[Haskell-cafe] Unambiguous choice implementation

I'm reading Conal Elliot's paper, Push-Pull FRP. At some point he needs an unambiguous choice operator, essentially to implement select: a future that waits for one of its future arguments to fire. His implementation of unamb creates two threads racing on a shared MVar. By his own admission, this is very inefficient. My question is, is there a better implementation? -- [:Bartosz:]