Re: Building STk on HP-UX

From: <Marc.Furrer_at_di.epfl.ch>
Date: Tue, 14 Feb 95 10:10:07 +0100

> I'm trying to build an application using STk and I ran into an unexpected
> (though documented) problem.
>
> Basically, the code bound to, say, a button widget that is to be executed when
> the button is pressed, is evaluated in the global environment.
>
> I want to be able to control the environment in which it is evaluated; I'd
> like to be able to do, for instance
>
> (let* ((foo 1)
> (bar 2))
> (button "Frob" :command (lambda ()
> (frob foo bar))))
>
> When I try it I get
>
> *** Read from string error:
> bad # syntax: "#"
>
> because STk cannot deal with the "#[closure 0xfdo92c]" it gets back from Tk.
>
> My question is, how hard would it be to support closures as widget actions?
> If it's a GC problem (in that the lambda might get collected before it's
> called, because Tk doesn't cooperate in GC) then why can't a thin layer of
> glue code notice that this closure is being bound to that button, and hold
> onto a live pointer so it won't be collected? It seems to me (though I
> haven't dived into the code to see) that it would not be hard for the
> interpreter evaluating the "string" of code from Tk to notice that it starts
> "#[closure..." and do the right magic closure-invoking stuff. Yes? No?
>
> Or perhaps the problem is more subtle (or more obvious) than I can see.
>
> --berry
>
> Berry Kercheval :: kerch_at_parc.xerox.com
>

I think, your facing two problems here:
local environment && lambda call-backs.
I only have an answer for the second part : (from the FAQ ;-)

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Subject : [3.3] Using lambda as call-backs


Using something like

   (.scrollbar 'configure :command '(lambda (pos) (display pos)))

won't work. This is due to the way Tk and STk communicate. For the
time being the call-back command are stored in strings. When Tk calls
the call-back, the string is sent to STk, if the string doesn't start
with a bracket it is bracketed first. Thus

    :command "callback" is called by (callback args)
    :command '(lambda (pos) ...) is called by (lambda ...) args
      which is obviously false.
    :command "apply (lambda ....)" is called by (apply (lambda..) args)
      which the only current working alternative.

Eventually, this problem will be solved in future release.

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Received on Tue Feb 14 1995 - 10:12:13 CET