[TUHS] v7 K&R C [really lexers]
scj at yaccman.com
scj at yaccman.com
Sun Jun 14 07:24:47 AEST 2020
I also gave up on lex for parsing fairly early. The problem was
reserved words. These looked like identifiers, but the state machine to
pick out a couple of dozen reserved words out of all identifiers was too
big for the PDP-11. When I wrote spell, I ran into the same problem.
I had some rules that wanted to convert plurals to singular forms that
would be found in the dictionary. Writing a rule to recognize .*ies
and convert the "ies" to "y" blew out the memory after only a handful of
patterns. My solution was to pick up words and reverse them before
passing them through lex, so I looked for the pattern "sei.*", converted
it to "y" and then reversed the word again. As it turned out, I only
owned spell for a few weeks because Doug and others grabbed it and ran
On 2020-05-16 18:23, Warner Losh wrote:
> On Sat, May 16, 2020, 6:05 PM Brantley Coile <brantley at coraid.com> wrote:
>> "The asteroid to kill this dinosaur is still in orbit."
>> --- Plan 9 lex man page
>> I always hand craft my lexers and use yacc to parse. Most code on plan 9 does that as well.
> Wow! That is the most awesome thing I've seen in a while....
> On May 16, 2020, at 8:00 PM, Jon Steinhart <jon at fourwinds.com> wrote:
> Steffen Nurpmeso writes:
> Tony Finch wrote in <alpine.DEB.2.20.2005142316170.3374 at grey.csi.cam.ac.uk>: |Larry McVoy <lm at mcvoy.com> wrote: |> |> It's got some perl goodness, regexps are part of the syntax, .... | |I got into Unix after perl and I've used it a lot. Back in the 1990s I saw |Henry Spencer's joke that perl was the Swiss Army Chainsaw of Unix, as a |riff on lex being its Swiss Army Knife. I came to appreciate lex |regrettably late: lex makes it remarkably easy to chew through a huge pile |of text and feed the pieces to some library code written in C. I've been |using re2c recently (http://re2c.org/), which is differently weird than |lex, though it still uses YY in all its variable names. It's remarkable |how much newer lexer/parser generators can't escape from the user |interface of lex/yacc. Another YY example: http://www.hwaci.com/sw/lemon/ P.S.: i really hate automated lexers. I never ever got used to use them. For learning i once tried to use flex/bison, but i failed really hard. I like
that blood, sweat and tears thing, and using a lexer seems so shattered, all the pieces. And i find them really hard to read. If you can deal with them they are surely a relief, especially in rapidly moving syntax situations. But if i look at settled source code which uses it, for example usr.sbin/ospfd/parse.y, or usr.sbin/smtpd/parse.y, both of OpenBSD, then i feel lost and am happy that i do not need to maintain that code. --steffen
> Wow, I've had the opposite experience. I find lex/yacc/flex/bison really
> easy to use. The issue, which I believe was covered in the early docs,
> is that some languages are not designed with regularity in mind which makes
> for ugly code. But to be fair, that code is at least as ugly with hand-crafted
> I believe that the original wisecrack was directed towards FORTRAN. My ancient
> experience was that it was using lex/yacc for HSPICE was not going to work so I
> had to hand-craft code for that.
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