Contents of macros

Appendix: Macros

This appendix describes an extension to Scheme that allows programs to define and use new derived expression types. A derived expression type that has been defined using this extension is called a macro.macro

Derived expression types introduced using this extension have the syntax
$\begin{scheme} (\hyper{keyword} \arbno{\hyper{datum}})% \end{scheme}$
where keyword is an identifier that uniquely determines the expression type. This identifier is called the syntactic keyword, or simply keyword, of the macro. The number of the datums, and their syntax, depends on the expression type.

Each instance of a macro is called a use of the macro. The set of rules, or more generally the procedure, that specifies how a use of a macro is transcribed into a more primitive expression is called the transformer of the macro.

The extension described here consists of three parts:

A set of expressions used to establish that certain identifiers are macro keywords, associate them with macro transformers, and control the scope within which a macro is defined,
a convenient pattern language that makes it easy to write transformers for most macros, and
a compatible low-level macro facility for writing macro transformers that cannot be expressed by the pattern language.

With this extension, there are no reserved identifiers. The syntactic keyword of a macro may shadow variable bindings, and local variable bindings may shadow keyword bindings. All macros defined using the pattern language are ``hygienic'' and ``referentially transparent'':hygienic referentially transparent

If a macro transformer inserts a binding for an identifier (variable or keyword), the identifier will in effect be 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.

This appendix is divided into three major sections. The first section describes the expressions and definitions used to introduce macros, i.e. to bind identifiers to macro transformers.

The second section describes the pattern language. This pattern language is sufficient to specify most macro transformers, including those for all the derived expression types from section $[*]$ . The primary limitation of the pattern language is that it is thoroughly hygienic, and thus cannot express macros that bind identifiers implicitly.

The third section describes a low-level macro facility that could be used to implement the pattern language described in the second section. This low-level facility is also capable of expressing non-hygienic macros and other macros whose transformers cannot be described by the pattern language, and is important as an example of a more powerful facility that can co-exist with the high-level pattern language.

The particular low-level facility described in the third section is but one of several low-level facilities that have been designed and implemented to complement the pattern language described in the second section. The design of such low-level macro facilities remains an active area of research, and descriptions of alternative low-level facilities will be published in subsequent documents.