What does it mean to say that a functional programming language is “strict” or “non-strict”?
• In a strict language, the arguments to a function are always evaluated before it is invoked. As a result, if the evaluation of an expression exp does not terminate properly (for example, because it generates a run-time error or enters an infinite loop), then neither will an expression of the form f(exp). ML and Scheme are both examples of this. • In a non-strict language, the arguments to a function are not evaluated until their values are actually required. For example, evaluating an expression of the form f(exp) may still terminate properly, even if evaluation of exp would not, if the value of the parameter is not used in the body of f. Miranda and Haskell are examples of this approach. There is much debate in the functional programming community about the relative merits of strict and non-strict languages. It is possible, however, to support a mixture of these two approaches; for example, some versions of the functional language Hope do this.
