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Chapter 15
`defmacro.ss`: Non-Hygienic Macros

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

`(`define-macro` `identifier expr`)` S Y N T A X

`(`define-macro` ``(`identifier . formals`)` expr `···`1`)` S Y N T A X

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 `expr`s 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 `expr`s 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`)` S Y N T A X

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.