[Haskell-cafe] Re: [Haskell] IVars

Conal Elliott conal at conal.net
Sun Dec 9 14:48:45 EST 2007


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 <lrpalmer at gmail.com> wrote:

> On Dec 9, 2007 5:09 PM, Conal Elliott <conal at conal.net> 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 <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.
> > > 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.
> > >
> > >
> > >
> > > > _______________________________________________
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> > > > Haskell at haskell.org
> > >
> > > > http://www.haskell.org/mailman/listinfo/haskell
> > > >
> > >
> > >
> > >
> >
> >
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>
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