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5.4 Macro utilities

Macro: syntax-error msg arg …
Macro: syntax-errorf fmt arg …

Signal an error. They are same as error and errorf (See section 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 expander. Because of the purpose, arg … are first passed to unwrap-syntax described below, to strip off the internal syntactic binding informations.

 
(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: (mymacro 1 2 3 4)"
Function: unwrap-syntax form

Removes internal syntactic information from form. In order to implement a hygienic macro, macro expander replaces symbols in the macro form for identifiers, which captures the syntactic environment where they are defined. Although it is necessary information for the compiler, it is rather confusing for users if it appears in the messages. This function replaces occurrences of identifiers in form to the original symbols.


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