17 Creating Languages
The macro facilities defined in the preceding chapter let a programmer define syntactic extensions to a language, but a macro is limited in two ways:
a macro cannot restrict the syntax available in its context or change the meaning of surrounding forms; and
a macro can extend the syntax of a language only within the parameters of the language’s lexical conventions, such as using parentheses to group the macro name with its subforms and using the core syntax of identifiers, keywords, and literals.
The distinction between the reader and expander layer is introduced in Lists and Racket Syntax.
That is, a macro can only extend a language, and it can do so only at the expander layer. Racket offers additional facilities for defining a starting point of the expander layer, for extending the reader layer, for defining the starting point of the reader layer, and for packaging a reader and expander starting point into a conveniently named language.