Set syntax

• define a macro g
• define a local macro f that uses g
• set! the identifier g to a procedure
• use the macro f

Will the system expand f using the macro g, the procedure g, or will it signal an error?

(define-syntax g
  (syntax-rules ()
    ((g 2) -3)))

(let-syntax ((f (syntax-rules ()
                  ((f 1) (g 2)))))
  (set! g (lambda (x) -1000))
  (f 1))

• These allow the redefinition, returning -3: Chicken, Gauche, Cyclone, STklos after version 1.70

• Kawa complains, saying identifiers bound to macros are immutable

• These complain about the set! not being correct: Chibi, Guile, Gambit, MIT, Peroxide, Loko, Chez, Sagittarius, Unsyntax, Ypsilon

• Bigloo acts as if we were trying to set! an undefined variable (which is also true!)

• STklos' behavior (up to version 1.70) is different - it complains about not being able to match a clause when expanding the macro (g was registered as a macro, and we set it to a procedure that does not do clause matching).

Back to Scheme Surveys

Page source (GitHub)