Signaling errors in macro transformers

(syntax-error {message} {args} ... ) syntax

syntax-error behaves similarly to error (6.11) except that implementations with an expansion pass separate from evaluation should signal an error as soon as syntax-error is expanded. This can be used as a syntax-rules template for a pattern that is an invalid use of the macro, which can provide more descriptive error messages. message is a string literal, and args arbitrary expressions providing additional information. Applications cannot count on being able to catch syntax errors with exception handlers or guards.

(define-syntax simple-let
  (syntax-rules ()
    ((_ (head ... ((x . y) val) . tail)
         body1 body2 ...)
     (syntax-error
       "expected an identifier but got"
       (x . y)))
    ((_ ((name val) ...) body1 body2 ...)
     ((lambda (name ...) body1 body2 ...)
       val ...))))