Basic Nowin self host. No dependencies on Microsoft.Owin.* assemblies.

Program.fs

open System
open System.IO
open System.Net
open System.Collections.Generic
open System.Threading.Tasks

let port = 8888
let endWithCompletedTask = fun x -> Task.FromResult(null) :> Task

type OwinEnvironment = {
    httpMethod: string;
    requestBody: Stream;
    responseBody: Stream;
    setResponseStatusCode: (int -> unit);
    setResponseReasonPhrase: (string -> unit)
}

let getOwinEnvironment (env: IDictionary<string, obj>) = {
    httpMethod = env.["owin.RequestMethod"] :?> string;
    requestBody = env.["owin.RequestBody"] :?> Stream;
    responseBody = env.["owin.ResponseBody"] :?> Stream;
    setResponseStatusCode = 
        fun (statusCode: int) -> env.["owin.ResponseStatusCode"] <- statusCode
    setResponseReasonPhrase = 
        fun (reasonPhrase: string) -> env.["owin.ResponseReasonPhrase"] <- reasonPhrase
}

let transform (request: string) : string =
    sprintf "%s transformed" request

let handleOwinEnvironment (owin: OwinEnvironment) : unit =
    use writer = new StreamWriter(owin.responseBody)
    match owin.httpMethod with
    | "POST" ->
        use reader = new StreamReader(owin.requestBody)
        writer.Write(transform(reader.ReadToEnd()))
    | _ ->
        owin.setResponseStatusCode 400
        owin.setResponseReasonPhrase "Bad Request"
        writer.Write("Only POST requests are allowed")

[<EntryPoint>]
let main argv = 

    use server = 
        Nowin.ServerBuilder
            .New()
            .SetEndPoint(new IPEndPoint(IPAddress.Any, port))
            .SetOwinApp(fun env -> 
                env 
                |> getOwinEnvironment 
                |> handleOwinEnvironment 
                |> endWithCompletedTask)
            .Build()

    server.Start() 

    printfn "Server listening on http://localhost:%i/ \nhit <enter> to stop." port
    Console.ReadLine() |> ignore

    0