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Date: Tue, 31 Jan 2012 10:09:37 +0100
From: Georg Bauhaus <rm.dash-bauhaus@futureapps.de>
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Subject: Re: OpenToken: Handling the empty word token
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Date: 2012-01-31T10:09:37+01:00
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On 31.01.12 01:56, Randy Brukardt wrote:
> "Stephen Leake"<stephen_leake@stephe-leake.org>  wrote in message
> news:82ehuibdwt.fsf@stephe-leake.org...
>> "Randy Brukardt"<randy@rrsoftware.com>  writes:
> ...
>>> I'd be surprised if OpenToken didn't have something similar;
>>
>> Not quite. Because OpenToken uses Ada types to build the grammar, we
>> need an explicit Epsilon token (full code below):
>
> I see. It appears that OpenToken uses an LL-derived parsing scheme. (It
> appears to be a version of recursive descent, which is an LL scheme.] And
> it's important to note that LL parsing cannot include epsilon tokens and
> have problems with left-recursion.

LL parsing can include empty words. But then, the grammar might
be such that predicting the next rule via FIRST and FOLLOW sets
does not lead to a single result. (In which case one could say
"cannot parse" or boldly move on and pick a production, try it,
and if it fails, try the next, I should think.) No guarantees.