functionMonoid.fs

open System

type IMonoid<'t> =
    abstract Zero : 't
    abstract Combine : 't -> 't ->  't

let sumMonoid : IMonoid<int> =
    { new IMonoid<int> with
        member __.Zero = 0
        member __.Combine x y = x + y }

let functionResultMonoid (resultMonoid : IMonoid<'r>) : IMonoid<'a -> 'r> =
    { new IMonoid<'a -> 'r> with
        member __.Zero = fun _ -> resultMonoid.Zero
        member __.Combine x y = fun k -> resultMonoid.Combine (x k) (y k) }

let endoMonoid : IMonoid<'a -> 'a> =
    { new IMonoid<'a -> 'a> with
        member __.Zero = fun x -> x
        member __.Combine f g = fun x -> f (g x) }

let functionOne : string -> int = fun (s : string) -> s.Length
let functionTwo : string -> int = fun (s : string) -> Seq.filter Char.IsDigit s |> Seq.length

[<EntryPoint>]
let main argv =
    let result = (functionResultMonoid sumMonoid).Combine functionOne functionTwo
    printfn "%A" (result "hello 123")

    0