VD>public Eval() : double
VD>{
VD>  match (this)
VD>  {
VD>    | Literal(value)    => value
VD>    | Call("max", [arg1, arg2]) => Math.Max(arg1.Eval(), arg2.Eval())
VD>    | Call("min", [arg1, arg2]) => Math.Min(arg1.Eval(), arg2.Eval())
VD>    | Call(_, _)                => throw InvalidOperationException(this.ToString());
VD>    | Plus(e1,  e2)             => e1.Eval() + e2.Eval()
VD>    | Minus(e1, e2)             => e1.Eval() - e2.Eval()
VD>    | Mul(e1,   e2)             => e1.Eval() * e2.Eval()
VD>    | Div(e1,   e2)             => e1.Eval() / e2.Eval()
VD>  }
VD>}
VD>

VD>>public variant Expr
VD>>{
VD>>  | Literal { value : double; }
VD>>  | Call    { name : string; parms : list[Expr]; }
VD>>  | Plus    { expr1 : Expr; expr2 : Expr; }
VD>>  | Minus   { expr1 : Expr; expr2 : Expr; }
VD>>  | Mul     { expr1 : Expr; expr2 : Expr; }
VD>>  | Div     { expr1 : Expr; expr2 : Expr; }
  
VD>>  public override ToString() : string
VD>>  {
VD>>    match (this)
VD>>    {
VD>>      | Literal(value)    => value.ToString()
VD>>      | Call(name, parms) => $"$name($(parms.Map(_.ToString()).ToString(\", \")))"
VD>>      | Plus(e1, e2) with op = "+" | Minus(e1, e2) with op = "-" 
VD>>      | Mul(e1, e2)  with op = "*" | Div(e1, e2)   with op = "/" => $"$e1 $op $e2"
VD>>    }
VD>>  }
VD>>

R>struct expr
R>{
R>    enum type {Literal, Call, Plus, Minus, Mul, Div} t;
R>    union
R>    {
R>        double value;
R>        struct {const char* name; expr* params[10]; int count;};
R>        struct {expr *expr1, *expr2;};
R>    } data;
R>};

R>void to_str(const expr* e)
R>{
R>    switch (e->t)
R>    {
R>    case expr::Literal: cout << e->data.value; break;
R>    case expr::Call: cout << e->data.name << "("; 
R>        for (int i = 0; i < e->data.count; ++i)
R>        {
R>            to_str(e->data.params[i]);
R>            cout << "," + !!i;
R>        }
R>        cout << ")"; break;
R>    case expr::Plus: to_str(e->data.expr1); cout << "+"; to_str(e->data.expr2); break;
R>    case expr::Minus: to_str(e->data.expr1); cout << "-"; to_str(e->data.expr2); break;
R>    case expr::Mul: to_str(e->data.expr1); cout << "*"; to_str(e->data.expr2); break;
R>    case expr::Div: to_str(e->data.expr1); cout << "/"; to_str(e->data.expr2); break;
R>    }
R>}
R>