sourcevault/luamail_lpeg.md

## luamail_lpeg.md

      
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              luamail_lpeg.md
            
          
    What is the idiomic way to create error messages in LPEG ?
To be specific I would like to create error messages for incomplete bracket completion.
Example :

[ 1 2 3     -- ERROR ! MISSING ] AT LINE 20


( 1 2 3     -- ERROR ! MISSING ) AT LINE 35


{ 1 2 3     -- ERROR ! MISSING } AT LINE 12


1 2 3 ]    -- ERROR ! EXTRA   ] AT LINE 42


[ ( 1 2 3 ] -- ERROR ! MISSING ) AT LINE 71

I have figured out that I need to use lpeg.Carg to pass a table to log line number but how can a pattern fail in a certain way as to let its error be know ?
local show = function(...)

  {tab,cap} = arg

  print (cap)

end


local inner = C(1 - P(']'))^0

local patt = P('[') * inner * P(']')

local test = Cmt((patt * (Carg(1))), show)

local out = test:match('[hello', 1, { })

return log(out) -- nil ??

The problem is the function show does not run if the pattern fails.
It also doen't not run if I use Cmt, but I think I am missing the larger picture.