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(define-compiler-syntax NAME)syntax
(define-compiler-syntax NAME TRANSFORMER)syntax

Defines what is usually called a compiler macro in Lisp: NAME should be the name of a globally or locally bound procedure. Any direct call to this procedure will be transformed before compilation, which allows arbitrary rewritings of function calls.

TRANSFORMER can be a syntax-rules expression or a transformer procedure (as returned by er-macro-transformer or ir-macro-transformer). Returning the original form in an explicit/implicit-renaming macro or simply "falling trough" all patterns in a syntax-rules form will keep the original expression and compile it normally.

In the interpreter this form does nothing and returns an unspecified value.

Compiler-syntax is always local to the current compilation unit and can not be exported. Compiler-syntax defined inside a module is not visible outside of that module.

define-compiler-syntax should only be used at top-level. Local compiler-syntax can be defined with let-compiler-syntax.

(define-compiler-syntax +
  (syntax-rules ()
    ((_) 1)
    ((_ x 0) x) ) )

If no transformer is given, then (define-compiler-syntax NAME) removes any compiler-syntax definitions for NAME.