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Expression types are categorized as primitive or derived. Primitive expression types include variables and procedure calls. Derived expression types are not semantically primitive, but can instead be defined as macros. Suitable syntax definitions of some of the derived expressions are given in Derived expression types.
The procedures force, promise?, make-promise, and
make-parameter are also described in this chapter because they
are intimately associated with the delay, delay-force,
and parameterize expression types.