Functor.fs

type Box<'T1,'T2> = { Value : obj }

module List =
    let Box (list: list<'T>) : Box<list<obj>,'T> = { Value = list }
    let Unbox (x: Box<list<obj>,'T>) : list<'T> = x.Value :?> _

type IFunctor<'T> =
    abstract member Map : ('T1 -> 'T2) -> Box<'T,'T1> -> Box<'T,'T2>

let ListFunctor =
    {
        new IFunctor<list<obj>> with
            member this.Map f x =
                List.Box << List.map f << List.Unbox <| x
    }