[Haskell-cafe] Re: [Haskell] IVars

[Lennart Augustsson]

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 <conal at conal.net> 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 <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.Environmentreturns
> > > > > values in IO.
> > > > >
> > > > > Paul.
> > > >
> > > >
> > > >
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> > > >
> > > >
> > >
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