[Haskell-cafe] Unambiguous choice implementation

[Bartosz Milewski]

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:]
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