Josef Wolf <j...@raven.inka.de> writes:
> Hello,
>
> I am trying to work through the little schemer book. In order to make it
> easier to go through the examples, I've come up with the following macro:
>
> ;;; Helper to visualize
>
> (define-macro (disp exp)
> (display exp) (newline)
> `(display ,exp)
> ; (newline) (newline)
> )
>
> This works great, except that when I uncomment the (newline) expressions,
> the result of the evaluation is omitted:
>
> guile> (define-macro (disp exp)
> ... (display exp) (newline)
> ... `(display ,exp)
> ... ; (newline) (newline)
> ... )
> guile> (disp (+ 3 4)) (newline)
> (+ 3 4)
> 7
> guile> (define-macro (disp exp)
> ... (display exp) (newline)
> ... `(display ,exp)
> ... (newline) (newline)
> ... )
> guile> (disp (+ 3 4)) (newline)
> (+ 3 4)
>
>
>
> guile>
>
> Any hints?
The body of a define-macro definition is supposed to return the code
that should be substituted in place of the original macro call. With
the newlines there, the body returns *unspecified*, because that is what
(newline) returns.
Do you want those newlines to happen at macro-expansion time or eval
time? If the latter (which I would guess), the code that you want is
(define-macro (disp exp)
(display exp) (newline)
`(begin
(display ,exp)
(newline) (newline)))
Regards,
Neil
