3.1.9 Procedure calls and syntactic keywords

Whereas (+ 23 42), (f 23), and ((lambda (x) (+ x 42)) 23) are all examples of procedure calls, lambda and let expressions are not. This is because let, even though it is an identifier, is not a variable, but is instead a syntactic keyword. A form that has a syntactic keyword as its first subexpression obeys special rules determined by the keyword. The let identifier in a definition is also a syntactic keyword. Hence, definitions are also not procedure calls.

The rules for the lambda keyword specify that the first subform is a list of parameters, and the remaining subforms are the body of the procedure. In let expressions, the first subform is a list of binding specifications, and the remaining subforms constitute a body of expressions.

Procedure calls can generally be distinguished from these special forms by looking for a syntactic keyword in the first position of a form: if the first position does not contain a syntactic keyword, the expression is a procedure call. (So–called identifier macros allow creating other kinds of special forms, but are comparatively rare.) The set of syntactic keywords of Scheme is fairly small, which usually makes this task fairly simple. It is possible, however, to create new bindings for syntactic keywords. Derived forms and macros