2 Jul
2021
2 Jul
'21
4:13 a.m.
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(a)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(a)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(a)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(a)math.utah.edu -
>>>> - 155 S 1400 E RM 233 beebe(a)acm.org
>>>> beebe(a)computer.org -
>>>> - Salt Lake City, UT 84112-0090, USA URL:
>>>> http://www.math.utah.edu/~beebe/ -
>>>>
>>>>
-------------------------------------------------------------------------------
>>>>
>>
>> --
>> ---
>> Larry McVoy lm at mcvoy.com [1]
http://www.mcvoy.com/lm
Links:
------
[1] http://mcvoy.com