A friend is making bindings to a C library that offers some fast math operations. The normal way to use the library is like this: int a = ...; int b = ...; int c = ...; int d = ...; int x = ...; int m, n; create_lookup_table(x); m = perform_math(a, b, x); n = perform_math(c, d, x); We see that the lookup table for x must be created before we can perform math in the field/ring/what-have-you defined by x. Once we have created the table, though, we're done. My friend would like to create a pure interface to this library. One thought was to write an interface to perform_math that checked if the table was created, created it if not, and did all this while locking an MVar so that no other instance could be called at the same time, trashing the table. Doing this behind unsafePerformIO would seem to be the ticket. We end up with an implementation like this: module FastMath where import Control.Concurrent import Foreign import Foreign.C foreign import ccall create_lookup_table :: CInt -> IO () foreign import ccall perform_math :: CInt -> CInt -> CInt -> IO CInt masterLock = unsafePeformIO (newMVar [CInt]) safe_perform_math a b x = do list <- takeMVar masterLock toPut <- if not (x `elem` list) then do create_lookup_table x return (x:list) else return list result <- perform_math a b x putMVar masterLock toPut return result performMath a b x = unsafePerformIO (safe_perform_math a b x) This does not compile but I think it gets the point across. Is this approach safe? The unsafePerformIO in conjunction with locking has me worried. -- Jason Dusek () ascii ribbon campaign - against html e-mail /\ www.asciiribbon.org - against proprietary attachments