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From: glenn@heaven.woodside.ca.us (Glenn Reid)
Newsgroups: comp.lang.postscript
Subject: Re: A question about //add
Message-ID: <329@heaven.woodside.ca.us>
Date: 23 Nov 90 19:28:11 GMT
References: <1990Nov12.195235.13908@light.uucp> <321@heaven.woodside.ca.us> <1990Nov14.170020.16233@light.uucp> <21745@well.sf.ca.us>
Reply-To: glenn@heaven.woodside.ca.us (Glenn Reid)
Organization: RightBrain Software, Woodside, CA
Lines: 48

In article <21745@well.sf.ca.us> shiva@well.sf.ca.us (Kenneth Porter) writes:
>// is particularly useful for re-defining system operators in
>heavily nested situations:
> 
>/showpage { stuff //showpage more-stuff } def
> 
>The problem here is that one has to test for whether the system operator
>has be re-defined before;  if so, then an exec has to be added after
>the name:
> 
>/systemname load operatortype { /systemname { //systemname } def } if

This second bit of code doesn't work, for one thing; you need to use
the "type" operator:

/systemname load type /operatortype eq { ... } if

And for another thing, it's not a particularly good idea.  You're
changing "systemname" from being an operator into being a procedure
that calls only a single operator, which is less efficient.  You're
much better off with "bind" in your "showpage" example; it has the
same strengths (and weaknesses, in that you have to make sure
"showpage" hasn't already been redefined).

Heavily-nested situations don't affect "bind"; it is applied recursively
to all the procedures nested within the outermost procedure.

In general, // is more dangerous than "bind" or "load".  The only situation
where I use it is if I have some constants that I want to define up front
in the program for readability (rather than hard-wiring them into the
code) but I don't want to suffer name lookup time whenever the constants
are used.  As long as they're my own names, I don't have to worry about
their having been redefined.  For example:

    /leftmargin 72 def
    /topmargin 792 72 sub def

    /startpage {
	//leftmargin //topmargin moveto
	/Times-Roman 12 selectfont
    } bind def

% Glenn

-- 
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