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{-# Language MagicHash, UnboxedTuples #-}
-- | Note: extensive testing of atomic operations is performed in the
-- "atomic-primops" library. Only extremely rudimentary tests appear
-- here.
import GHC.IO
import GHC.IORef
import GHC.ST
import GHC.STRef
import GHC.Prim
import GHC.Base
import Data.Primitive.Array
import Data.IORef
import Control.Monad
------------------------------------------------------------------------
casArrayST :: MutableArray s a -> Int -> a -> a -> ST s (Bool, a)
casArrayST (MutableArray arr#) (I# i#) old new = ST$ \s1# ->
case casArray# arr# i# old new s1# of
(# s2#, x#, res #) -> (# s2#, (isTrue# (x# ==# 0#), res) #)
casSTRef :: STRef s a -- ^ The 'STRef' containing a value 'current'
-> a -- ^ The 'old' value to compare
-> a -- ^ The 'new' value to replace 'current' if @old == current@
-> ST s (Bool, a)
casSTRef (STRef var#) old new = ST $ \s1# ->
-- The primop treats the boolean as a sort of error code.
-- Zero means the CAS worked, one that it didn't.
-- We flip that here:
case casMutVar# var# old new s1# of
(# s2#, x#, res #) -> (# s2#, (isTrue# (x# ==# 0#), res) #)
-- | Performs a machine-level compare and swap operation on an
-- 'IORef'. Returns a tuple containing a 'Bool' which is 'True' when a
-- swap is performed, along with the 'current' value from the 'IORef'.
--
-- Note \"compare\" here means pointer equality in the sense of
-- 'GHC.Prim.reallyUnsafePtrEquality#'.
casIORef :: IORef a -- ^ The 'IORef' containing a value 'current'
-> a -- ^ The 'old' value to compare
-> a -- ^ The 'new' value to replace 'current' if @old == current@
-> IO (Bool, a)
casIORef (IORef var) old new = stToIO (casSTRef var old new)
------------------------------------------------------------------------
-- Make sure this Int corresponds to a single object in memory (NOINLINE):
{-# NOINLINE mynum #-}
mynum :: Int
mynum = 33
main = do
putStrLn "Perform a CAS within an IORef"
ref <- newIORef mynum
res <- casIORef ref mynum 44
res2 <- casIORef ref mynum 44
putStrLn$ " 1st try should succeed: "++show res
putStrLn$ " 2nd should fail: "++show res2
------------------------------------------------------------
putStrLn "Perform a CAS within a MutableArray#"
arr <- newArray 5 mynum
res <- stToIO$ casArrayST arr 3 mynum 44
res2 <- stToIO$ casArrayST arr 3 mynum 44
putStrLn$ " 1st try should succeed: "++show res
putStrLn$ " 2nd should fail: "++show res2
putStrLn "Printing array:"
forM_ [0..4] $ \ i -> do
x <- readArray arr i
putStr (" "++show x)
putStrLn ""
------------------------------------------------------------
putStrLn "Done."
|
