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From: Joel de Guzman (joel_at_[hidden])
Date: 2004-12-24 20:58:04

David Abrahams wrote:

>>>Is first/follow really linked to determinism? It seems to me that you
>>>can use first/follow sets to prune unproductive searches in a
>>>nondeterministic parse, too (?)
>>Right. First-follow makes a parser predictive and thus deterministic,
> Okay, either I'm "right" and first-follow can be used to prune
> unproductive parses in a nondeterministic parse *or* using first-follow
> makes a parser deterministic. It can't be both :)

Using first-follow makes a parser deterministic, it can't be both.
An LL(1) parser, for instance, doesn't do backtracking. It always
has one and only one choice when faced with alternatives. It's
roughly like the switch (deterministic) versus the cascading ifs

However... (see below)

> As I understand "nondeterministic" it just means you're doing
> backtracking when a parse gets into trouble, unlike in an NFA where
> you're conceptually moving along all viable paths in parallel. I don't
> see why the use of first/follow should preclude backtracking. You can
> always define ambiguous grammars with dynamic failure conditions.
>>typically with one symbol lookahead (LL(1)). A strictly predictive
>>parser is a lot more restrictive in the grammars that one can use,
>>however. There's no way to make it parse adhoc grammars like the
>>C pre-processor or, ahem, c++. What I am inclined to do is to mix
>>determinism with classic spirit's non-determinism. Exactly the
>>recipe that you are alluding to.
> I'm not sure if it is or not -- I'm just getting more confuseder. ;^)

Classic LL(1) does not have the "dynamic failure conditions" that
you mention. LL(1) grammars cannot have ambiguity. If the grammar
is ambiguous, first follow computation will fail (first follow

I'm developing a scheme where you can mix both predictive LL(1) with
classic spirit's non-determinism. The scheme involves using a
predictive front, backed up by a non-predictive fall-back step when
ambiguity arises. It's roughly like combining the switch with

It's still too early if such a scheme will hold water. AFAIK, no one
has done it yet. An alternative scheme is to let the user switch
strategies (possibly using spirit directives).

Another scheme, used by PCCTS, is to make the parser predictive
by default, and allow the user to use syntactic and semantic
predicates with arbitrary lookahead on specific points in
the grammar.


Joel de Guzman

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