Simple expression evaluator

The macro source code, macro.n:

using Nemerle.Compiler;

macro myeval (expr)
{
  def r = System.Random ();

  def eval (expr) {
    | <[ $e1 + $e2 ]> => eval (e1) + eval (e2)
    | <[ $e1 * $e2 ]> => eval (e1) * eval (e2)
    | <[ $e1 / $e2 ]> => eval (e1) / eval (e2)
    | <[ $e1 - $e2 ]> => eval (e1) - eval (e2)
    | <[ rand ]> => r.Next () % 42
    | <[ $(k : int) ]> => k
    | e =>
      Message.Error ($ "invalid expression to myeval: $e");
      0
  }

  def res = eval (expr);

  Message.Warning ($"myeval($expr) = $res");

  <[ $(res : int) ]>
}

Example usage, use.n:

System.Console.WriteLine (myeval (2 + 2));
System.Console.WriteLine (myeval (2 + 8 * rand));

Compilation and running:

ncc -r:Nemerle.Compiler -t:dll -o:macro.dll macro.n
ncc -r:macro.dll use.n
./out.exe

and the results, first warnings from the compiler:

use.n:1:27: warning: myeval(2 + 2) = 4
use.n:2:27: warning: myeval(2 + 8 * rand) = 162

and then the output:

4
162