3.9.2 Macros

Libraries and top–level programs can define and use new kinds of derived expressions and definitions called syntactic abstractions or macros. A syntactic abstraction is created by binding a keyword to a macro transformer or, simply, transformer.

The transformer determines how a use of the macro (called a macro use) is transcribed into a more primitive form.

Most macro uses have the form:

(?keyword ?datum …)

where ?keyword is an identifier that uniquely determines the kind of form. This identifier is called the syntactic keyword, or simply keyword. The number of ?datums and the syntax of each depends on the syntactic abstraction.

Macro uses can also take the form of improper lists, singleton identifiers, or set! forms, where the second subform of the set! is the keyword:

(?keyword ?datum … . ?datum)
?keyword
(set! ?keyword ?datum)

The define-syntax, let-syntax and letrec-syntax forms create bindings for keywords, associate them with macro transformers, and control the scope within which they are visible.

The syntax-rules and identifier-syntax forms create transformers via a pattern language. Moreover, the syntax-case form allows creating transformers via arbitrary Scheme code.

Keywords occupy the same name space as variables. That is, within the same scope, an identifier can be bound as a variable or keyword, or neither, but not both, and local bindings of either kind may shadow other bindings of either kind.

Macros defined using syntax-rules and identifier-syntax are “hygienic” and “referentially transparent” and thus preserve Scheme’s lexical scoping.

• If a macro transformer inserts a binding for an identifier (variable or keyword) not appearing in the macro use, the identifier is in effect renamed throughout its scope to avoid conflicts with other identifiers.
• If a macro transformer inserts a free reference to an identifier, the reference refers to the binding that was visible where the transformer was specified, regardless of any local bindings that may surround the use of the macro.

Macros defined using the syntax-case facility are also hygienic unless datum->syntax is used.