The F# Zoo

[<AbstractClass>]
type Animal(name : string) as this =
    do
        let t = this.GetType()
        printfn "Creating an animal called %s of type %A!" name t

    abstract member say: unit -> string

type Giraffe(name) =
    class
        inherit Animal(name)
        let sound = "cough"
        override this.say() = sound
    end

type Tiger(name) =
    class
        inherit Animal(name)
        let sound = "growl"
        override this.say() = sound
    end

let doEvil (a : Animal byref) =
    a <- new Tiger("Tony")

let main =
    let mutable g = Giraffe("Fred") :> Animal
    doEvil(&g)
    printfn "The animal says, \"%s\"!" (g.say())
    ()