In Chapter 2 we mentioned the importance of supplying a means for abstraction in a programming language. The procedure was the earliest major form of abstraction and continues to be a central concept, so it certainly deserves a chapter of its own. A procedural abstraction is a mapping from a set of inputs to a set of outputs that can be described by a specification. The specification must show how the outputs relate to the inputs, but it need not reveal or even imply the way the outputs are to be computed. We can view the procedure concept in a programming language as an abstraction because it allows a user to focus only on what is done (at the point of call) and not on how it is accomplished.
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