Subject: Re: How to implement following procedures in Scheme?
From: (Rob Warnock)
Date: 1999/08/05
Newsgroups: comp.lang.lisp
Message-ID: <7obk7f$>
Kent M Pitman  <> wrote:
| It's obviously the case that Schemers could write a rest list and use
|  (adjoin-dispenser 'test equal? 'key identity)

This is exactly what Elk [a Scheme] does with it's X binding. Sample code
clipped from a simple Xlib app:

  (let* ((dpy (open-display))
         (black (black-pixel dpy)) (white (white-pixel dpy))
         (width (list-ref geom 0)) (height (list-ref geom 1))
         (win (create-window 'parent (display-root-window dpy)
                             'width width 'height height
                             'x (list-ref geom 2) 'y (list-ref geom 3)
                             'background-pixel white
                             'border 0
                             'event-mask '(exposure)))
         (gc (create-gcontext 'window win 'function 'xor
                              'background black 'foreground white))
	 ...and-so-on... )
     ...the-main-app... )

There's a utility routine "apply-with-keywords" that matches the &rest list
with a pattern of required positional args (and their defaults), and then
calls the underlying primitive [written in C] with purely positional args.
E.g., for the "create-window" call above, the glue routine [with some added
annotations] is:

	(define (create-window . args)
	    'create-window		; for error messages
	    xlib-create-window		; underlying primitive
	    ; positional args required by underlying primitive:
	    '((parent) (x 0) (y 0) (width) (height) (border 2))
	    ; table of additional permitted optionals:
	    'set-window-attributes set-window-attributes-slots

Yes, a bit hacky, but once it's in place you *can* use keyword-style
calls pretty much anywhere they make sense...

| and even then they could leave out arguments they didn't want, but 
| the language doesn't offer anything to encourage them to do it, which
| means it will be too much trouble for some, too hard to remember
| for others, and potentially irregularly done (e.g., varying in 
| calling details on a per-facility or per-author basis).

Agreed. Had I not seen how Elk does it in its X binding, I probably
would have continued to avoid the "keyword" style. But now, having
been given one well-worked-out example...


Rob Warnock, 8L-855
Applied Networking
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