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Re: Postscript in 4tH

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anton@mips.complang.tuwien.ac.at (Anton Ertl) writes:
> Kragen Javier Sitaker <kragen@canonical.org> writes:
>>PostScript has several other key similarities to Lisp.  Referencing Paul
>>Graham's "What Made Lisp Different" <https://paulgraham.com/diff.html>:
>
> Given your claim below that Forth has only #1, #3, #9, let's look at
> the others:
>
>>2. A function type.  “In Lisp, functions are first class objects--
>>they’re a data type just like integers, strings, etc, and have a literal
>>representation, can be stored in variables, can be passed as arguments,
>>and so on.”
>
> Forth has execution tokens.

I thought about this when posting.  Xts certainly can be stored in
variables or passed around as arguments or results.  But orthodox Forth
doesn’t have Joy-like quotations, which I think is the language feature
that allows you to claim that functions “have a literal representation”,
although it does have `:noname`.  Orthodox Forth is almost the same as C
in this sense, in that you must define all of your functions at the top
level.  Function pointers such as Forth xts provide a very significant
gain in expressiveness, but function *literals* add so much
expressiveness that you can use them to define custom control
structures, as in Smalltalk (or PostScript).

Lisp programs also commonly create new functions at runtime, although in
modern Lisp this is largely limited to creating closures.  Forth has
closures in the form of `create does>`, but because that appends to the
dictionary, you don’t normally do it willy-nilly, usually only when you
are loading a program.

>>4. A new concept of variables. “In Lisp, all variables are effectively
>>pointers. Values are what have types, not variables, and assigning or
>>binding variables means copying pointers, not what they point to.”
>
> In Forth, neither variables nor values have types that are known to
> the Forth system.  The way to use that is that the programmer must
> know the type, and that typically means always storing the same type
> into a variable.

That is certainly true, but to my eye, Forth is much more similar to
assembly, Fortran, or C in this sense than to Lisp.  You might `create`
an array and give it a name, for example, and you can think of that name
as referring to the beginning of the array (like a label in assembly
language) or to the entire array (as in Fortran or C), but it definitely
does not refer to a cell somewhere holding a *pointer to* the array, the
way it does in Lisp and PostScript (or Python, or JS, or Lua, or
Smalltalk, etc.)

Forth uses the term `variable` for single-cell variables, and those are
often used to store integers that are not pointers.  Some Lisps (and,
for example, Lua and Squeak) avoid “boxing” their integers in a memory
allocation by packing type bits into a pointer, but this trick is
invisible to the language semantics, while it is very visible in Forth.

>>6. Programs composed of expressions.  Not really, but closer than any of
>>the currently popular languages.
>
> Forth is like Postscript here: Programs composed of words (operators
> in Postscript), which work on values on the stack.

Yes.  D’Oliveiro suggested that what PG intended here was that there’s
no distinction between statements and expresions, which is plausible;
but, as I see it, in neither PostScript nor Forth do we have clear-cut
expressions in the syntax, so even if PostScript (and Forth) fit what PG
intended to say, they do not fit what he actually said.

>>7. A symbol type.  Yes, PostScript distinguishes /names from strings,
>
> Forth distinguishes name tokens from strings, but they do not play the
> role they play in Lisp or Postscript; in particular, you have to
> create new named words outside a colon definitions, and if you use a
> named word to get a unique value, you tend to use its body address,
> not its name token.

I had forgotten about name tokens; thank you for the reminder.

>>8. A notation for code using trees of symbols.  Yes.  The only
>>difference is that in conventional Lisp they’re binary trees, while in
>>PostScript they’re N-ary ordered trees.
>
> As mentioned in <2026Sep9.084205@mips.complang.tuwien.ac.at>, one
> might consider the indirect-threaded code representation and its
> modern equivalents for inlining to be such a notation, but in Forth
> many control structures become branches, whereas in Postscript the
> controlled stuff is a procedure (in Forth terminology, a quotation).

As you say, Forth flattens the tree into an array at compile time.
Also, though, it is entirely omitted from the standards, so that Forths
that do not use threaded code can still count as “compliant”.

The things in the array are generally not name tokens, but execution
tokens.  PostScript supports either one, and people usually do `bind`
their subroutines, replacing the symbols with subroutine pointers, so
that they run faster:

    GS>/square {dup mul} def
    GS>4 square =
    16
    GS>/square load ===
    {dup mul}
    GS>/square load bind ===
    {--dup-- --mul--}

>>Forth hits #1, #3, and #9.
>
> And also #2, and partially #6, #7, and #8.

