LPeg parsing errors (Re: A Review of MoonScript 0.2.0)

[David Kolf <kolf@...>]

Alex Bradbury wrote:
> On 15 December 2011 10:46, steve donovan<steve.j.donovan@gmail.com>  wrote:
> > Still immature, of course; languages take time and getting good error
> > messages out of LPeg grammars is a bit of an art.
>
> When playing with lpeg grammars that's always been the part I've found
> less easy (and of course the part which is easiest to just ignore).
> Does anyone know of any lpeg examples which do a particularly good job
> of this?

This thread is a bit old by now, but for my JSON parser 
<http://chiselapp.com/user/dhkolf/repository/dkjson/> I used match-time 
captures to generate error messages. The error message is saved in a 
state table which is passed as argument to the match function.

  local function loc (str, where)
    local line, pos, linepos = 1, 1, 1
    while true do
      pos = strfind (str, "\n", pos, true)
      if pos and pos < where then
        line = line + 1
        linepos = pos
        pos = pos + 1
      else
        break
      end
    end
    return "line " .. line .. ", column " .. (where - linepos)
  end

  local g = require ("lpeg")
  local P, S, R, V = g.P, g.S, g.R, g.V

  local function ErrorCall (str, pos, msg, state)
    if not state.msg then
      state.msg = msg .. " at " .. loc (str, pos)
      state.pos = pos
    end
    return false
  end

  local function Err (msg)
    return g.Cmt (g.Cc (msg) * g.Carg (2), ErrorCall)
  end

  -- Usage examples:

  local EscapeSequence =
    (P"\\" * g.C (S"\"\\/bfnrt" + Err "unsupported escape sequence")) /
    escapechars
  local String = P"\"" * g.Cs (Char ^ 0) * (P"\"" +
    Err "unterminated string")
  local Pair = g.Cg (Space * String * Space * (P":" +
    Err "colon expected") * ExpectedValue)