[TUHS] [tuhs] Dennis Ritchie's couch
Rob Pike
robpike at gmail.com
Fri Jul 2 13:30:17 AEST 2021
To show you what I mean, here is a parser I wrote recently for a simple Go
expression evaluator, in Go. It has all Go's precedence levels and
operators. The one odd detail is that >= etc. can be combined, as in 1 >= 2
> 3, but that doesn't matter in the context of this program and is easily
avoided with a few more lines in cmpList.
I'm using a screen grab because GMail refuses to leave my indentation
alone. I left factor off. It's got all the usual details but it's the leaf
of the grammar and of no interest here.
Note that this could easily have been made a table instead of a bunch of
one-line functions.
Parsers are easy to write. It took us a generation (or more) to understand
that, but it's remarkable nonetheless. The first big step might have been
realizing that recursion was a good idea, even if you weren't writing LISP,
if the data structure is itself recursive.
-rob
[image: image.png]
On Fri, Jul 2, 2021 at 12:13 PM <scj at yaccman.com> wrote:
> I found Dennis's compiler to be a thing of beauty when compiling
> statements, but hell on wheels when doing expressions. One of the
> motivations for writing Yacc was that I wanted to add the exclusive or into
> the expression syntax and we had agreed on the character (^) and the
> precedence. Practically the whole expression grammar needed to be
> rewritten, and small typos created un-debuggable chaos. The state of the
> art at the time was to write multi-layered grammars for each precedence
> level. A paper was published on how to shrink the resulting parsing tables
> by eliminating redundant states. I realized that the same results could
> be obtained by writing an ambiguous expression grammar and using a
> precedence table to resolve the ambiguities. The initial response in the
> academic community to programming with ambiguous grammars was somewhere
> between skeptical and horrified -- as if I had shown porn to a Victorian.
> So Al and I worked out a proof that we would get the same optimized parser
> in a much more intuitive way.
>
> I do agree with Rob that some of the languages that Yacc gave birth to
> should never have been born. Remember, though, that the dominant language
> at the time was FORTRAN, and it had all sorts of strange implementation
> issues in their hand-written compilers. Things like subscript indices had
> to be single variables in some places, and in others could have a constant
> added to the index. One of Yacc's best features was that it made
> consistency of usage the path of least resistance when designing the
> language, and the grammar was often easier to understand than the
> programming manual. At Bell Labs, Barbara Ryder wrote a program that would
> read FORTRAN and detect things that would not work on one or more of the
> six major FORTRAN's at the time. It was an inspiration for me, later, do
> the same thing with Lint.
>
> I do suggest that having languages like C++ that have bloated up to over
> 1000 pages in the programmer reference doesn't feel like a real advance,
> especially since the real language problems of today are how to program
> very large numbers of processor-like objects on a single chip. We need new
> ways to think, and I doubt that we'll get there by making C++ require a
> 2000-page manual.
> ---
>
>
>
> On 2021-05-25 23:52, Rob Pike wrote:
>
> 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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