module Program open System type Expr = Cons of int | Neg of Expr | Add of Expr * Expr | Mul of Expr * Expr | Div of Expr * Expr let rec eval e = match e with | Cons i -> i | Neg e -> -(eval e) | Add(e1,e2) -> eval e1 + eval e2 | Mul(e1,e2) -> eval e1 * eval e2 | Div(e1,e2) -> eval e1 / eval e2 let Puzzle e = // what value of e can cause exceptions? ask Pex to find out! let v = eval e Console.WriteLine v