[Haskell-cafe] Re: [Haskell] IVar

[Conal Elliott]

> main = do
>     --irrelevant  x <- newIVar
>     let y = last [1..]
>     print "test"  --was irrelevant  writeIVar x 3
>     print y

Exactly.  The termination concern doesn't seem to have to do with readIVar.

On Dec 4, 2007 11:56 AM, Isaac Dupree <isaacdupree at charter.net> wrote:

>
> >> A pure readIVar would be just like lazy I/O, with similar drawbacks.
> >> With readIVar, the effect that lets you observe the evaluation order
> >> is writeIVar; with hGetContents it is hClose.  Conclusion: it's
> >> probably no worse than lazy I/O.
> >
> > Actually, it's considerably better.
>
> +: implementation may not evaluate something that may terminate/block,
> anyway
>
> IVar may change from blocking to non-blocking, unlike most pure
> expressions.  This is only meaningful if trying to evaluate it would
> prevent it from gaining a value.  It can only have such an effect
> (helping the IVar gain a value) if the same thread would do any IO (if
> it wasn't for the unnecessary evaluation).  IO is generally observable,
> so getting stuck on a normal non-terminating expression wouldn't be any
> more acceptable than getting stuck on an IVar in that case.
>
> ... all assuming unsafeInterleaveIO doesn't exist, because it's unsafe,
> so the semantics are undefined of when it gets evaluated relative to
> later normally-sequenced IO actions, if it will be eventually evaluated
> for sure -- right?  Nevertheless its intended use generally assumes that
> it will be evaluated as if its evaluation might not terminate, i.e. as
> late as possible.  If it is in fact (a finite amount of) non-blocking
> IO, then evaluating it early will only have consequences on when the IO
> happens (and therefore its effect/result), not directly on the program's
> non-termination or actions.
>
> It gets more confusing the more I think about it!
>
>
> to modify Simon's example, it looks to me like treating readIVar as
> potentially non-terminating, and writeIVar as potentially having
> side-effects, is enough:
>
> main = do
>     --irrelevant  x <- newIVar
>     let y = last [1..]
>     print "test"  --was irrelevant  writeIVar x 3
>     print y
>
> Isaac
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