[Haskell-cafe] Re: no sparks?

[Jamie Morgenstern]

Thank you for all of the responses! The amb package is something like what I
want; though, as aforementioned, the right and left rules won't return the
same proof and so we can't really use it here.

I've been thinking about this problem generally, not just in the Haskell
setting. It makes sense (in the very least, with theorem proving)
to allow

a p|| b

to return the value of a or b, whichever returns first, wrapped in a
constructor which would allow you to case analyze which result returned

case (a p|| b) of
 (1, Xa) = ...
 (2, Xb) = ...


On Sun, Dec 20, 2009 at 8:52 PM, Benedikt Huber <benjovi at gmx.net> wrote:

> Daniel Fischer schrieb:
>
>> Am Sonntag 20 Dezember 2009 23:25:02 schrieb Jamie Morgenstern:
>>
>>> Hello;
>>>
>>> Also, I was wondering if something akin to a "parallel or" exists. By
>>> this,
>>> I mean I am looking for a function which, given x : a , y : a, returns
>>> either, whichever computation returns first.
>>>
>>
>> This wouldn't be easy to reconcile with referential transparency.
>> You can do that in IO, roughly
>>
>> m <- newEmptyMVar
>> t1 <- forkIO $ method1 >>= putMVar m
>> t2 <- forkIO $ method2 >>= putMVar m
>> rs <- takeMVar m
>> killThread t1
>> killThread t2
>> return rs
>>
>
> And in this case (returning (Maybe Proof)), you are not happy with any of
> the results, but with one of the proofs. So you would need something like
>
> solve :: Ctx -> Prop -> Int -> IO (Maybe Proof)
> solve ctx goal n = amb leftRight rightLeft
>  where
>    leftRight = m1 `mplus` m2
>    rightLeft = m2 `mplus` m1
>
>    m1 = (tryRight ctx goal n)
>    m2 = (tryLeft ctx goal n)
>
> I think the idea of directly supporting this kind of thing is quite
> interesting.
>
> benedikt
>
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