Re: How detect cycle in grammar ?

On Nov 24, 12:24 pm, Borneq <a.modera...@gmail.com> wrote:
> On 21 Lis, 19:20, Gene <gene.ress...@gmail.com> wrote:
>
> > As I recall the algorithm starts with the lhs's of rules that expand
> > entirely to terminals (including epsilon) and recursively marks
> > nonterminals that have a rule where the entirely right hand side is
> > either marked or terminal. When you're done marking in this manner,
> > the unmarked nonterminals are useless. All rules involving them can
> > be deleted without changing the represented language. If the start
> > symbol is unmarked, the language is empty.
>
> I foundhttp://programming4.us/desktop/408.aspxsection "4 Reduction
> of Grammar"
> I interpret this: we mark nonterminal as usable if any its production
> contains only terminals or marked previously nonterminals.
> But what if nonterminal has one using production and one useless? For
> example
> A->A
> A->B
> B->b
> How eliminate A->A ?


Once you know the useless nonterminals, you can delete any rule with a
right hand side consisting entirely of them.


It's also worth noting that any grammar with A->A in it is ambiguous
in a way that should cause LR table generation to fail. How would a
parser know how many times to reduce A->A?

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