The problem I see here is how to proove the IO in safeIO is indeed safe. Perhaps "UnsafeIO" is a better name, as infact the IO is still unsafe - the compiler has to take special notice of this type and not inline its definitions.
Your oneShot function has the same problem - if the compiler inlines the funtion you get two 'oneShot' functions.
Keean.
Adrian Hey wrote:
On Friday 05 Nov 2004 7:03 pm, MR K P SCHUPKE wrote:
Could someone give an example of what these things are that need to be
initialised and that are safe.
Here's a utility I've concocted for dealing with partial ordering constraints on initialisation of foreign libraries..
oneShot :: IO a -> IO (IO a) oneShot io = mdo mv <- newMVar $ do a <- io let loop = do putMVar mv loop return a loop return $ do act <- takeMVar mv act
The idea being that oneShot takes a real initialising action as argument and returns a new action which will perform the real initialisation at most once, no matter how many times it's used.
Suppose I want to use this to create a userInit (which is exported) from a realInit (which isn't exported).
Currently I have to write..
userInit :: IO <whatever> userInit = unsafePerformIO $ oneShot realInit
but I think what I would really like is something like this perhaps..
-- For use from SafeIO monad oneShotSafeIO :: IO a -> SafeIO (IO a) <same definition>
-- For use from IO monad oneShotIO :: IO a -> IO (IO a) oneShotIO io = liftSafeIO $ oneShotSafeIO io
userInit :: IO <whatever> userInit <- oneShotSafeIO realInit
Though this could be simplified if SafeIO could be made a sub-type of IO I guess (but I don't know a way to do this).
Regards -- Adrian Hey
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