A conflict between call-by-need computation and parallelism

Abstract

In functional language implementation, there is a folklore belief that there is a conflict between implementing call-by-need semantics and parallel evaluation. In this note we illustrate this by proving that reduction algorithms of a certain general and commonly used form which give call-byneed semantics offer very little parallelism. The analysis of lazy pattern-matching which leads to the above result also suggests an efficient sequential algorithm for the evaluation of a class functional programs satisfying certain constraints, an algorithm which respects the mathematical semantics of the program considered as a term rewrite system.