Question

I am reading the real world haskell Chapter 9.
There is a maybeIO function wraps a IO function result into a Maybe:

    maybeIO :: IO a -> IO (Maybe a)
    maybeIO act = handle(\_ -> return Nothing) ( liftM Just act)

but this code wont work in GHC 7 or +, i wonder how to edit this code so it will work in GHC 7, i tried

    import Control.Exception (bracket, handle, SomeException(..))
    maybeIO :: IO a -> IO (Maybe a)
    maybeIO act = handle(\(SomeException e) -> return Nothing) ( liftM Just act)

but i got a Parse error: naked expression at top level

Answer 1

I didn’t get any such parse error – the only error I got was from missing functions:

so3.hs:3:62: Not in scope: `liftM'

You need to import Control.Monad which is where the liftM function is defined. That or use fmap instead of liftM.

Your parse error might be due to incorrect indentation, but a copy and paste of your code results in no such issues for me.