Alternatively, in MLs (this sample specifically is OCaml) :

    type expression =
    | Number of float
    | Add of expression * expression
    | Subtract of expression * expression
    | Multiply of expression * expression
    | Divide of expression * expression

    let rec eval expr = 
        match expr with
        | Number n -> n
        | Add (expr1, expr2) -> (eval expr1) +. (eval expr2)
        | Subtract (expr1, expr2) -> (eval expr1) -. (eval expr2)
        | Multiply (expr1, expr2) -> (eval expr1) *. (eval expr2)
        | Divide (expr1, expr2) -> (eval expr1) /. (eval expr2)

Hey look, I just built a calculator DSL. And everything is statically typed and typechecked (and if I left off the "Divide" case in my "eval" method, it would warn me).