Well, that’s certainly a defensible position, but I think it’s still
debatable.

Thank you very much for your thought-provoking post.

Kragen

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Thread

Postscript in 4tH Hans Bezemer <the.beez.speaks@gmail.com> - 2026-09-05 19:55 +0200
  Re: Postscript in 4tH Paul Rubin <no.email@nospam.invalid> - 2026-09-05 15:54 -0700
    Re: Postscript in 4tH albert@spenarnc.xs4all.nl - 2026-09-06 11:08 +0200
      Re: Postscript in 4tH Paul Rubin <no.email@nospam.invalid> - 2026-09-07 09:24 -0700
        Re: Postscript in 4tH albert@spenarnc.xs4all.nl - 2026-09-07 23:47 +0200
      Re: Postscript in 4tH Kragen Javier Sitaker <kragen@canonical.org> - 2026-09-08 02:09 -0300
        Re: Postscript in 4tH Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-09-08 20:34 +0000
          Re: Postscript in 4tH Kragen Javier Sitaker <kragen@canonical.org> - 2026-09-08 22:38 -0300
            Re: Postscript in 4tH Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-09-09 03:05 +0000
              XML in Pythong (was: Re: Postscript in 4tH) Johann 'Myrkraverk' Oskarsson <johann@myrkraverk.invalid> - 2026-09-09 15:00 +0800
              Re: Postscript in 4tH Kragen Javier Sitaker <kragen@canonical.org> - 2026-09-10 01:06 -0300
                Re: Postscript in 4tH Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-09-10 07:38 +0000
                Re: Postscript in 4tH Peter Flass <Peter@Iron-Spring.com> - 2026-09-10 07:39 -0700
                Re: Postscript in 4tH antispam@fricas.org (Waldek Hebisch) - 2026-09-15 16:04 +0000
                Re: Postscript in 4tH Paul Rubin <no.email@nospam.invalid> - 2026-09-15 09:24 -0700
            Re: Postscript in 4tH Johann 'Myrkraverk' Oskarsson <johann@myrkraverk.invalid> - 2026-09-09 15:15 +0800
              Re: Postscript in 4tH Kragen Javier Sitaker <kragen@canonical.org> - 2026-09-10 01:11 -0300
            Re: Postscript in 4tH anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2026-09-09 07:35 +0000
              Re: Postscript in 4tH Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-09-09 08:10 +0000
                Re: Postscript in 4tH Hans Bezemer <the.beez.speaks@gmail.com> - 2026-09-10 17:58 +0200
              Re: Postscript in 4tH Kragen Javier Sitaker <kragen@canonical.org> - 2026-09-10 01:44 -0300
            Re: Postscript in 4tH Paul Rubin <no.email@nospam.invalid> - 2026-09-09 03:01 -0700
            Re: Postscript in 4tH albert@spenarnc.xs4all.nl - 2026-09-09 14:05 +0200
          Re: Postscript in 4tH anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2026-09-09 06:42 +0000
            Re: Postscript in 4tH Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-09-09 08:28 +0000
              Re: Postscript in 4tH anton@mips.complang.tuwien.ac.at (Anton Ertl) - 2026-09-09 10:03 +0000
                Re: Postscript in 4tH Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-09-09 22:17 +0000
              Re: Postscript in 4tH albert@spenarnc.xs4all.nl - 2026-09-09 14:42 +0200
                Re: Postscript in 4tH antispam@fricas.org (Waldek Hebisch) - 2026-09-09 19:17 +0000
                Re: Postscript in 4tH albert@spenarnc.xs4all.nl - 2026-09-10 12:22 +0200
                Re: Postscript in 4tH Lawrence D’Oliveiro <ldo@nz.invalid> - 2026-09-09 21:54 +0000
                Re: Postscript in 4tH Paul Rubin <no.email@nospam.invalid> - 2026-09-09 22:14 -0700
                Re: Postscript in 4tH albert@spenarnc.xs4all.nl - 2026-09-10 12:30 +0200
            Re: Postscript in 4tH albert@spenarnc.xs4all.nl - 2026-09-09 14:36 +0200
    Re: Postscript in 4tH Hans Bezemer <the.beez.speaks@gmail.com> - 2026-09-06 14:27 +0200
      Re: Postscript in 4tH Hans Bezemer <the.beez.speaks@gmail.com> - 2026-09-08 17:18 +0200

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