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RE: [Haskell-cafe] Re: no sparks?

Subject: RE: [Haskell-cafe] Re: no sparks?
From: "Sittampalam, Ganesh"
Date: Mon, 21 Dec 2009 17:07:20 -0000
\a b -> Left a `amb` Right b


From: [email protected] [mailto:[email protected]] On Behalf Of Jamie Morgenstern
Sent: 21 December 2009 16:50
To: Benedikt Huber
Cc: [email protected]
Subject: [Haskell-cafe] Re: no sparks?

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 <[email protected]> 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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