interpreter-m.n

// REFERENCE: Nemerle.Compiler

using Nemerle.Compiler;
using Nemerle.Compiler.Parsetree;
using Nemerle.Collections;

public class Robot
{
  public mutable Orientation : byte;
  public mutable X : int;
  public mutable Y : int;

  public IsDown : bool
  {
    get { Orientation == 1 }
  }

  public override ToString () : string
  {
    $"($X, $Y)"
  }
}

public variant Expr {
  | MoveBy { steps : int; }
  | Left
  | Right
  | Value { prop : string; }
  | If { cond : Expr.Value; then : Expr; els : Expr; }
}

public module Scripts
{
  public Run (obj : Robot, expr : Expr) : void
  {
    def check_value (val) {
      System.Convert.ToBoolean (obj.GetType ().GetProperty (val.prop).GetValue (obj, null))
    }
    
    match (expr) {
      | Expr.MoveBy (steps) =>
        match (obj.Orientation) {
          | 0 => obj.X += steps
          | 1 => obj.Y += steps
          | 2 => obj.X -= steps
          | _ => obj.Y -= steps
        }

      | Expr.Left => obj.Orientation = ((obj.Orientation + 3) % 4) :> byte;
      | Expr.Right => obj.Orientation = ((obj.Orientation + 1) % 4) :> byte;

      | Expr.Value as val => _ = check_value (val)
        
      | Expr.If (val, e1, e2) =>
        if (check_value (val))
          Run (obj, e1)
        else
          Run (obj, e2)
    }
  }

  public Run (obj : Robot, name : string) : void
  {
    def script = GetScript (name);
    foreach (e in script) Run (obj, e)
  }
  
  public GetScript (name : string) : list [Expr]
  {
    | "myscript1" =>
      [Expr.Right (), Expr.MoveBy (5),
       Expr.If (Expr.Value ("IsDown"), Expr.MoveBy (3), Expr.Left ())]

    | _ => throw System.ArgumentException ($"unknown script $name")
  }

  public GenerateRun (obj : PExpr, expr : Expr) : PExpr
  {
    def check_value (val) {
      <[ $obj.$(val.prop : dyn) ]>
    }
    
    match (expr) {
      | Expr.MoveBy (steps) =>
        <[ match ($obj.Orientation) {
             | 0 => $obj.X += $(steps : int) 
             | 1 => $obj.Y += $(steps : int) 
             | 2 => $obj.X -= $(steps : int) 
             | _ => $obj.Y -= $(steps : int)
           }
        ]>

      | Expr.Left => <[ $obj.Orientation = (($obj.Orientation + 3) % 4) :> byte ]>;
      | Expr.Right => <[ $obj.Orientation = (($obj.Orientation + 1) % 4) :> byte ]>;

      | Expr.Value as val => <[ _ = $(check_value (val)) ]>
        
      | Expr.If (val, e1, e2) =>
        <[ if ($(check_value (val)))
             $(GenerateRun (obj, e1))
           else
             $(GenerateRun (obj, e2))
        ]>
    }
  }
}

macro GenerateRun (obj, name : string)
{
  def script = Scripts.GetScript (name);
  def exprs = NList.Map (script, fun (e) { Scripts.GenerateRun (obj, e) });
  <[ { ..$exprs } ]>
}