Dear Guile users,
syntax-case macros can have cool side effects at expansion time.
However, they are still draped in a veil of mystery to me.
Consider this code:
(define-syntax hello
(lambda (stx)
(syntax-case stx ()
(_
(begin
(format (current-error-port) "Hello!\n")
(datum->syntax #f "hello"))))))
(define-syntax world
(lambda (stx)
(syntax-case stx ()
(_
(begin
(format (current-error-port) "World!\n")
(datum->syntax #f "world"))))))
(define-syntax hello-world
(lambda (stx)
(syntax-case stx ()
(_
#`(string-append #,hello " " #,world)))))
,expand hello-world
Running it gives me:
Hello!
World!
$1 = (string-append "hello" " " "world")
Cool! Now, I suspect there are no clear rules to guess the order of
expansion (whether Hello! is printed before World! or after). I would
very much like to twist that order and reliably get World! and then
Hello!. How can I achieve that?
(define-syntax hello-world
(lambda (stx)
(syntax-case stx ()
(_
(let* ((w #'world)
(h #'hello))
#`(string-append #,h " " #,w))))))
With this modification, I get the same order:
Hello!
World!
$1 = (string-append "hello" " " "world")
So my guess is that syntax objects are expanded lazily. Is there
something I can do to get #'world expanded before #'hello?
Best regards,
Vivien
