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 section 7.3 (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.