Kragen is right that PostScript is a lot more like Lisp or Smalltalk than Forth, especially when you use Owen Densmore's object oriented PostScript programming system (which NeWS was based on). PostScript is semantically very different and much higher level that Forth, and syntactically similar to Forth but uses totally different names (exch instead of swap, pop instead of drop, etc).

But its most obvious similarities with Forth are based on generic Computer Science concepts like stacks, which is not something particularly unique to Forth itself, so saying PostScript was influenced by Forth ignores other much more influential languages like Interpress, JaM, and Design System, which directly influenced PostScript and were designed by the same people. Also, PostScript is not threaded like Forth. But it's homoiconic, unlike Forth and Lisp.

https://en.wikipedia.org/wiki/Threaded_code

https://en.wikipedia.org/wiki/Homoiconicity

https://medium.com/@donhopkins/the-shape-of-psiber-space-oct...

The heavily commented PostScript source code of PizzaTool for NeWS shows Owen's OOP PostScript system in action:

https://donhopkins.com/home/archive/NeWS/pizzatool

PostScript was a direct descendent of and heavily influenced by Xerox PARC's Interpress, by John Warnock and Charles Geschke, which itself was a direct descendent of JaM, which stands for "John and Martin", by John Warnock and Martin Newall. And JaM was a descendent of Evans and Sutherland's Design System, by John Warnock and John Gaffney.

https://en.wikipedia.org/wiki/John_Warnock

http://historyofinformation.com/detail.php?entryid=1213

https://en.wikipedia.org/wiki/Martin_Newell_(computer_scient...

https://en.wikipedia.org/wiki/Charles_Geschke

https://en.wikipedia.org/wiki/Interpress

https://en.wikipedia.org/wiki/Evans_%26_Sutherland

Brian Reid (whose brother is Glenn Reid, author of several books on PostScript from Adobe) wrote up an excellent historical summary in 1985 on the laser-lovers mailing list of the influences and evolution of PostScript.

https://en.wikipedia.org/wiki/Brian_Reid_(computer_scientist...

http://glennreid.blogspot.com/

Here's a post I wrote earlier:

https://news.ycombinator.com/item?id=19874245

>DonHopkins 8 months ago [-]

>Brian Reid wrote about page independence, comparing Interpress' and PostScript's different approaches. Adobe's later voluntary Document Structuring Conventions actually used PostScript comments to make declarations and delimit different parts of the file -- it wasn't actually a part of the PostScript language, while Interpress defined pages as independent so they couldn't possibly affect each other:

https://groups.google.com/forum/#!topic/fa.laser-lovers/H3us...

>By now you can probably see the fundamental philosophical difference between PostScript and Interpress. Interpress takes the stance that the language system must guarantee certain useful properties, while PostScript takes the stance that the language system must provide the user with the means to achieve those properties if he wants them. With very few exceptions, both languages provide the same facilities, but in Interpress the protection mechanisms are mandatory and in PostScript they are optional. Debates over the relative merits of mandatory and optional protection systems have raged for years not only in the programming language community but also among owners of motorcycle helmets. While the Interpress language mandates a particular organization, the PostScript language provides the tools (structuring conventions and SAVE/RESTORE) to duplicate that organization exactly, with all of the attendant benefits. However, the PostScript user need not employ those tools.

There's more interesting discussion about the relationship between PostScript and Forth on c2:

http://wiki.c2.com/?ForthPostscriptRelationship

Here's a lot more stuff I've written about PostScript and Forth (two of my favorite topics!):

https://news.ycombinator.com/item?id=11479364

https://news.ycombinator.com/item?id=13198492

https://news.ycombinator.com/item?id=13705664

https://news.ycombinator.com/item?id=13705739

https://news.ycombinator.com/item?id=18317280

https://news.ycombinator.com/item?id=18692144

https://news.ycombinator.com/item?id=18695275

https://news.ycombinator.com/item?id=18696116

https://news.ycombinator.com/item?id=19873630

https://news.ycombinator.com/item?id=19874245

https://news.ycombinator.com/item?id=19882301