Using your own `mainIO'

Normally, the GHC runtime system begins things by called an internal function

        mainIO :: IO ()

which, in turn, fires up your `Main.main'. The standard definition of `mainIO' looks like this:

        mainIO = catch Main.main 
                   (\err -> error ("I/O error: " ++ 
                                        showsPrec 0 err "\n"))

that is, all it does is run `Main.main', catching any I/O errors that occur and displaying them on standard error before exiting the program.

To subvert the above process, you need only provide a `mainIO' of your own (in a module named `GHCmain').

Here's a little example, stolen from Alastair Reid:

        module GHCmain ( mainIO ) where
        
        import GlaExts
        
        mainIO :: IO ()
        mainIO = do
                 sleep 5
                 _ccall_ printf "%d\n" (14::Int)
        
        sleep :: Int -> IO ()
        sleep t = _ccall_ sleep t