<div dir="ltr">Several very elegant FRP approaches are emerging, most visibly FRP.Reactive, which rely on blocking on multiple variables at once, continuing when the *first* of them is available. . . inside an unsafePerformIO, so the beautiful STM "orElse" solution is not available. The current solution is to race threads against each other, and have the one that finishes first kill the other one. This is implemented, for example, in Data.Unamb. However, our empirical tests have shown that the GHC scheduler is not *quite* good enough to handle this efficiently, and ends up introducing too much latency and nondeterminacy.<br>
<br>The <a href="http://hackage.haskell.org/cgi-bin/hackage-scripts/package/data-ivar-0.1">Data.IVar</a> module, just uploaded to hackage, provides an alternative to thread racing as a solution to this problem. It provides *write-once* variables which can be blocked on in parallel, without forking any threads or using STM (so it is safe to use in unsafePerformIO). Example usage from the documentation:<br>
<br style="font-family: courier new,monospace;"><span style="font-family: courier new,monospace;"> import qualified Data.IVar as IVar</span><br style="font-family: courier new,monospace;"><span style="font-family: courier new,monospace;"> import Control.Concurrent</span><br style="font-family: courier new,monospace;">
<br style="font-family: courier new,monospace;"><span style="font-family: courier new,monospace;"> main = do</span><br style="font-family: courier new,monospace;"><span style="font-family: courier new,monospace;"> iv <- IVar.new</span><br style="font-family: courier new,monospace;">
<span style="font-family: courier new,monospace;"> iv' <- IVar.new</span><br style="font-family: courier new,monospace;"><span style="font-family: courier new,monospace;"> forkIO $ threadDelay 10000000 >> writeIVar iv' "my spoon is too big"</span><br style="font-family: courier new,monospace;">
<span style="font-family: courier new,monospace;"> let merger = IVar.read iv `mplus` IVar.read iv'</span><br style="font-family: courier new,monospace;"><span style="font-family: courier new,monospace;"> print =<< IVar.nonblocking merger -- most likely "Nothing"</span><br style="font-family: courier new,monospace;">
<span style="font-family: courier new,monospace;"> print =<< IVar.blocking merger -- waits a while, then prints</span><br style="font-family: courier new,monospace;"><span style="font-family: courier new,monospace;"> writeIVar iv' "i am a banana" -- throws error "IVar written twice"</span><br>
<br>Enjoy!<br><br>Luke</div>