LR(1) grammar/parser and lookahead-restrictions

Waldemar Horwat waldemar at
Tue Feb 7 00:32:19 UTC 2017

On 02/04/2017 07:20, Michael Dyck wrote:
> On 17-02-03 05:32 PM, Waldemar Horwat wrote:
>> On 02/03/2017 08:17, Michael Dyck wrote:
>>> On 17-02-02 06:23 PM, Waldemar Horwat wrote:
>>>> Lookahead restrictions fit very well into an LR(1) engine
>>> Again: Great, but how? E.g., do you pre-process the grammar, modify the
>>> construction of the automaton, and/or modify the operation of the parser?
>> For each state × token combination, the automaton describes what happens
>> when you're in state S and see a token T.  The lookahead restrictions remove
>> possible transitions; without them there would be ambiguities where a given
>> state × token combination would want to do two incompatible things.
> Okay, so do you generate the automaton (ignoring lookahead restrictions) and then remove transitions (using lookahead restrictions)? Or do you integrate the lookahead-restrictions into the generation of the automaton?

It's integrated.  You can't generate a valid automaton without the lookahead restrictions.

>> That's different from parametrized rules, which simply macro-expand into
>> lots of individual rules.
> Yup.
>>> But the context-dependentness of lexing is a parse-time problem, not a
>>> validation-time problem, right?
>> No.
>>> The syntactic level can just assume a stream of (correctly lexed) input
>>> elements.
> (I should have said "*Validation* of the syntactic level [etc]")
>> No!  It's impossible to create a stream of correctly lexed input elements
>> without doing syntactic level parsing.
> I quite agree. I didn't mean to suggest otherwise. What I mean is that, once you've generated the automaton for the syntactic grammar, you can just look at each state's set of expected terminals, and from that deduce the goal symbol that the lexer will need to use to get the next token when the parser is in that state. The point being that you can do that *after* generating the syntactic automaton. So the context-dependentness of lexing doesn't have to affect the process of generating the syntactic automaton.

That's correct.

> (That's assuming an LR(1)-ish parser, and an approach where you don't try to combine the syntactic and lexical grammars to generate a single [scannerless] automaton. Which may not be your approach.)

The parser and lexer stay separate, other than the lexer providing tokens to the parser and the parser selecting one of several top-level lexer goal symbols for lexing the next token.  I do not use any kind of unified parser-lexer grammar; that could run into issues such as the syntactic grammar making lexing non-greedy: a+++++b lexing as a ++ + ++ b instead of the correct a ++ ++ + b (which would then generate a syntax error at the syntactic level).

>> The validator looks for problems such as the syntactic grammar giving the
>> lexer contradictory instructions. An example would be any syntactic
>> grammar automaton state where one outgoing syntactic transition would
>> swallow a regexp token and another outgoing syntactic transition from the
>> same state would swallow a / token. If any such state exists, the grammar
>> is broken.
> Yup, the spec asserts the non-existence of such states ("syntactic grammar contexts").
> -Michael


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