### 4.3 Macros

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 <datum>s, 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.

The macro definition facility consists of two 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, and
- a pattern language for specifying macro transformers.

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 [Kohlbecker86], [ hygienic], [Bawden88], [macrosthatwork], [syntacticabstraction]:

- 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. Note that a `define` at top level may or may not introduce a binding; see section Definitions.
- 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.