I would claim that it's fine to use the type readIVar :: IVar a -> a if you're willing to give the "right" semantics to newIVar :: IO (IVar a) The semantics is that sometimes when you create an IVar you'll get one that always returns _|_ when read, sometimes you'll get a proper one. Now if you happen to read an IVar and it deadlocks your program, well, sorry, you were unlucky and got a bad IVar that time.
So it's possible to explain away the deadlock as something non-deterministic in the IO monad. Doing so comes at a terrible price though, because you can no longer reason about your program. -- Lennart On Dec 9, 2007 7:48 PM, Conal Elliott <[EMAIL PROTECTED]> wrote: > Thanks, Luke. I'd been unconsciously assuming that the IVar would get > written to (if ever) by a thread other than the one doing the reading. > (Even then, there could be a deadlock.) > > - Conal > > > On Dec 9, 2007 9:37 AM, Luke Palmer <[EMAIL PROTECTED]> wrote: > > > On Dec 9, 2007 5:09 PM, Conal Elliott <[EMAIL PROTECTED]> wrote: > > > (moving to haskell-cafe) > > > > > > > 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". > > > > > > The same argument applies any to pure function, doesn't it? For > > instance, a > > > non-IO version of succ is unnecessary. My question is why make > > readIVar a > > > blocking IO action rather than a blocking pure value, considering that > > it > > > always returns the same value? > > > > But I don't think it does. If we're single-threaded, before we > > writeIVar on it, > > it "returns" bottom, but afterward it returns whatever what was written. > > It's > > a little fuzzy, but that doesn't seem referentially transparent. > > > > Luke > > > > > - Conal > > > > > > On Dec 8, 2007 11:12 AM, 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.Environmentreturns > > > > > values in IO. > > > > > > > > > > Paul. > > > > > > > > > > > > > > > > > _______________________________________________ > > > > > Haskell mailing list > > > > > [EMAIL PROTECTED] > > > > > > > > > http://www.haskell.org/mailman/listinfo/haskell > > > > > > > > > > > > > > > > > > > > > > > > > > _______________________________________________ > > > Haskell-Cafe mailing list > > > [email protected] > > > http://www.haskell.org/mailman/listinfo/haskell-cafe > > > > > > > > > > > _______________________________________________ > Haskell-Cafe mailing list > [email protected] > http://www.haskell.org/mailman/listinfo/haskell-cafe > >
_______________________________________________ Haskell-Cafe mailing list [email protected] http://www.haskell.org/mailman/listinfo/haskell-cafe
