Syntax Mutation

Can we change a variable from a transformer into a procedure?

R7RS explains that (define <variable> <expression>) is the same as (set! <variable> <expression>) if <variable> is bound to a non-syntax value -- but if <variable> is a syntactic keyword, then the definition would bind <variable> to a new location.

We tested:

Some of the tested systems do not aim at R7RS compliance, but it is interesting to see the results nevertheless.

1. re-defining a macro

(define-syntax f
  (syntax-rules ()
    ((_) -1)))

(define (f) -2)

(display (f))
(newline)
system REPL batch/compiled
Bigloo -1
Chibi -2 -2
Chicken -1 -2
Chez -2 -2
Cyclone -2 -1
Gambit -1 (error on define) error on define
Gauche -2 -2
Guile -2 -2
Kawa -2 ?
LIPS -2
Loko -2
MIT -2 error (Variable reference to a syntactic keyword)
Peroxide -2
Racket -2
Sagittarius -2 -2
Scheme48 -2
STklos -1 -1
scheme9 -1
Unsyntax -2 duplicate definition of identifier
Ypsilon -2

The same program was translated into Elisp, Common Lisp and Clojure:

system result
elisp -2
Clojure -2
Common Lisp -2

2. re-setting a macro to a value

(define-syntax f
  (syntax-rules ()
    ((_) -1)))

(define (change-f)
  (set! f (lambda () -2)))

(display (f))
(newline)
system REPL batch/compiled
Bigloo Unbound variable
Chez invalid syntax
Chibi invalid use of syntax
Cyclone -2 Error: Unbound variable
Chicken -1 -2
Gambit Macro name can't be used as a variable: f Macro name can't be used as a variable: f
Gauche -2 -2
Guile set! - not a transformer set! - not a transformer
Kawa error - constant variable f is set! unexpected exception while compiling: java.lang.NullPointerException
LIPS -2 n/a
Loko invalid syntax
MIT Variable required
Peroxide cannot set non-variable f
Racket set!: cannot mutate syntax identifier
Sagittarius syntax error n/a
Scheme48 -1 (after warning: invalid assignment)
STklos -1 (after error: variable 'f' unbound) error ('f' unbound)
Scheme9 -1 n/a
Unsyntax -1 (after error: not a variable transformer 'f')
Ypsilon error in set!: misplaced syntactic keyword as variable

The same program was translated into Elisp, Common Lisp and Clojure:

system result
elisp -1
Clojure -2
Common Lisp -2

(In Common Lisp, the function value of f was changed with (setf (symbol-function 'f) (lambda () -2))

n/a = no compiler, or no difference between "compiled" and "running from the REPL".


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