Re: [Chicken-users] backtracking in abnf/lexgen

[Ivan Raikov]

Hi Nathaniel,

    I don't understand what you mean by "different levels" here. Can
you express your grammar in ABNF notation?

   -Ivan


On Thu, Sep 14, 2017 at 4:06 AM, Nathaniel Rudavsky-Brody
<address@hidden> wrote:
> Hi,
>
> I was wondering if anyone might have some advice for a beginner's question
> with abnf/lexgen. Clearly it has something to do with backtracking, and  I
> hope I'm just missing something simple -- 1500 lines into my parser, I'd
> hate to have to start from scratch with another approach!
>
>
> For matching a string of a's and b's that ends with an "a", clearly
>
>
> (lex (seq (star (bar (lit "a") (lit "b")))
>                (lit "a"))
>     error "ababa")  ; => error
>
>
> For patterns on the same level, I can do manual backtracking:
>
>
> (define-syntax vac
>   (syntax-rules ()
>     ((_ fn) (lambda args (apply fn args)))))
>
> (define (left-star-seq p1 p2)   ; p1*p2
>   (vac
>    (bar
>     (seq p1 (left-star-seq p1 p2))
>     p2)))
>
> (lex (left-star-seq
>           (bar (lit "a") (lit "b"))
>           (lit "a"))
>   error "ababa")   ; =>  ((#\a #\b #\a #\b #\a) ())
>
>
> but what if p1 and p2 are on completely different levels, something like:
>
>
> (define p1 (star (bar (lit "a") (lit "b"))))
> (define p2 (lit "a"))
> ...
> (define q2 (seq q1 p1))
> (define q4 (seq q3 q2))
> (define Word (seq q4 p2))
>
>
> Is there way to do "composable" backtracking with these tools? (and is the
> question even the right one?)
>
> Thanks for any ideas,
>
> Nathaniel
>
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