// match-failed handler written by Eric Kedji
// The arrow pointing to the error position
// in the error message works better when
// using a fixed-width font, like in a shell.

matchFailed = function (grammar, errorPos) {
  var lines = grammar.input.lst.split('\n');
  var pos = 0, l = 0;
  var msg = ["Execution error: input matching failed at position: " + errorPos];

  while (pos < errorPos) {
    var line = lines[l], length = line.length;
    if (pos + length >= errorPos) {
      var c = errorPos - pos;
      msg.push("  line:" + (l + 1) + ", column:" + c);
      msg.push(line)
      // replicate str n times
      function replicate(str, n) {
        if (n < 1) return "";
        var t = [];
        for (var i=0; i<n; i++) t.push(str);
        return t.join('');
      }
      msg.push(replicate('-', c) + '^');
    }
    pos += length + 1;
    l++;
  }
  alert(msg.join('\n'));
}

ometa M {
  start = letter+ '.' digit+
}
M.matchAll("hello .1234", "start", [], matchFailed)