[TUHS] [tuhs] Dennis Ritchie's couch

[Rob Pike]

I enjoy writing recursive descent parsers, and I enjoy the grammars that
result from such parsers when cleanly done.

I do agree though that if you grammar is being invented as you go, yacc can
be a boon. But in a sense, that's also it's biggest failing: it makes it
too easy to write bad grammars.

-rob


On Wed, May 26, 2021 at 4:22 PM Bakul Shah <bakul at iitbombay.org> wrote:

> Many existing programming languages do have a simple enough
> syntax to make it easy to write a recursive descent parser for them
> but not alll.
>
> Handwritten recursive descent parsers are often LL(1) with may be
> a separate operator-precedence parsing for expressions.
>
> If you are defining a new language syntax you can make sure parsing
> is easy but not all languages are LL(1) (which is a subset of LALR(1),
> which is a subset of LR(1), which is a subset of GLR). Handwritten
> parsers for these more general grammars are bound to get more
> complicated.
>
> Even *we* understand parsing, writing a parser for some existing
> languages  which grew "organically" can become tedious, or
> complicated or adhoc. Often such languages have no well specified
> grammar (the code is the specification!). A yacc grammar would help.
>
> Often one writes a yacc grammar while a new language & its syntax
> is evolving. Changing a yacc file is more localized & easier than fixing
> up a handwritten parser. Even Go has such a grammar initially.
>
> -- Bakul
>
> > On May 25, 2021, at 8:03 PM, Larry McVoy <lm at mcvoy.com> wrote:
> >
> > You do, I don't.  I'm not alone in my lack of understanding.
> >
> > I think that all the things that yacc solved, Steve gets some kudos.
> > I've used it a bunch and I did not need to be as smart as you or
> > Steve to get the job done.
> >
> > You getting past that is cool but it doesn't make his work less.
> >
> > On Wed, May 26, 2021 at 10:37:45AM +1000, Rob Pike wrote:
> >> And today, we understand parsing so well we don't need yacc.
> >>
> >> -rob
> >>
> >>
> >> On Wed, May 26, 2021 at 10:20 AM Nelson H. F. Beebe <
> beebe at math.utah.edu>
> >> wrote:
> >>
> >>> The last article of the latest issue of the Communications of the ACM
> >>> that appeared electronically earlier today is a brief interview with
> >>> this year's ACM Turing Award winners, Al Aho and Jeff Ullman.
> >>>
> >>> The article is
> >>>
> >>>        Last byte: Shaping the foundations of programming languages
> >>>        https://doi.org/10.1145/3460442
> >>>        Comm. ACM 64(6), 120, 119, June 2021.
> >>>
> >>> and it includes a picture of the two winners sitting on Dennis
> >>> Ritchie's couch.
> >>>
> >>> I liked this snippet from Jeff Ullman, praising fellow list member
> >>> Steve Johnson's landmark program, yacc:
> >>>
> >>>>> ...
> >>>>> At the time of the first Fortran compiler, it took several
> >>>>> person-years to write a parser.  By the time yacc came around,
> >>>>> you could do it in an afternoon.
> >>>>> ...
> >>>
> >>>
> >>>
> -------------------------------------------------------------------------------
> >>> - Nelson H. F. Beebe                    Tel: +1 801 581 5254
> >>>    -
> >>> - University of Utah                    FAX: +1 801 581 4148
> >>>    -
> >>> - Department of Mathematics, 110 LCB    Internet e-mail:
> >>> beebe at math.utah.edu  -
> >>> - 155 S 1400 E RM 233                       beebe at acm.org
> >>> beebe at computer.org -
> >>> - Salt Lake City, UT 84112-0090, USA    URL:
> >>> http://www.math.utah.edu/~beebe/ -
> >>>
> >>>
> -------------------------------------------------------------------------------
> >>>
> >
> > --
> > ---
> > Larry McVoy                        lm at mcvoy.com
> http://www.mcvoy.com/lm
>
>
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