Syntax objects as traditional pairs

[Jean Abou Samra <jean@abou-samra.fr>]

Hi,

I am lost as to when a syntax object whose syntax->datum is a pair
can be manipulated as a plain Scheme pair. For example:


(define synt1 #'(a b))
(pk 'syntax synt1 'pair? (pair? synt1))
(pk 'car (car synt1))

(define-syntax mysyntax
   (lambda (stax)
     (syntax-case stax ()
       ((_ thing)
        (begin
          (pk 'syntax #'thing 'pair? (pair? #'thing))
          (pk 'car (car #'thing)))))))

(mysyntax (c d))


results in


;;; (syntax (#<syntax a> #<syntax b>) pair? #t)

;;; (car #<syntax a>)

;;; (syntax #<syntax:test.scm:13:10 (c d)> pair? #f)
Backtrace:
            7 (primitive-load "/home/jean/repos/lilypond/build/test.s…")
In ice-9/eval.scm:
    721:20  6 (primitive-eval (mysyntax (c d)))
In ice-9/psyntax.scm:
   1229:36  5 (expand-top-sequence (#<syntax:test.scm:13:0 (mysynta…>) …)
   1121:20  4 (parse _ (("placeholder" placeholder)) ((top) #(# # …)) …)
   1342:32  3 (syntax-type (mysyntax #<syntax:test.scm:13:10 (c d)>) # …)
   1562:32  2 (expand-macro #<procedure 7fa6fa335320 at ice-9/eval.s…> …)
In ice-9/eval.scm:
     159:9  1 (_ #(#(#<directory (guile-user) 7fa6fa3dfc80>) #<synt…>))
     155:9  0 (_ _)

ice-9/eval.scm:155:9: In procedure car: Wrong type argument in position 
1 (expecting pair): #<syntax:test.scm:13:10 (c d)>


Why does one syntax object look like a pair and not the other?
What is the difference between the two cases?

Thanks,
Jean