[antlr-interest] Re: a new paper on ANTLR style grammars

[Oliver Zeigermann]

Loring, thanks for the substantial input and taking this seriously :)

--- In antlr-interest at yahoogroups.com, "lgcraymer" <lgc at m...> wrote:
> You can do better by deferring the actions--basically, build a 
> monster case statement including all of the possible actions in a 
> grammar--and execute them after matching a rule. 

*After* exactly is the problem. It is pretty easy to execute actions
after a derivation / reduce, but not while shifting. This may be
desirable though...

> Then you can 
> trigger a set of actions at "commit" points.  Functional languages 
> make this sort of lazy evaluation easier.  

Where should those commit points be?

> Rolling back actions is 
> trickier--you have to have some sort of mechanism to record state, 
> or the cost for checkpoint/rollback is very high.

Rolling back / forward does not come for free, agreed! But, if
implemented reasonably expenses may at most be doubled. Compared to
worst case exponential costs of backtracking this is not so bad.

> For that matter, you can defer all actions until the entire grammar 
> is recognized as long as the actions do not affect the parse.  
> Editing of a generated tree might be trickier, but not that 
> difficult--you just insert code for that in the list of deferred 
> actions.

We discussed this before - but still - writing to and afterwards
reading from a symbol table is quite a usual thing. This can not be
expressed merely by means of CFGs (or less), you need actions and
semantic checks (i.e. predicates) here.

Other than that, to show my colors: I am a big fan of ASTs in genereal
and ANTLR tree transformation in particular :)

Oliver

> --Loring
> 
> 
> --- In antlr-interest at yahoogroups.com, "Oliver Zeigermann" 
> <oliver at z...> wrote:
> > What I wanted to say was: If you have sematic actions associated
to
> > your grammar that can be inserted (and of course executed) at any
> > point and you have a table driven approach you are in trouble. 
> This is
> > because what I understand as the precomputation of a search tree 
> into
> > a table combining certain *search* states. Extrapolating from
what 
> I
> > know about LR you have a problem when youe have a grammar like 
> this:
> > 
> > a : A A { do something here } A A B ;
> > b : A A { do something different here } A A C ;
> > 
> > upon input
> > 
> > AAAAC
> > 
> > This is because the parser has no idea which action to execute 
> here.
> > Now, my idea was to execute both and roll back the action of rule 
> a as
> > soon as it is clear that rule b actually matches.
> > 
> > A bit clearer now?
> > 
> > Oliver
> > 
> > --- In antlr-interest at yahoogroups.com, Terence Parr <parrt at c...> 
> wrote:
> > > 
> > > On Wednesday, November 19, 2003, at 03:12 PM, Oliver Zeigermann 
> wrote:
> > > 
> > > > Actually made it through the paper while getting nervous with 
> the
> > > > proofs ;)
> > > >
> > > > While he has linear time "backtracking" performance, ANTLR is 
> worst
> > > > case exponential. I was wondering why: ANTLR does not combine 
> its
> > > > depth first search (aka backtracking in this context) into a 
> table
> > > > while Bryan's approach does (at least I understand it this 
> way). The
> > > > problem Bryan will come across (given my understanding is 
> halfway
> > > > correct) is ACTIONS. As with LR and combined states, the 
> problem is
> > > > when to execute associated semantic actions. The drawback is 
> well
> > > > known and and leads to reduction in parsing power.
> > > >
> > > > Might sound weird, but I thought if we still combined states 
> even
> > > > though they are associated with different actions and simple 
> execute
> > > > all actions, there would be no loss of power :) Silly? Not if 
> you have
> > > > a transactional language that allows you to roll back actions 
> that
> > > > turn out to be invalid later and roll forward the valid ones.
> > > >
> > > > Technically this is possible. Does it make sense as well? Am
I 
> slowly
> > > > going crazy? ;)
> > > 
> > > You are already crazy like me ;) <snicker, snort>.  Just got 
> mail from 
> > > him. :)  Hope it's ok to repeat part here:
> > > 
> > > > - Packrat parsing guarantees linear-time parsing on all the 
> types of 
> > > > grammars
> > > > it supports, which amounts to everything that fits the 
> formalism or
> > > > "conceptual model" of parsing expression grammars.  But 
> the "pure"
> > PEG 
> > > > model
> > > > doesn't directly support "stateful" grammars like those of C 
> and C++, 
> > > > in
> > > > which you have to build up symbol tables and such that 
> effectively 
> > > > modify the
> > > > grammar mid-stream as the parser scans the input from left to 
> right.  
> > > > From
> > > > what I've seen so far, it appears fundamentally difficult or 
> > > > impossible to
> > > > make a packrat parser support stateful grammars efficiently 
> without
> > > > effectively turning it into a deterministic (e.g., LR) parser.
> > > 
> > > So, the actions are the problem for everyone :)
> > > 
> > > Ter
> > > --
> > > Professor Comp. Sci., University of San Francisco
> > > Creator, ANTLR Parser Generator, http://www.antlr.org
> > > Co-founder, http://www.jguru.com
> > > Co-founder, http://www.knowspam.net enjoy email again!
> > > Co-founder, http://www.peerscope.com pure link sharing


 

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