5.8 Macro utilities ¶

Macro: syntax-error msg arg … ¶

Macro: syntax-errorf fmt arg … ¶

Signal an error. They are same as error and errorf (see Signaling exceptions), except that the error is signaled at macro-expansion time (i.e. compile time) rather than run time.

They are useful to tell the user the wrong usage of macro in the comprehensive way, instead of the cryptic error from the macro transformer. Because of the purpose, arg … are first passed to unwrap-syntax to strip off the internal syntactic binding informations (see Identifiers).

(define-syntax my-macro
  (syntax-rules ()
    ((_ a b)   (foo2 a b))
    ((_ a b c) (foo3 a b c))
    ((_ . ?)
     (syntax-error "malformed my-macro" (my-macro . ?)))))

(my-macro 1 2 3 4)
  ⇒ error: "malformed my-macro: (my-macro 1 2 3 4)"