4.11. Parallel Or

``Parallel or'' is a classic idiom of parallel programming. The ``parallel or'' operation executes two expressions F and G in parallel, each of which may publish a single boolean, and returns the disjunction of their publications as soon as possible. If one of the expressions publishes true, then the disjunction is true, so it is not necessary to wait for the other expression to publish a value. This holds even if one of the expressions is silent.

The ``parallel or'' of expressions F and G may be expressed in Orc as follows:

val result =
  val a = F
  val b = G
  Ift(a)  >>  true | Ift(b)  >>  true | (a || b)


The expression (a || b) waits for both a and b to become available and then publishes their disjunction. However if either a or b is true we can publish true immediately regardless of whether the other variable is available. Therefore we run Ift(a) >> true and Ift(b) >> true in parallel to wait for either variable to become true and immediately publish the result true. Since more than one of these expressions may publish true, the surrounding val is necessary to select and publish only the first result.