Chapter 16

defmacro.ss: Non-Hygienic Macros

To load: (require (lib "defmacro.ss"))

(define-macro identifier expr)      SYNTAX

(define-macro (identifier . formals) expr ···1)      SYNTAX

Defines a (non-hygienic) macro identifier as a procedure that manipulates S-expressions (as opposed to syntax objects). In the first form, expr must produce a procedure. In the second form, formals determines the formal arguments of the procedure, as in lambda, and the exprs are the procedure body. In both cases, the procedure is generated in the transformer environment, not the normal environment (see section 12 in PLT MzScheme: Language Manual).

In a use of the macro,

(identifier expr ···)

syntax-object->datum is applied to the expression (see section 12.2.2 in PLT MzScheme: Language Manual), and the macro procedure is applied to the cdr of the resulting list. If the number of exprs does not match the procedure's arity (see section 3.12.1 in PLT MzScheme: Language Manual) or if identifier is used in a context that does not match the above pattern, then a syntax error is reported.

After the macro procedure returns, the result is compared to the procedure's arguments. For each value that appears exactly once within the arguments (or, more precisely, within the S-expression derived from the original source syntax), if the same value appears in the result, it is replaced with a syntax object from the original expression. This heuristic substitution preserves source location information in many cases, despite the macro procedure's operation on raw S-expressions.

After substituting syntax objects for preserved values, the entire macro result is converted to syntax with datum->syntax-object (see section 12.2.2 in PLT MzScheme: Language Manual). The original expression supplies the lexical context and source location for converted elements.

(defmacro identifier formals expr ···1)      SYNTAX

Same as (define-macro (identifier . formals) expr ···1).

Important: define-macro is still restricted by MzScheme's phase separation rules. This means that a macro cannot access run-time bindings because it is executed in the syntax expansion phase. Translating code that involves define-macro or defmacro from an implementation without this restriction usually implies separating macro related functionality into a begin-for-syntax or a module (that will be imported with require-for-syntax) and properly distinguishing syntactic information from run-time information.