lua-users home
lua-l archive

[Date Prev][Date Next][Thread Prev][Thread Next] [Date Index] [Thread Index]

Hi Spc,

So essentially what you are saying is the '/' function syntax is just syntax sugar ? without having much value to creating a parser ?

I got introduced to '/' first which caused a lot of frustration, I only understood how to use Cmt later on.

Cmt is so much better since it behaves the way you want it to. What is the return to '/' ?

Is there a function equivalent to '/' ?

When would you use / INSTEAD of Cmt ?

I am really enjoying your stack technique using Cmt , thanks for showing me !

I was stuck trying to use Cb ( back referencing ) and Cg - which are confusing.

Then I read that Cb is experimental.

Cmt seems to be the main way to run function in your LPEG code.

Cheers !! and thanks

On Thu, Apr 4, 2019 at 4:51 AM Sean Conner <> wrote:
It was thus said that the Great joy mondal once stated:
> Just a side question.
> Under what design consideration do we use 'Cmt(patt,f) ' vs 'patt/f' ?
> I noticed you used Cmt , why not 'patt/f' ?

  The docs say this of lpeg.Cmt():

        Creates a match-time capture. Unlike all other captures, this one is
        evaluated immediately when a match occurs (***even if it is part of
        a larger pattern that fails later***). It forces the immediate
        evaluation of all its nested captures and then calls function.

        (emphasis added)

  If I didn't, the error checking wouldn't work as expected.  Changing the
WS production to:

        local WS = lpeg.P" "
                 + lpeg.P"\t"
                 + lpeg.P"\n" * lpeg.Carg(1)
                 / function(state)
                     state.line = state.line + 1

causes my code to produce the following output as an error:

        Missing ']' at line 1 (start line 1)

even though I see the diagnostic print() statements printed three times:

        >>>     1
        >>>     2
        >>>     3

  I think it has something to do with lazy evaluation in LPEG in this case.

  I've also used it to fail a pattern that would otherwise match.  It is
possible to come up with a pattern that matches the numbers between 0 and
255 but it's quite involved and part of it looks like:

        dec_octet = P"25" * R"05"
                  + P"2" * P"04" * P"09"
                  + P"1" * ...

I found it easier to use Cmt() instead:

        local dec_octet = Cmt(DIGIT^1,function(_,position,capture)
          local n = tonumber(capture)
          if n < 256 then
            return position

When the string of digits is greater than 255, this returns nil, which
causes the match to fail.  Doing this:

        dec_octet = DIGIT^1
                  / function(c)
                      local n = tonumber(c)
                      if n < 256 then
                        return c

won't cause the match to fail---instead it will return nil as the captured