[Haskell-cafe] Proof that Haskell is RT

Dan Doel dan.doel at gmail.com
Thu Nov 13 07:14:44 EST 2008

On Wednesday 12 November 2008 7:05:02 pm Jonathan Cast wrote:
> I think the point is that randomIO is non-deterministic (technically,
> pseudo-random) but causal --- the result is completely determined by
> events that precede its completion.  unsafeInterleaveIO, by contrast, is
> arguably (sometimes) deterministic --- but is, regardless, definitely
> not causal.

Sure, it's weird if you actually know how it's allegedly making its decisions 
(predicting the future, but not really). But all you can actually observe is 
that sometimes it gives (1,1), and sometimes it gives (2,1), and maybe it 
coincidentally always seems to give (1,1) to f, but to g, which is 
observationally equal to g, excepting bottoms, it always seems to give (2,1). 
That's a weird situation, but if you make things opaque enough, I think you 
can fall back on IO's nondeterminism.

Another weird situation (it seems to me) goes like:

    foo = unsafeInterleaveIO $ fail "foo"

    do x <- foo
       putStrLn "bar"
       return $! x

where you can't simply fall back to saying, "foo throws an exception," because 
it doesn't prevent "bar" from printing. So what is actually throwing an 
exception is some later IO action that doesn't have any relation to foo 
besides using a 'pure' result from it. However, I suppose you can push off such 
oddities in a similar way, saying that IO actions can throw delayed, 
asynchronous exceptions, and what do you know, it always happens to end up in 
the action that evaluated x. :)

Of course, those sorts of explanations might be good enough to conclude that 
unsafeInterleaveIO doesn't break referential transparency, but they may make 
for a very poor operational semantics of IO and such. foo throws some delayed 
asynchronous exception? Well, that doesn't tell when or how. And it doesn't 
explain how weirdTuple 'nondeterministically' chooses between (1,1) and (2,1). 
So that would probably lead you to another semantics (a more operational one) 
that brings up how weirdTuple reads the future, or even that mutation happens 
as a result of evaluating pure values. But, the point is, I think, that such a 
semantics doesn't indicate that referential transparency is being violated 
(which would be a notion from another sort of semantics where there's no way 
to notice something weird is going on).

Anyhow, I'm done rambling. (Hope it didn't go on too long; I know we weren't 
fundamentally in any kind of disagreement.) :)

-- Dan

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