### 3.3 Macro by Example ¶

`(require 'macro-by-example)`

A vanilla implementation of Macro by Example (Eugene Kohlbecker, R4RS) by Dorai Sitaram, (dorai @ cs.rice.edu) using `defmacro`.

- generating hygienic global `define-syntax` Macro-by-Example macros cheaply.
- can define macros which use `...`.
- needn’t worry about a lexical variable in a macro definition clashing with a variable from the macro use context
- don’t suffer the overhead of redefining the repl if `defmacro` natively supported (most implementations)

#### 3.3.1 Caveat ¶

These macros are not referentially transparent (see Macros in Revised(4) Scheme). Lexically scoped macros (i.e., `let-syntax` and `letrec-syntax`) are not supported. In any case, the problem of referential transparency gains poignancy only when `let-syntax` and `letrec-syntax` are used. So you will not be courting large-scale disaster unless you’re using system-function names as local variables with unintuitive bindings that the macro can’t use. However, if you must have the full r4rs macro functionality, look to the more featureful (but also more expensive) versions of syntax-rules available in slib Macros That Work, Syntactic Closures, and Syntax-Case Macros.

Macro: define-syntax keyword transformer-spec ¶

  The keyword is an identifier, and the transformer-spec should be an instance of `syntax-rules`.

  The top-level syntactic environment is extended by binding the keyword to the specified transformer.

      (define-syntax let*
        (syntax-rules ()
          ((let* () body1 body2 ...)
           (let () body1 body2 ...))
          ((let* ((name1 val1) (name2 val2) ...)
             body1 body2 ...)
           (let ((name1 val1))
             (let* (( name2 val2) ...)
               body1 body2 ...)))))

Macro: syntax-rules literals syntax-rule … ¶

  literals is a list of identifiers, and each syntax-rule should be of the form

  `(pattern template)`

  where the pattern and template are as in the grammar above.

  An instance of `syntax-rules` produces a new macro transformer by specifying a sequence of hygienic rewrite rules. A use of a macro whose keyword is associated with a transformer specified by `syntax-rules` is matched against the patterns contained in the syntax-rules, beginning with the leftmost syntax-rule. When a match is found, the macro use is trancribed hygienically according to the template.

  Each pattern begins with the keyword for the macro. This keyword is not involved in the matching and is not considered a pattern variable or literal identifier.