lennart at augustsson.net
Sun Dec 9 04:25:10 EST 2007
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 <coeus at gmx.de> 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.
> 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:
> 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 at haskell.org
> > http://www.haskell.org/mailman/listinfo/haskell
> Haskell mailing list
> Haskell at haskell.org
-------------- next part --------------
An HTML attachment was scrubbed...
More information about the Haskell