type SmartList<'State, 't> = ('State -> 't -> 'State) -> 'State -> 'State

let fromList L folder state = Seq.fold folder state L

let fold folder state L = L folder state
let map f L folder state = L (fun s el -> folder s (f el)) state
let filter p L folder state =
    let updatedFolder s el =
        if p el then folder s el
        else s
    L updatedFolder state

// check
let result = fromList [1; 2; 3; 4; 5] |> filter (fun x -> x > 2) |> fold (+) 0
printfn "%A" result