Simon Peyton-Jones
This question from John Meacham is really a Libraries Team question. Should the MonadIO class get an extra method? His case looks plausible to me.
And to me. If the operations only involved 'Monad', I would resist strengthening the constraint but since it involves the IO monad and exceptions are an essential part of IO (the real world often fails to behave as planned), I see no problem. The only question I see is whether it is best done by adding a superclass: class Monad m where ... class Monad m => MonadCatch m where catch :: ... class MonadCatch m => MonadIO m where liftIO :: IO a -> m a -- Alastair ps This reminds me of an observation Mark P Jones made in the Hugs source code when he implemented Haskell 1.3 IO. He pointed out a very pleasing symmetry between the success operations (>>= and return) and the failure operations (catch and fail). They have similar types and similar semantics. This symmetry just screams at you if you implement the IO monad using continuation passing (as Hugs did until last week).