Hi Mark,
Brilliant work, thank you!
I was playing with your arithmetic parser from tutorial, and noticed the
following definition for add and sub:
"expr = add-sub
<add-sub> = mul-div | add | sub
add = add-sub <'+'> mul-div
sub = add-sub <'-'> mul-div
...
And I realize now that the ordering "add-sub <'+'> mul-div" here is
very deliberate.
What should be my intuition for your parser for
default ambiguity resolution rules? (sorry, I'm not familiar [yet] with
GLL parsers)
Is it guaranteed to be non-greedy, i.e. it consumes as little as possible?
The docs section "No Grammar Left Behind" is somewhat fuzzy about it.
If I were writing LALR parser I would do (along with defining precedence
and associativity rules for op) something like:
expr = expr op expr | ...
And for LL parser I would do:
add = mul-div <'+'> add-sub
(i.e. reverse ordering compared to yours).
But your parser rules are somewhat new to me.
Both variations are accepted:
add = add-sub <'+'> add-sub
add = mul-div <'+'> add-sub
And in both cases some generated parsers are correct (arithmetically
speaking :-) ), but I'd like to understand rules for the first/default
parser.
Could you clarify it a little please?
Great docs, BTW.
Thanks, Dmitry.
On Monday, April 8, 2013 10:18:39 PM UTC-7, puzzler wrote:
>
> Instaparse is an easy-to-use, feature-rich parser generator for Clojure.
> The two stand-out features:
>
> 1. Converts standard EBNF notation for context-free grammars into an
> executable parser. Makes the task of building parsers as lightweight and
> simple as working with regular expressions.
>
> 2. Works with *any* context-free grammar. This means you don't have to
> learn the esoteric subtleties of LR, LL, LALR or any other specialized
> subset. Left-recursion, right-recursion, ambiguous grammars -- instaparse
> handles it all.
>
> Example:
>
> (def as-and-bs
> (parser
> "S = AB*
> AB = A B
> A = 'a'+
> B = 'b'+"))
>
> => (as-and-bs "aaaaabbbaaaabb")
> [:S
> [:AB [:A "a" "a" "a" "a" "a"] [:B "b" "b" "b"]]
> [:AB [:A "a" "a" "a" "a"] [:B "b" "b"]]]
>
> https://github.com/Engelberg/instaparse for full feature list and
> extensive tutorial.
>
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