Re: Elegant design for creating error messages in LPEG parser

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 <sean@conman.org> 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)
print(">>>",state.line)
state.line = state.line + 1
end

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
end
end)

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
end
end

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

-spc