Twan van Laarhoven wrote:
If MonadIO were in base, then the base library itself could also use it. For example the functions in System.IO could be lifted to work on any MonadIO monad. Whether that is a good idea is completely orthogonal to this discussion, however.
The main problem is that exceptions don't work well with MonadIO in GHC. So really MonadIO is currently only a toy and cannot be used in production code. The reason for this is that just about any operation involving exceptions ultimately depends (via the source code of base library functions) on the function block :: IO a -> IO a and that type is hard-wired in a GHC primitive. Additional primitives to support things like startBlocking :: IO () stopBlocking :: IO () which would play well with MonadIO, could be added to GHC, but they would lose important optimizations. I'm not sure about the order of magnitude of the cost - whether it would just make things run more slowly, or render them completely unusable. If the former, I am in favor of this proposal, but only combined with the addition of GHC support for startBlocking and stopBlocking. If the latter, then there is no point to this proposal. Regards, Yitz