Re: Questions about Lpeg (semantics of captures)

[roberto@... (Roberto Ierusalimschy)]

> I still don't understand lpeg very well, and I have the (naive?)
> impression that patterns-with-captures are implemented on top of
> patterns-without-captured in a way that even allows "projecting" a
> pattern-with-captures into the lowel level, by discarding all the
> information about captures... also, matching a pattern-with-captures
> involves some backtracking, and some operations on the captures - like
> "patt / function" - should only be performed after the (super)pattern
> succeeds; so, in a first moment lpeg.match keeps backtracking
> information and instructions for performing captures; at some point
> the pattern is "closed", the backtracking information is dropped, and
> the instructions for performing captures are executed...
> 
> Is that mental model correct?

Yes, although I think it is not "healthy" to use captures with side
effects (that is, that depend on when they are executed).


> Is there a way to force a subpattern to be closed, and its captures
> performed?

Not now.


> Now let me show why I stumbled on that question, and why I was
> somewhat surprised when I discovered that the execution of the
> function in "patt / function" is delayed.
> 
> [...]
> 
> but then I discovered that the the "/ setsymbol" part was being
> executed after the "lpeg.P(isheadsymbol)", not before...
> 
> My current solution (which works!) is like this - again, I'm
> reconstructing this from from memory; the real implementation is more
> complex:
> 
>    SSymbol = lpeg.R("AZ", "az", "09") + lpeg.S("-+_")
> 
>    headsymbols = { ["info"]=true, ["man"]=true }
> 
>    setmark = function (subj, pos)
>        mark = pos
>        return pos
>      end
>    isheadsymbol = function (subj, pos)
>        local symbol = string.sub(subj, mark, pos - 1)
>        return headsymbols[symbol] and pos
>      end
> 
>    SHeadSymbol = lpeg.P(setmark) * SSymbol * lpeg.P(isheadsymbol)

This is one option. Another option is to write a function that matches
the whole SHeadSymbol inside it (by calling lpeg again to match
SSymbol). Something like this:

  SHeadSymbol = lpeg.P(function (subj, pos)
    local e = lpeg.match(SSymbol, subj, pos)
    if not e then return nil end
    local symbol = string.sub(subj, pos, e - 1)
    return headsymbols[symbol] and pos
  end)


-- Roberto