Topic summary
Non-strict evaluation
Extracted from the Wikipedia article Evaluation strategy.
Non-strict evaluation
A non-strict evaluation order is an evaluation order that is not strict, that is, a function may return a result before all of its arguments are fully evaluated. The prototypical example is normal order evaluation, which does not evaluate any of the arguments until they are needed in the body of the function. Normal order evaluation has the property that it terminates without error whenever any other evaluation order would have terminated without error. The name "normal order" comes from the lambda calculus, where normal order reduction will find a normal form if there is one (it is a "normalizing" reduction strategy). Lazy evaluation is classified in this article as a binding technique rather than an evaluation order. But this distinction is not always followed and some authors define lazy evaluation as normal order evaluation or vice-versa, or confuse non-strictness with lazy evaluation.