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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