Pegged, From EBNF to PEG

[Dmitry Olshansky]

On 14.03.2012 2:49, Philippe Sigaud wrote:
> On Tue, Mar 13, 2012 at 17:17, Dmitry Olshansky<dmitry.olsh at gmail.com>  wrote:
>
>> PEG defines order of alternatives, that is pretty much like a top-down
>> recursive descent parser would parse it. Alternatives are tried from left to
>> right, if first one fails, it tries next and so on.
>
> Yes.
>
>> In an example I give B is always picked first and so C is never ever looked
>> at.
>
> That's one of the caveats on PEG. That and greedy operators.
>
> 'a*a' never succeeds because 'a*' consumes all the available a's.
>

Hey, wait, I thought there has to be backtrack here, i.e. when second 
'a' finally fails?

>
>>> Literal<- ("'" .+ "'") / ('"' .+ '"')
>>>
>> This needs escaping. Plain '.+' in pattern asks for trouble 99% of time.
>
> This is an example were PEG would munch anything till the end, and
> fail (since """ is not found in an empty string).
> The standard PEG way to do that is
>
> (!EndMarker .)+ EndMarker
>
> It's a common enough pattern I tend to define a parameterized rule for that:
>
> Until(Expr)<- (!Expr .)* Expr
>
>
> Gotta love parametrized rules! 15' to code, a neverending stream of joy.

Nice!

>
>
>> In fact grammars are usually devised the other way around, e.g.
>> Start:
>>   Program ->  ...
>> Ehm... what the whole program is exactly ? Ok, let it be Declaration* for
>> now. What kind of declarations do we have? ... and so on. Latter grammars
>> get tweaked and extended numerous times.
>>
>> At any rate production order has no effect on the grammar, it's still the
>> same. The only thing of importance is what non-terminal considered final (or
>> start if you are LL-centric).
>
> Yes. The PEG standard seem to be that the first rule is the start
> (root) rule. Pegged let you decide which rule you use for a start. The
> rest is automatic.
> I might change that.


-- 
Dmitry Olshansky