Opportunistically Parallel Lambda Calculus(dl.acm.org)

  3.4 Evaluation Strategy

  The confluence of our semantics enables us to soundly step statements in unusual orders. However, for an implementation, we need an actual evaluation strategy. Sequential, call-by-value evaluation would step statements in order. Lazy evaluation would work backward from the return variable, stepping statements needed to turn it into a value. By contrast, the idea of opportunistic evaluation is to step as many statements as we can, anywhere we can. Some may not be able to step yet, e.g. a function call where the function does not yet have a known definition, but we will step all the rest.