Bryan Donlan wrote:
{-# NOINLINE hDevRandom #-} hDevRandom = unsafePerformIO $ openFile "/dev/random" ReadMode
I wrote:
The NOINLINE guarantees that openFile is called only once. But does it guarantee that openFile is NOT called if we do not need it? We could check what the compilers actually do, but I am not sure we have a guarantee here.
There's commentary in GHC/Conc.lhs that this is the case: {-# NOINLINE pendingEvents #-} {-# NOINLINE pendingDelays #-} (pendingEvents,pendingDelays) = unsafePerformIO $ do startIOManagerThread reqs <- newIORef [] dels <- newIORef [] return (reqs, dels) -- the first time we schedule an IO request, the service thread -- will be created (cool, huh?) I don't know if this is a documented guarentee however.
Hmm. I'm not sure what that comment means. They are doing just what I did - creating only an empty IORef, with the actual resource allocated only when needed. Also, this is located inside a module that is explicitly GHC-specific. Regards, Yitz