Re: Fw: Macro Question

Hi again!

Let me for completeness add that the "fix" shown in the previous email is
more an explanation than a "solution" to the problem.

A "solution" (with the possibility of a default value and a specified
value) would be something like:

(module mm)

(define-class foo a)

(define-syntax identity
    (syntax-rules ()
        ((_ x) x)))

(define-syntax darn
    (syntax-rules ()
        ((_  ) (let ((v 1))
                    (instantiate::foo ((identity a) v))))
        ((_ x) (let ((v x))
                    (instantiate::foo ((identity a) v))))))

(print (darn))
(print (darn 10))


Best regards

Bent

2017-01-06 0:34 GMT+01:00 Bent Phillipsen <[email protected]>:

> Hello Joe!
>
> IMO this is expected behavior - but doesn't work, as the variable in let
> will be generated:
>
> ------------------------------------------------------------
> -----------------------------
>
> (define-syntax bad-macro
>         (syntax-rules ()
>                 ((_ val body) (let ((i val)) body))))
>
> (bad-macro 5 (* 2 i))  ;does not work - "i" in let will be a newly
> generated variable
>
>
> (define-syntax good-macro
>         (syntax-rules ()
>                 ((_ i val body) (let ((i val)) body))))
>
> (good-macro i 5 (* 2 i))  ;does work
>
> ------------------------------------------------------------
> -----------------------------
>
>
> So you probably need:
>
> (module mm)
>
> (define-class foo a)
>
> (define-syntax darn
>    (syntax-rules ()
>       ((_ a )
>        (let ((v 1))
>           (instantiate::foo (a v))))))
>
> (print (darn a))
>
> ... which compiles and gives the output:
>
> #|foo [a: 1]|
>
>
> Best regards
>
> Bent
>
>