 5.04.2
The real problem above is that I forgot to enable extensions. It would
be helpful if the error message indicated that as the problem.
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