marmar@mtk.UUCP (Mark Martino) (05/23/89)
I was reading an article about the innards of PostScript and the author mentioned an operator whose label is "//". I put it into a file an ran it through version 43.0. It seemed to be recognized by the interpreter but did not have any effect I could detect. Is this a real operator? I could not find it in the red, blue, or green book. If it is real, what is it supposed to do?
batcheldern@icosa.dec.com (Ned Batchelder) (05/27/89)
The // construct in PostScript is not an operator; it is more like a name modifier.
When a name is preceded by the double-slash, its current value is used instead of the
/a 17 def
/foo { a } def
/bar { /a } def
/baz { //a } def
"foo" is now a procedure which when executed will push the latest value of "a" on the
stack. The procedure actually contains the name "a", and it is looked up when the
procedure is executed.
"bar" is now a procedure which when executed will push the literal name "a" on the stack. No name lookup is performed.
"baz" is now a procedure which when executed will push 17 on the stack. The procedure actually contains the integer 17, because that is the value that "a" had at the time the procedure was defined. The double-slash means "lookup this name, and use its value just as it had appeared in the input". Of course, some values for names may not have a representation
in the input stream, such as dictionaries.
Be careful when using the double-slash (if at all), because older PostScript interpreters don't have it, and there is no way your program will work properly on them.
Ned Batchelder, Digital Equipment Corp., BatchelderN@Hannah.DEC.comgeof@apolling (Geof Cooper) (06/06/89)
In article <2586@shlump.dec.com> batcheldern@icosa.dec.com (Ned Batchelder) writes: >The // construct in PostScript is not an operator; it is more like a name modifier. >When a name is preceded by the double-slash, its current value is used instead of the >name. Software developers, be careful how you use this feature. It is not possible to test for the existence of a feature if you want to use // in relation to it. For example, the latest Apple LaserPrep contains code of the form: if <I'm a LaserWriter> then <do some nifty things with funny operators in serverdict> This is all very well, but the "nifty thing" references LaserWriter operators in serverdict using the // convention. This means that those names must resolve to valid objects on ALL machines, not just the LaserWriter, since the names are resolved in the scanner independent of the result of the conditional. - Geof
alo@kampi.hut.fi (Antti Louko) (06/06/89)
In article <3903@imagen.UUCP> geof@apolling (Geof Cooper) writes: >Software developers, be careful how you use this feature. It is not >possible to test for the existence of a feature if you want to use // in >relation to it. For example, the latest Apple LaserPrep contains code >of the form: > > if <I'm a LaserWriter> then > <do some nifty things with funny operators in serverdict> > >This is all very well, but the "nifty thing" references LaserWriter >operators in serverdict using the // convention. This means that those >names must resolve to valid objects on ALL machines, not just the >LaserWriter, since the names are resolved in the scanner independent >of the result of the conditional. > >- Geof Software developers, you can use this little hack I just put together: skipblock is a procedure which skips a %%BeginProcSet .. %%EndProcSet region so that the interpreter won't scan the skipped text. The only problem is how to get big companies to use this procedure... Antti -------------------------------- %! %%BeginProcSet: skipblock 1.0 0 /stoppedreadline { /savedhandler errordict /rangecheck get def errordict /rangecheck { pop stop } put { readline } stopped errordict /rangecheck /savedhandler load put { exch pop true dup } { false } ifelse } bind def /readlinehead { /-str- exch def /-file- exch def -file- -str- stoppedreadline { { -file- (..............) stoppedreadline not { pop pop exit} if pop pop } loop } if } bind def /skipblock { /-save- save def /str 600 string def /str2 600 string def currentfile str readlinehead not { stop } if (%%BeginProcSet:) anchorsearch { pop /-name- exch def } { stop } ifelse { currentfile str2 readlinehead not { stop } if (%%EndProcSet:) anchorsearch { pop -name- eq { exit } if } { pop } ifelse } loop -save- restore } bind def %%EndProcSet: skipblock 1.0 0 %example skipblock %%BeginProcSet: foobar (This stuff should be skipped) == (This stuff should be skipped) == (This stuff should be skipped) == (This stuff should be skipped) == %%EndProcSet: foobar (This stuff should not be skipped) == --------------------------------
alo@kampi.hut.fi (Antti Louko) (06/06/89)
PS to my earlier posting:
This is how you can tell if your PostScript interpreter has // feature:
%%BeginProcSet: immediate-test 1.0 0
/has-immediate
(//add) token % If immediately evaluated names are not implemented
% this parses as / and /add
{ exch pop / eq not }
{ false } ifelse % token should always succeed
def
%%EndProcSet: immediate-test 1.0 0