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From: carlglassberg@gmail.com
Newsgroups: comp.compilers
Subject: Re: A minimal LL(1) parser generator ?
Date: Sun, 5 Jan 2020 11:59:45 -0800 (PST)
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On Thursday, January 2, 2020 at 10:21:53 AM UTC-8, Anton Ertl wrote:
...
> (( a b && c d && ))

1. For concatenation of these, I would write, in Gray:
   ((a b &&))((c d & &&)) for EBNF: a { b a } c { d c }
because wouldn't
   ((a b && c d & &&))
be the same as?
   a { b a } c { d a { b a } c }

2. Let us compare separator-list notation.

In Waite and Goos, "Compiler Construction", there is an infix operator, "||", for separator list, not to be confused with Gray's meta-symbol for "or".
And Eiffel has a special notation for separator list.

In comparison, Anton's Gray has a postfix operator, "&&", for separator-list.

/Comparison of separator-list notation in
Waite/Goos EBNF, Eiffel EBNF (BNF-E), and Gray EBNF/

                               Wirth EBNF         Gray EBNF:
-------------------------------------------------------------------
Waite/Goos: x || y ==========> x { y x }          x y &&

In Eiffel:  { x y ...}* ===> [ x { y x } ]     (( x y && )) ??
            { x y ...}+ ===>   x { y x }          x y &&
-------------------------------------------------------------------

What is better about Gray is that it only uses braces in action semantics,
in doubled form {{ ... }}, and no one is likely to be confused by the use of
braces.

Also it's postfix separator-list operator, "&&", is easier to remember than
Eiffel's more verbose special variation of standard EBNF braces.

And Gray's "&&" separator-list operator is just another postfix op, consistent with it's use of "??", "**" and "++"

Gray better separates the 2 styles of notation, it's "regular expression" style versus standard bracketed EBNF style.

The 2 styles ideally should not be mixed, or at least confusion should be avoided by not introducing yet another similiar notation, in the form of a special variation of standard EBNF braces for separator lists.

One minor criticism of Gray:

I do wish Gray had a production rule terminator, say semicolon ";" or stop "."

The end of 1 rule would be clearly distinguishable from the beginning of the next without special processing of end-of-line which makes scanning and
parsing space sensitive. We would avoid the need for LL(2) parsing or other
parsing and scanning techniques.

Carl
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