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 Wednesday, April 22, 2009
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One of the issues I’ve been struggling with in building MCsla is that the mgx tool that is used to compile a DSL into M doesn’t output valid M in all cases.

A couple of the issues are simply unsolvable (I mentioned these in my last blog post as well).

  1. If your grammar includes any Boolean values, they come out as ‘ true’ and ‘false’ (including the single quotes), when they should come out as true and false (no quotes).
  2. If your grammar includes any lists of values, a single-item list generates invalid M, while multi-item lists generate valid M (though I’m not sure you can describe the result using an MSchema – but at least the result is “valid”).

As I say, these problems are “unsolvable” because the M rendering API is what creates the bad result, and that’s a black box.

The solution, such as it is, requires taking the text output, the M code, and doing some text processing to fix the issues. In the olden days I’d have whipped up an awk script to do this, but these days C# is the tool (even if it is a lot more verbose). Here’s the code I’m using to fix the two issues – kind of a post-processor to the mgx tool:

public static class MFix
{
  /// <summary>
  /// Fix single-quote and single-list-item
  /// issues with a generated M file.
  /// </summary>
  /// <param name="fileName">Path to the file to fix</param>
  public static void FixFile(string fileName)
  {
    var output = new System.Text.StringBuilder();
    File.ReadAllLines(fileName).ToList().ForEach((line) =>
    {
      output.AppendLine(FixQuotes(FixSingleNode(line)));
    });
    File.WriteAllText(fileName, output.ToString());
  }

  /// <summary>
  /// Fix single-quote issue with M generated by
  /// mgx.exe.
  /// </summary>
  /// <param name="input">One line of text input.</param>
  /// <returns>Fixed line of text from input.</returns>
  private static string FixQuotes(string input)
  {
    var sb = new System.Text.StringBuilder(input.Length);
    int state = 0;
    for (int i = 0; i < input.Length; i++)
    {
      var sub = input.Substring(i, 1);
      if (sub == "\"")
      {
        if (state == 0)
          state = 1;
        else
          state = 0;
        sb.Append(sub);
      }
      else if (sub == "\\" && state == 1)
      {
        sb.Append(sub);
        if (i < input.Length - 1)
        {
          i++;
          sb.Append(input.Substring(i, 1));
        }
      }
      else if (sub == "'" && state == 0)
      {
        // do nothing and eat next char if it is a ' '
        i++;
        if (input.Substring(i, 1) != " ")
          i--;
      }
      else
        sb.Append(sub);
    }
    return sb.ToString();
  }

  /// <summary>
  /// Fix single element array issue with M generated by
  /// mgx.exe.
  /// </summary>
  /// <param name="input">One line of text input.</param>
  /// <returns>Fixed line of text from input.</returns>
  private static string FixSingleNode(string input)
  {
    var trm = input.TrimStart();
    if (!trm.StartsWith("=")) return input;

    var pos = input.IndexOf("=");
    var sb = new System.Text.StringBuilder(input.Length);

    sb.Append(input.Substring(0, pos));
    sb.Append(" {");
    sb.Append(input.Substring(pos + 1, input.Length - pos - 1));
    sb.Append(" } ");

    return sb.ToString();
  }
}

It is kind of ironic that I had to write a subset of an M parser to fix issues with the M generator API.

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