2.11 Macros

(This section is largely taken from the Revised^4 Report on the Algorithmic Language Scheme. The section on Syntactic Closures is derived from a document written by Chris Hanson. The section on Explicit Renaming is derived from a document written by William Clinger.)

Scheme programs can define and use new derived expression types, called macros. Program-defined expression types have the syntax

(keyword datum …)

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 that specifies how a use of a macro is transcribed into a more primitive expression is called the transformer of the macro.

MIT/GNU Scheme also supports anonymous syntactic keywords. This means that it’s not necessary to bind a macro transformer to a syntactic keyword before it is used. Instead, any macro-transformer expression can appear as the first element of a form, and the form will be expanded by the transformer.

The macro definition facility consists of these 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 standard high-level pattern language for specifying macro transformers, introduced by the syntax-rules special form.
• Two non-standard low-level languages for specifying macro transformers, syntactic closures and explicit renaming.

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” and thus preserve Scheme’s lexical scoping:

• 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.