The unamb package

unamb [1] contains the "unambiguous choice" operator "unamb" [2], which
wraps thread racing up in a purely functional, semantically simple
wrapper.  Given any two arguments u and v that agree unless bottom, the
value of unamb u v is the more terminating of u and v.  Operationally, the
value of unamb u v becomes available when the earlier of u and v does.
The agreement precondition ensures unamb's referential transparency.

unamb was originally a part of Reactive [3].  I moved it to its own
package in order to encourage experimentation.

Please share any comments & suggestions on the discussion (talk) page at
[1].

You can configure, build, and install all in the usual way with Cabal
commands.

  runhaskell Setup.lhs configure
  runhaskell Setup.lhs build
  runhaskell Setup.lhs install

References:

[1] http://haskell.org/haskellwiki/unamb
[2] http://conal.net/papers/simply-reactive
[3] http://haskell.org/haskellwiki/reactive