What about the following? It does use unsafePerformIO, but only to wrap newMVar in this specific case. once :: Typeable a => IO a -> IO a once m = let {-# NOINLINE r #-} r = unsafePerformIO (newMVar Nothing) in do y <- takeMVar r x <- case y of Nothing -> m Just x -> return x putMVar r (Just x) return x The "Typeable" constraint forces the return value to be monomorphic, which prevents the following from happening (the first line doesn't type check under the constraint):
let ref = once (newIORef []) :t ref ref :: forall a. IO (IORef [a]) ref >>= flip writeIORef "foo" ref >>= readIORef >>= (\(x::[Bool]) -> print x) [Illegal instruction
Additionally, I'd like to repeat the point that "once" (whether
defined my way or Keean's) is
not just a consequence of module initialization; it can actually
replace it in most cases!
For example:
myRef :: IO (IORef Char)
myRef = once (newIORef 'a')
readMyRef :: IO Char
readMyRef = myRef >>= readIORef
writeMyRef :: Char -> IO ()
writeMyRef c = myRef >>= flip writeIORef c
A library interface might consist of readMyRef and writeMyRef, while hiding
myRef itself from the user. However, what happens in IO stays in the
IO monad; myRef is an action, so the IORef is not initialized until
the first time
that one of read/writeMyRef is called. Indeed, any action wrapped by once
will only be run in the context of the IO monad. IMO, this is the primary
advantage of a function like once over the proposal for top-level
x <- someAction
where the exact time someAction is evaluated is unspecified.
Are there any applications of module initialization for which once
does not suffice?
-Judah
On Wed, 10 Nov 2004 17:11:31 +0000, Keean Schupke
I have written a small library for supporting one-shot without using unsfePerformIO... The library uses SYSV semaphores under linux to make sure the functional argument of "once" is only ever run once. It uses the ProcessID as the key for the semaphore, so will even enforce the once-only property accross different Haskell threads. Some semaphore functions are also exported, allowing other constraints to be used (for example, once only over multiple processes by using a constant ID rather than the processID.
I have attached the source for the library incase anyone is interested. If people think it is useful I could put it up on a website (let me know). Also attached is an example, which can be compiled with:
ghc -o test NamedSem.hs Test.hs -package posix
Keean.
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