Am Montag 21 Dezember 2009 01:00:10 schrieb Felipe Lessa: > On Mon, Dec 21, 2009 at 12:39:06AM +0100, Daniel Fischer wrote: > > Am Sonntag 20 Dezember 2009 23:25:02 schrieb Jamie Morgenstern: > > > 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 > > > > But in pure code, I think not. > > There's 'unamb' in Hackage, however I think you should carefully > understand its implementation details before using it. Not that > I use it myself.
Which is a (much) refined version of the above, wrapped in unsafePerformIO. It is your obligation to call it only when legit, because it is *not* referentially transparent: Prelude Data.Unamb> let go :: Int -> Bool -> Bool; go 0 b = b; go n b = let c = not b in c `seq` go (n-1) c Prelude Data.Unamb> :set +s Prelude Data.Unamb> let fun n = go n True (0.00 secs, 0 bytes) Prelude Data.Unamb> unamb (fun $ 10^6) (fun $ 10^6+1) Loading package unamb-0.2.2 ... linking ... done. True (2.38 secs, 75324960 bytes) Prelude Data.Unamb> unamb (fun $ 10^6) (fun $ 10^6+1) False (2.29 secs, 73940848 bytes) > > Link: http://hackage.haskell.org/package/unamb > > -- > Felipe. _______________________________________________ Haskell-Cafe mailing list Haskell-Cafe@haskell.org http://www.haskell.org/mailman/listinfo/haskell-cafe
