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Re: pseudoöperators?

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On Friday, December 2, 2016 at 6:36:27 PM UTC-6, Dan Coby wrote:
> On 11/29/2016 3:12 PM, luser droog wrote:
> > I remember a while back discussing interpretation with
> > Helmar who at the time had just gotten pseudoöperators
> > to work, which is a procedure that acts like an operator.
> >
> > And I remember not being very enthusiastic because I
> > didn't see the point of doing that. Procedures already
> > work like operators if you just call them.
> >
> > But I noticed in the ghostscript docs that gs also has
> > this ability to make a proc quack like an oper.
> >
> > Is this commonly done in ps interpreters? What is it
> > used/useful for?
> >
> Since no one else has responded, I will stick in my 2 cents.
> 
> It has been a decade since I did anything with Ghostscript but my memory 
> is that the issue is how pseudooperators handle errors. If an error 
> occurs within a typical PS procedure, then the contents of the stack 
> could be whatever happened to be there when the error occurred. With a 
> pseudooperator, the stack is restored to a known state. (I think that 
> the stack was returned to its state when the pseudooperator was called.)
> 
> Dan

Thanks. I hadn't thought of that. That's actually really compelling.

If we take the "pseudo-operator" I posted in another thread,

/show {
    graphicsdict /currgstate get /currfont get
    dup /FontType get 3 ne { pop //show }{
    gsave
        currentpoint translate
        newpath
        dup /FontMatrix get concat
        exch
        %pstack()=
        {
            gsave
                1 index /BuildChar get exec
            grestore
            graphicsdict /currgstate get /currfont get
            dup /xadvance get 1 index /yadvance get
            translate
            %hook
        } forall
        pop
        0 0 transform  % use device-space coordinates to save location past grestore
    grestore
    itransform moveto

    } ifelse
} def

As-is, this would be an excellent candidate for such a thing.
To users it should appear that `show` has hit an error. 

The gyrations necessary for such a procedure to manage its own
internal errors are pretty cumbersome:

/show {
    1 dict begin /ARG 1 index def { mark exch
    graphicsdict /currgstate get /currfont get
    dup /FontType get 3 ne { pop //show }{
    gsave
        currentpoint translate
        newpath
        dup /FontMatrix get concat
        exch
        %pstack()=
        {
            gsave
                1 index /BuildChar get exec
            grestore
            graphicsdict /currgstate get /currfont get
            dup /xadvance get 1 index /yadvance get
            translate
            %hook
        } forall
        pop
        0 0 transform  % use device-space coordinates to save location past grestore
    grestore
    itransform moveto

    } ifelse
} stopped {
  cleartomark ARG end /show cvx $error/errorname get signalerror
}{ cleartomark end } ifelse
} def

And this just passes-on whatever the internal error was,
which may not be meaningful for the operator.

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Thread

pseudoöperators? luser droog <luser.droog@gmail.com> - 2016-11-29 15:12 -0800
  Re: pseudoöperators? Dan Coby <adcoby@earthlink.net> - 2016-12-02 16:35 -0800
    Re: pseudoöperators? luser droog <luser.droog@gmail.com> - 2016-12-02 19:53 -0800
    Re: pseudoöperators? luser droog <luser.droog@gmail.com> - 2016-12-02 20:01 -0800

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