Lenient evaluation is neither strict nor lazy

What is a non-strict functional language? Is a non-strict language necessarily lazy? What additional expressiveness brings non-strictness, with or without laziness? This paper tries to shed some light on these questions. First, in order to characterize non-strictness, di1erent evaluation strategies are introduced: strict, lazy, and lenient. Then, using program examples, how these evaluation strategies di1er from each other is examined, showing that non-strictness, even without laziness, allows a more general use of recursive de3nitions. We also report on a small experiment that we performed to examine how, in practice, laziness was used in a number of lazy benchmark programs. c © 2001 Elsevier Science Ltd. All rights reserved.