[Jscheme-user] Re: Questions on jscheme

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At 08:54 AM 9/29/2004 -0400, Michael Thome wrote:
>I've never used scheme enough to know if these are stupid questions, but:

They're not.

>1.  why does "define" only act in the dynamic context where "set!" works will merely prefer the dynamic context?  I would have expected define to always apply to the global context...
>> (let ((x 1)) (define blab (lambda () (set! x (+ x 1)))))
>(lambda ?? ()...)
>> (blab)
>(blab )
>  ====================================
>SchemeException: ERROR: undefined variable "blab"
>> 

5.2.2 Internal definitions

Definitions may occur at the beginning of a <body> (that is, the body of a lambda, let, let*, letrec.  These definitions are local to be body.

>2. why can't you use define wherever you want? e.g.:
>> (let () (print "who") (define (me) 1))
>SchemeException: Jscheme requires all embedded defines to appear first in procedure bodies
>You must move (define (me) 1) up

Because the defines at the top of the body are collected and turned into a letrec.

I once wrote a (def) macro that let you put defines anywhere.  It grouped defines into nested letrec's.  While a real Schemer might find this strange, it acts more like java and tends to keep things less nested.  I'll see if i can come up with it again. 

>3. is there a destructuring-bind hiding in there somewhere?

No.  But i have used a simple macro:
(define-macro (dbind var pattern . body)
  ;; pattern must be valid lambda argument list.
  `(apply (lambda ,pattern ,@body) ,var))

(dbind '(1 2 3 4 5) (a b . c) (+ a b))

-> 3.

For the notam project, look at src/scm/matcher.scm for a simple matcher here's an example:
(case-match
             e
             (`((xsd:group  . ,x))) ; ignore
             (`((xsd:element (name ,name) (type ,type) . ,notes)
                (xsd:annotation
                 (xsd:documentation
                  ,doc)))
              (set! properties (cons (makeProperty name type doc notes)
                                     properties)))
            ...)

Keep the questions coming! 