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Unbalanced [

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  Unbalanced [ anavi8r@gmail.com - 2013-09-11 09:16 -0700
    Re: Unbalanced [ Mark Carroll <mtbc@bcs.org> - 2013-09-11 17:28 +0100
    Re: Unbalanced [ Helge Blischke <h.blischke@acm.org> - 2013-09-11 18:34 +0200
    Re: Unbalanced [ luser- -droog <mijoryx@yahoo.com> - 2013-09-11 12:16 -0700
      Re: Unbalanced [ Helge Blischke <h.blischke@acm.org> - 2013-09-12 11:31 +0200
        Re: Unbalanced [ anavi8r@gmail.com - 2013-09-18 10:11 -0700

#1616 — Unbalanced [

Hi everyone,

I am writing a PostScript interpreter for a product I sell, and while working with some sample .EPS files, I ran into the following:

/xpdfm
{
	{
		dup 0 get/Label eq
		{
			aload length[exch 1 add 1 roll/PAGELABEL
		}{
			aload pop
			[{ThisPage}<<5 -2 roll>>/PUT
		}ifelse
		pdfmark_5
	}forall


This code snippet contains two unbalanced opening square brackets ([). 

Is this normal? How should this be treated by a PostScript interpreter?

Thanks in advance.

-- Peter

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#1617

anavi8r@gmail.com writes:

> Is this normal? How should this be treated by a PostScript interpreter?

[ just pushes a mark onto the stack. I guess that procedures that look
back on the stack for the mark might find it useful, like counttomark. 
] looks back for the mark to pop the elements for putting into an array.
All marks are the same, they're usually used as a delimiter for the end
of a list for consumption by something else.

-- Mark

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#1618

anavi8r@gmail.com wrote:

> Hi everyone,
> 
> I am writing a PostScript interpreter for a product I sell, and while
> working with some sample .EPS files, I ran into the following:
> 
> /xpdfm
> {
> {
> dup 0 get/Label eq
> {
> aload length[exch 1 add 1 roll/PAGELABEL
> }{
> aload pop
> [{ThisPage}<<5 -2 roll>>/PUT
> }ifelse
> pdfmark_5
> }forall
> 
> 
> This code snippet contains two unbalanced opening square brackets ([).
> 
> Is this normal? How should this be treated by a PostScript interpreter?
> 
> Thanks in advance.
> 
> -- Peter

The "[" is a synonym for the "mark" operator. Its purpose is
to process the following objects on the operand stack back
up to the mark (and removing the mark object) without the necessity
to know how many objects to process.
See e.g. the cleartomark operator from the PLRM as a simple
example, or the array operator.

Helge

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#1619

On Wednesday, September 11, 2013 11:16:13 AM UTC-5, ana...@gmail.com wrote:
> Hi everyone,
> 
> I am writing a PostScript interpreter for a product I sell, and while working with some sample .EPS files, I ran into the following:
> 
> /xpdfm
> {
> 	{
> 		dup 0 get/Label eq
> 		{
> 			aload length[exch 1 add 1 roll/PAGELABEL
> 		}{
> 			aload pop
> 			[{ThisPage}<<5 -2 roll>>/PUT
> 		}ifelse
> 		pdfmark_5
> 	}forall
> 
> 
> This code snippet contains two unbalanced opening square brackets ([). 
> 
> Is this normal? How should this be treated by a PostScript interpreter?
> 
> Thanks in advance.
> 

As others have said, the left bracket [ is defined in systemdict as a 
marktype object (same as the name /mark). Since only one of the procedure-
bodies passed to ifelse will be executed, there is really only one mark 
without a matching right bracket ]. This style is used in much the same
way as a vararg function in C, to pass an open-ended set of values, 
presumably to pdfmark_5. I'm not familiar with the '_5' part, but pdfmark
is used to pass data into a generated pdf file.

If your output device is not a pdf-writer, you can probably safely 
define

    /pdfmark { cleartomark } def

By the way, the << is also a mark, so you will sometimes see [ used instead.

    [ /this (is) /a (dictionary) >>

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#1620

luser- -droog wrote:

> On Wednesday, September 11, 2013 11:16:13 AM UTC-5, ana...@gmail.com
> wrote:
>> Hi everyone,
>> 
>> I am writing a PostScript interpreter for a product I sell, and while
>> working with some sample .EPS files, I ran into the following:
>> 
>> /xpdfm
>> {
>> {
>> dup 0 get/Label eq
>> {
>> aload length[exch 1 add 1 roll/PAGELABEL
>> }{
>> aload pop
>> [{ThisPage}<<5 -2 roll>>/PUT
>> }ifelse
>> pdfmark_5
>> }forall
>> 
>> 
>> This code snippet contains two unbalanced opening square brackets ([).
>> 
>> Is this normal? How should this be treated by a PostScript interpreter?
>> 
>> Thanks in advance.
>> 
> 
> As others have said, the left bracket [ is defined in systemdict as a
> marktype object (same as the name /mark). Since only one of the procedure-
> bodies passed to ifelse will be executed, there is really only one mark
> without a matching right bracket ]. This style is used in much the same
> way as a vararg function in C, to pass an open-ended set of values,
> presumably to pdfmark_5. I'm not familiar with the '_5' part, but pdfmark
> is used to pass data into a generated pdf file.
> 
> If your output device is not a pdf-writer, you can probably safely
> define
> 
>     /pdfmark { cleartomark } def
> 
> By the way, the << is also a mark, so you will sometimes see [ used
> instead.
> 
>     [ /this (is) /a (dictionary) >>

The ample snippet is from the prolog section Acrobat 
(Adobe Reader as well) generates when printing to PostScript printers, 
and /pdfmark_5 is there defined like:

/currentdistillerparams where{pop currentdistillerparams/CoreDistVersion get 5000 lt}{true}ifelse
{
        /pdfmark_5{cleartomark}bind def
}{
        /pdfmark_5{pdfmark}bind def
}ifelse

Helge

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#1625

Thanks a lot to everyone who replied! Your insight made my realize that my original design was all wrong :-)

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