| type MVar# s elt -- primitive
|
| newMVar# :: State# s -> (# State# s, MVar# s elt #)
| takeMVar# :: SynchVar# s elt -> State# s -> (# State# s, elt #)
| putMVar# :: SynchVar# s elt -> State# s -> State# s
Bad idea to look at the primops. The important things are
the MVar type
the operations newMVar, takeMVar, putMVar
the forkIO operation
Their semantics are described (completely I think) in my "Tackling the
awkward squad" tutorial.
| As another aside, in the hierarchical libraries docs (package base,
| Control.Concurrent.MVar) there appears to be an ambiguity:
|
| readMVar :: MVar a -> IO a
| This is a combination of takeMVar and putMVar; ie. it takes the
| value from the MVar, puts it back, and also returns it.
|
| It specifically avoids describing the combination as atomic, so I
| am uncertain whether the thread can be pre-empted between the 'take'
| and the 'put'? Normally the answer would be yes, but then there is no
| benefit from having a separate 'readMVar' to combine the operations,
| so I am guessing that it /should/ be atomic.
False. readMVar is completely described by
readMVar m = do { v <- takeMVar m; putMVar m v; return v }
If it gets pre-empted between the take and the put, no matter; anyone
trying to take from that same MVar will block.
Simon
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