Re: [Chicken-users] unbound variable: or

[Peter Bex]

On Sat, May 30, 2015 at 03:42:41PM +0200, Michele La Monaca wrote:
> On Sat, May 30, 2015 at 4:23 AM, John Cowan <address@hidden> wrote:
> > Jinsong Liang scripsit:
> >
> >> I want to learn some basic macro programming in Chicken. However, it seems
> >> there are multiple macro definition APIs in Chicken: define-syntax,
> >> syntax-rules, syntax-case, define-macro. Which one should I start with?
> >
> > Define-macro is completely obsolete and not supported in Chicken 4 or
> > any modern Scheme.
> 
> Gambit has it. Too bad Chicken 4 dropped it.
> 
> I don’t think describing define-macro "obsolete” is 100% correct.

As has been pointed out time and again, it is fundamentally broken.
See also below:

> Let’s say someone had the (bad?) idea to masquerade/embed it in other
> more baroque forms. For example in Chicken you have to write:
> 
> (define-syntax apply-any
>   (er-macro-transformer
>     (lambda (form rename compare)
>       (let ((x (cadr form)) (lst (caddr form)))
>         (eval `(cons ',x ,lst))))))
> 
> instead of:
> 
> (define-macro apply-any
>  (lambda (x lst)
>    (eval `(cons ‘,x ,lst))))

Looks like you are mixing up two things: on one hand, define-macro
offers the syntactic convenience of not having to pick apart the input
form.  On the other hand, define-macro does not support hygienic
renaming at all (except for gensym, which only works on identifiers
created by the macro itself).

If it's the syntactic convenience you're after, take a look at the
"bindings" egg by Juergen Lorenz.  It offers various improvements
on the basic er-macro-transformer and ir-macro-transformer syntax.

> Oh, and it is quite powerful. I’ve used Gambit to
> practice. Only when you dominate it, I suggest to move on to
> syntax-rules. The latter works at a higher layer (see below), enforces
> hygiene and introduces you to pattern matching (which is not a macro
> specific topic).
> 
> (define-syntax apply-any
>   (syntax-rules ()
>     ((_ x lst) (eval (cons 'x lst)))))
> 
> (define l '(#f (+ 1 2) (print "hello")))
> 
> (apply-any or l)
> 
> => 3
> 
> (apply-any + (list 1 2 3))
> 
> => 6
> 
> (let ((x 9)) (apply-any or '(#f (+ x 1) (print "hello"))))
> 
> Error: unbound variable: x
> 
> Oops.

Eval does not have access to the lexical scope where it is
invoked.

Cheers,
Peter

signature.asc
Description: Digital signature