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Date: 2007-04-29 (04:43)
From: skaller <skaller@u...>
Subject: Re: [Caml-list] menhir
On Sat, 2007-04-28 at 18:50 +0200, Francois Pottier wrote:

> > Third the generated ml file was 4.5 Meg.  Ocamlopt on amd64 hung for so long
> > I almost posted a bug report for Ocamlopt, but finally it finished.
> Yup, I am somewhat disappointed that ocamlopt does not seem to have linear
> time complexity, but I shouldn't complain too loud, my boss may be listening
> :)
> An option to generate tables like ocamlyacc would clearly be useful.

Although that defeats one of the future advantages of Menhir:
embedding. To understand what I mean you might look at the Felix
parser generator. In Felix you can write parsers in any scope,
and the user actions bind to the containing scope.

This means you can think about mixing programmatic recursive 
descent with the parser automaton. An executable recursive
descent parser is easily extended dynamically (but has woeful
error handling ;(

For Menhir, embedding would be nice. However even without
it being able to use recursion and actually pass arguments --
which ocamlyacc does not support -- should allow decoupling
of some of your grammar into smaller subgrammars, and this
in turn could reduce compile time.

It is possible (but I'm no expert!!) that the non-linearity
in compiling Menhir generated code is due to a very 
large let rec/and/... being generated. Some extra analysis 
might fix that, i.e. using recursion only when necessary,
and let/and/in otherwise (though I have no idea how to
do that).

For embedding the 'one token overshoot' is a bit nasty.

OTOH with GLR parser, extra functionality is gained,
and user actions must have no side effects, so
the idea to 'push back' the overshoot token cannot be used
(since it is a side-effect), at least without some thought.. 
hmmm ..

An extensible GLR+ parser would be interesting. This is 
an extensible list of GLR parsers that run in 'parallel',
using the GLR idea, but the list being dynamically extensible.
Exactly how you'd manage it i don't know.

The theoretical underpinnings should be the same as
for open-recursion idea: make a grammar with a parameter
representing extra productions, then you can fix it
to close the grammar. This should at least be possible
statically (at compile time).

> > Basically: when Ocamlyacc reduces a production, it sometimes
> > ends on the last token, and sometimes it overshoots by 1.
> I don't know the details of your grammar, but our (perhaps naive) view is that
> you should modify your grammar to avoid end-of-stream conflicts (and Menhir's
> conflict reports will help you do that). Then, Menhir will not overshoot.

Actually in my grammar there is a definite user defined ENDMARKER
for the main grammar, and 'end-of-expression' and 'end-of-statements'
set of tokens for the extensible parts. The input is not a file,
it's a list of tokens built by other means.

So the conflicts are spurious: end of stream can't be parsed
(but of course Menhir doesn't know that). This is good,
because Mehir CANNOT detect end of stream, since my lexbuf
is a dummy and is not used at all.

> > The semantics used are: when the exprx is processed the
> > action arranges to push the terminator token back
> > into the token stream.
> How is this done? 

The lexer function is bound to a class object which contains
a mutable list of tokens. The actual lexbuf is ignored,
its a dummy. So pushing token onto the list is just

	ts <- last_token :: ts


> It is possible that this hack is not compatible with Menhir,
> because Menhir keeps a local cache of the next token, which is not properly
> updated when you push your old token back.

That's what I would have guessed.

In order to use an LR parser recursively, which I do, it must
be possible to control the lookahead token somehow. My code
just uses an implementation detail of Ocamlyacc discovered
by accident, and the detail differs for Menhir.

John Skaller <skaller at users dot sf dot net>
Felix, successor to C++: http://felix.sf.net