[Haskell-cafe] Re: no sparks?

[Benedikt Huber]

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