The Vocabulary of Advanced Scheme

The foundation of any language is its vocabulary. In Beginning Student Scheme, we distinguish four categories of words: variables, constants, primitive functions, and keywords. The classification ignores parentheses but we know that every compound phrase is surrounded by a pair of parentheses, and that every atomic phrase stands on its own.

Advanced Student Scheme respects this basic classification, though it contains four important new keywords: local, lambda, set!, and begin. The first two are important for organizing and abstracting programs; the last two are important for the computation of effects. Still, keywords per se have no meaning. They are road signs that tell us what is ahead so that we can orient ourselves. It is the grammar and the meaning of a language that explains the role of the keywords.