module

Modularity, Objects, and State

State 狀態

3.1.3 The Costs of Introducing Assignment

As we have seen, assignment enables us to model objects that have local state. However, this advantage comes at a price. Our programming language can no longer be interpreted in terms of the substitution model of function application that we introduced in section 1.1.5. Moreover, no simple model with “nice” mathematical properties can be an adequate framework for dealing with objects and assignment in programming languages

So long as we do not use assignments, two evaluations of the same function with the same arguments will produce the same result, so that functions can be viewed as computing mathematical functions. Programming without any use of assignments, as we did throughout the first two chapters of this book, is accordingly known as functional programming.

Environment

Once we admit assignment into our programming language, such a definition is no longer adequate. In particular, section 3.1.3 argued that, in the presence of assignment, a name cannot be considered to be merely representing a value. Rather, a name must somehow designate a “place” in which values can be stored. In our new model of evaluation, these places will be maintained in structures called environments.