16.2 General Macro Transformers

The define-syntax form creates a transformer binding for an identifier, which is a binding that can be used at compile time while expanding expressions to be evaluated at run time. The compile-time value associated with a transformer binding can be anything; if it is a procedure of one argument, then the binding is used as a macro, and the procedure is the macro transformer.

The syntax-rules and syntax-id-rules forms are macros that expand to procedure forms. For example, if you evaluate a syntax-rules form directly (instead of placing on the right-hand of a define-syntax form), the result is a procedure:

> (syntax-rules () [(nothing) something])

#<procedure>

Instead of using syntax-rules, you can write your own macro transformer procedure directly using lambda. The argument to the procedure is a value that represents the source form, and the result of the procedure must be a value that represents the replacement form.