Re: How detect cycle in grammar ?

On Thu, Dec 1, 2011 at 2:46 AM, Paul B Mann <paul@paulbmann.com> wrote:


> The LRSTAR parser generator reports two errors in this grammar:
>
> S -> A|B
> A -> A
> B -> t
>
>
> LRSTAR Basic 3.0.137 Copyright 2011 Compilerware.
>
> silly.grm(6) : Useless production.
>
> silly.grm(6) : Nonterminal symbol in cycle, cannot derive anything.
>
> 0 min 0.000 sec, 0.728 MB, 0 warnings, 2 errors.


The Meta-S parser generator reports two errors:


eV005: Cyclic Recursion (on A->A since Meta-S is top-down, A->A is
illegal cyclic recursion)
eV008: Class expected as parameter (on S->A|B, since A did not
compile, and therefore A|B does not resolve A to any valid rule)


A variation of the above grammar that generates the same errors is:


> S -> A|B
> A -> [B] A
> B -> t


Because A obviously expands to:


A -> B A
A -> A


Which ends up in the same left-recursive hole.


A -> C A
C -> [B]


of course generates the same error because effectively it's just
another way to say A -> [B] A and can lead to potential recursion.


I suppose the reason I'm bringing this up is that I mentioned earlier
that in adaptive grammars it is possible to write grammars that have
rules that are not known until parse-time to derive terminals on any
given rule. Because the parse is top down, however, A -> A is a case
of a rule that generates an error because it is malformed per the
Adaptive-K algorithm.


That said:


S -> A|B
A -> C A
C -> x.y
B -> "t"


Is a case where x.y *may* contain the empty string at parse time, or
may not (depending on input), and so is not thrown away with an error.
This is where writing adaptive grammars becomes a creative exercise.


Quinn

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