Delta Engine の Sample/EmptyGame を F# で無理やり mutable をなくして実装。

EmptyGame.fs

open DeltaEngine.Core
open DeltaEngine.Datatypes
open DeltaEngine.Platforms

type LerpColor = { Current: Color; Next: Color; Percentage:float32 }

[<CompilationRepresentation(CompilationRepresentationFlags.ModuleSuffix)>] 
module LerpColor =
    let initialState = {Current=Color.Black; Next=Color.Black; Percentage=0.0f}

    let update state = 
        match (state.Percentage + Time.Current.Delta) with
        | t when t >= 1.0f -> {Current=state.Next; Next=Color.GetRandomColor(); Percentage=0.0f}
        | t -> {state with Percentage=t}

    let draw (state:LerpColor) (window:Window) =
        window.BackgroundColor <- Color.Lerp(state.Current, state.Next, state.Percentage)

module EmptyGame =
    let subscribe window =          
            Observable.scan(fun state source -> LerpColor.update state) LerpColor.initialState
            >> Observable.subscribe(fun state -> LerpColor.draw state window)
            >> ignore

    type EmptyGame (window:Window) =
        let runEvent = new Event<_>()
        let run() = runEvent.Trigger()
        do runEvent.Publish |> subscribe window

        interface Runner with
            member this.Run() = run()
        member this.Run() =  run()

    [<CompiledName("Main")>]
    let run () =
        let app = new App()
        app.Start<EmptyGame>();


module Program =
    [<EntryPoint>]
    let run args =
        EmptyGame.run()
        0