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 of state. Side effects are introduced as a mechanism for effecting such decompositions. We find that the notion of side effect is inextricably wound up with the notion of identity. Dynamic scoping is retrospectively viewed as a restricted kind of side effect.

With this we summarize and conclude with many tantalizing questions yet unanswered.

In Part Three (in a separate paper) we will find that the introduction of side effects forces the issue of the order of evaluation of expressions. We will contrast call-by-name and its variants with call-by-value, and discuss how these control disciplines arise as a consequence of different models of packaging. In particular, call-by-name arises naturally from the syntactic nature of the Algol 60 copy rule. As before, many little interpreters for these disciplines will be exhibited.

In Part Four we will be led to generalize the notion of a syntactic package. We will discuss meta-procedures, which deal with the representations of procedures. The distinction between a procedure and its representation will be more carefully considered. Macro processors, algebraic simplifiers, and compilers will be considered as meta-procedures. Various interpreters, compilers, and simplifiers will be exhibited.