On Thu, Apr 25, 2013 at 6:53 PM, Patrick Walton <[email protected]> wrote:
> On 4/25/13 9:23 AM, Felix S. Klock II wrote:
>
>> On 25/04/2013 18:12, Graydon Hoare wrote:
>>
>>> I've been relatively insistent on LL(1) since it is a nice
>>> intersection-of-inputs, practically guaranteed to parse under any
>>> framework we retarget it to.
>>>
>> I'm a fan of this choice too, if only because the simplest efficient
>> parser-generators and/or parser-composition methodologies I know of take
>> an LL(1) grammar as input.
>>
>> However, Paul's earlier plea on this thread ("Please don't do this
>> [grammar factoring] to the official parser!") raised the following
>> question in my mind:
>>
>> Are we allowing for the possibility of choosing the semi-middle ground
>> of: "There *exists* an LL(1) grammar for Rust that is derivable from the
>> non-LL(1)-but-official grammar for Rust." ? Or do we want to go all the
>> way to ensuring that our own grammar that we e.g. use for defining the
>> syntactic classes of the macro system etc is strictly LL(1) (or perhaps
>> LL(k) for some small but known fixed k)?
>>
>
> I'm not sure we can do the latter. There are too many issues relating to
> `unsafe`, `loop`, the `self` argument, etc. to make the LL(1) derivable
> from the human-readable grammar in an automated fashion, in my eyes. At
> least, I'm pretty sure that if we did want to go down that route, we'd
> probably be doing months of parser research (and I do mean *research*, as
> far as I know).
>
> Patrick
>
>
> On the other hand, should you content yourself with LL(2), and actually
have a tool like yapp2 guarantee that it is indeed LL(2) (and does not
degenerate), would it not be sufficient ?
(in case LL(1) really is gruesome compared to LL(2))
-- Matthieu
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