Clojurians Log v2

I have a question about greedy parsing and ambiguous grammars, I have the following grammar:

(insta/parser
    "layout   = (align | delim | repeat)+
     repeat   = <'{'> (align | delim)+ <'}'>
     delim    = (fill | padding)+
     fill     = 'F'
     padding  = #'[^\\[\\]{}fF]*'
     align    = <'['> ('L' | 'C' | 'R' | 'V') <']'>"
    :string-ci true))

@mbjarland you could change (align | delim)+ to delim? (align delim?)*, that would force the parser to alternate between delim and align, effectively making the delim rule greedy

and this is why I love the clojure community, thank you for the fast reply! : ) are we talking about the first rule layout = ...? In that case I would have to cook up a repeating pattern with 3 elements

so what I’m doing here is defining a column layout

Actually I was talking about the repeat rule. But you made a good point that we'd also have to apply a similar solution to the layout rule to get a similar effect

where the repeat says “if the user comes in with more columns than we have defined, use the repeating group to fill out the layout”

Column layout? Not sure what you mean

never mind, that is really application related and not related to the grammar

thought actually explaining what the thing does might clarify, but I think it just muddles the waters even more : )

I also thought about the / ordered parsing syntax, but I can not really see how to apply that to these repeating patterns

layout = delim? ((align | repeat) delim?)*
repeat = <'{'> delim? (align delim?)* <'}'>

: )

Since align and repeat both require nonzero characters to be present in order for the rule to succeed, wedging them in the repeat rule enforces some regularity in the repetition and thus unambiguates the parsing

that works, I tried it with insta/parses and my standard examples and it comes out with a single interpretation

I will have to meditate on the exact mechanics

Yeah, ordered choice doesn't really help here unless delim was competing with some other rule and you wanted to establish the priority. But in this case delim is just competing with itself, in a way.

exactly, just a question on what level in the bnf the repetition happens

Yeah

Fortunately that's easily solved by restructuring the combinators, no fancy PEG combinators necessary

this is actually a higher level pattern, I will make sure to grok this properly for the next time I run into an ambiguous repetition

thanks a ton!

No problem

@mbjarland I just thought of another way you could have solved it: ignore the above changes I proposed and instead change delim rule to:

delim = (fill | padding)+ !delim

Pretty sure my first solution would be more performant, albeit more complex

what does the !delim do?