Before we can talk about what right and wrong we need to know what the semantics of IVars should be.
On Dec 8, 2007 7:12 PM, Marc A. Ziegert <[EMAIL PROTECTED]> wrote: > many many answers, many guesses... > let's compare these semantics: > > readIVar :: IVar a -> IO a > readIVar' :: IVar a -> a > readIVar' = unsafePerformIO . readIVar > > so, we do not need readIVar'. it could be a nice addition to the > libraries, maybe as "unsafeReadIVar" or "unsafeReadMVar". > but the other way: > > readIVar v = return $ readIVar' v > > does not work. with this definition, readIVar itself does not block > anymore. it's like hGetContents. > and... > > readIVar v = return $! readIVar' v > > evaluates too much: > it wont work if the stored value evaluates to 1) undefined or 2) _|_. > it may even cause a 3) deadlock: > > do > writeIVar v (readIVar' w) > x<-readIVar v > writeIVar w "cat" > return x :: IO String > > readIVar should only return the 'reference'(internal pointer) to the read > object without evaluating it. in other words: > readIVar should wait to receive but not look into the received "box"; it > may contain a nasty undead werecat of some type. (Schrödinger's Law.) > > - marc > > > > > > Am Freitag, 7. Dezember 2007 schrieb Paul Johnson: > > Conal Elliott wrote: > > > Oh. Simple enough. Thanks. > > > > > > Another question: why the IO in readIVar :: IVar a -> IO a, instead > > > of just readIVar :: IVar a -> a? After all, won't readIVar iv yield > > > the same result (eventually) every time it's called? > > Because it won't necessarily yield the same result the next time you run > > it. This is the same reason the stuff in System.Environment returns > > values in IO. > > > > Paul. > > _______________________________________________ > > Haskell mailing list > > Haskell@haskell.org > > http://www.haskell.org/mailman/listinfo/haskell > > > > > > _______________________________________________ > Haskell mailing list > Haskell@haskell.org > http://www.haskell.org/mailman/listinfo/haskell > >
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