[TUHS] Knuth and Unix

[arnold at skeeve.com]

You can't lay the blame for that on Steve.

The GNU Awk grammar has only 36 shift-reduce conflicts and no
reduce-reduce conflicts.

The mawk 1.3.4 grammar has an amazing count of only *four* shift-reduce
conflicts.  (But then, Mike Brennan is an amazing programmer.)

I have no idea what happens if you run the POSIX awk grammar through
yacc / bison, but it'd be an interesting experiment.

Arnold

Rob Pike <robpike at gmail.com> wrote:

> So you're the reason (Plan 9) awk has 83 reduce-reduce conflicts (and
> 42 shift-reduce).
>
> -rob
>
> On Wed, Jan 16, 2019 at 9:39 AM Steve Johnson <scj at yaccman.com> wrote:
> >
> > I remember reading Knuth's paper, and certainly heard DeRemer's name, but it didn't affect much of what I did.  There was a paper out of Stanford about that time that influenced me greatly -- it was about pattern matching languages, and proposed separating two ideas: 1.  "Here are the patterns that match this tree".  And 2.  "If more than one pattern matches, here's how to decide which one to use."   Given the constraints of size on the PDP 11, anything but LR(1) was infeasable.  But using ambiguous grammars and broadening the shift/reduce test to trest operator precedence fit right into that pattern.   Another thing that I think was unique to Yacc at the time was introducing symbols that matched the empty string whose reduction caused program actions.  Many similar parser systems at the time could not deal with these "empty" symbols.
> >
> > Steve
> >
> >
> >
> > ----- Original Message -----
> > From: "Bakul Shah" <bakul at bitblocks.com>
> > To:"Steve Johnson" <scj at yaccman.com>
> > Cc:<arnold at skeeve.com>, <ecashin at noserose.net>, <dave at horsfall.org>, <tuhs at tuhs.org>
> > Sent:Sat, 12 Jan 2019 20:40:11 -0800
> > Subject:Re: [TUHS] Knuth and Unix
> >
> >
> > On Sat, 12 Jan 2019 19:41:26 -0800 "Steve Johnson" <scj at yaccman.com> wrote:
> > > One connection Knuth had to Unix was inventing LALR parsing, the basic
> > > algorithm used in Yacc. I added some things (notably, the precedence
> > > mechanism) and had to do a lot of engineering to be able to handle large
> > > grammars (e.g. F77) on a PDP-11. But the underlying algorithm (taught to
> > > my be Al Aho) was all Knuth.
> >
> > Knuth invented LR parsing but IIRC it was DeRemer who came up
> > with LALR parsing. In 78-79 I was implementing a LALR(1)
> > parser generator in Pascal on strength of which I got my first
> > real job. At that job I used DeRemer and Pennello's 1979
> > paper to reimplement the parser generator.
> >