Mounting Suave in an Owin application

Question

I have an existing owin application written in C# and would like to mount a suave application as a middleware but since I am relatively new to F# I am finding it quite difficult to navigate how this should be done. I think I'm looking for something like:

// in F# land
module MySuaveApp.ApiModule

let app =
  choose
    [ GET >=> choose
        [ path "/hello" >=> OK "Hello GET"
          path "/goodbye" >=> OK "Good bye GET" ]
      POST >=> choose
        [ path "/hello" >=> OK "Hello POST"
          path "/goodbye" >=> OK "Good bye POST" ] ]

  let getSuaveAsMiddleware() =
    ... magic goes here ...

// in Startup.cs
app.Use(MySuaveApp.ApiModule.getSuaveAsMiddleware())

As for what that magic should be I think it's a combination of OwinApp.ofAppFunc or OwinApp.ofMidFunc, but I can't for the life of me figure out what it should be.

Answer 1

There is no easy magic.¹ofAppFunc and ofMidFunc are here for creating WebParts out of OWIN components, i.e. OWIN -> Suave, whereas you want Suave -> OWIN.

The following works for your 'application' and serves as an example what would be needed to get it working:

open System.Runtime.CompilerServices

[<Extension>]
module Api = 
    open Suave
    open Successful
    open Filters
    open Operators
    open Microsoft.Owin
    open System.Threading.Tasks

    let app = 
        choose [ GET >=> choose [ path "/hello" >=> OK "Hello GET"
                                  path "/goodbye" >=> OK "Good bye GET" ]
                 POST >=> choose [ path "/hello" >=> OK "Hello POST"
                                   path "/goodbye" >=> OK "Good bye POST" ] ]

    let withCtx (ctx : IOwinContext) webpart =
        async {
            let request =
                { HttpRequest.empty with
                    headers = ctx.Request.Headers |> List.ofSeq |> List.map (fun kvp -> kvp.Key, kvp.Value |> String.concat ",")
                    host = ctx.Request.Host.Value 
                    ``method`` = HttpMethod.parse ctx.Request.Method
                    url = ctx.Request.Uri }
            let! res = webpart { HttpContext.empty with request = request }
            res |> Option.iter (fun r ->
                ctx.Response.StatusCode <- r.response.status.code
                match r.response.content with
                | Bytes bs -> ctx.Response.Write bs
                | _ -> failwith "Not supported")
            return res
        }

    type SuaveMiddleware(n) =
        inherit OwinMiddleware(n)
        override __.Invoke(context : IOwinContext) =
            let res = withCtx context app |> Async.RunSynchronously
            match res with
            | Some _ -> Task.CompletedTask
            | None -> base.Next.Invoke context

    [<Extension>]
    let UseSuave(app : Owin.IAppBuilder) =
        app.Use(typeof<SuaveMiddleware>)

The main works is delegated to withCtx that tries to fulfill a request given a IOwinContext and a WebPart. It does so mainly by converting back and forth between Suave and OWIN context and related entities. Note that this code is a PoC (Proof-of-Concept) and not fit for production. The SuaveMiddleware forwards request to the next middleware if Suave cannot fulfill the request.

Using from C# is easy then:

using MySuave;
using Owin;

namespace Main
{
    using System.Web.Http;

    public class Startup
    {
        public static void Configuration(IAppBuilder appBuilder)
        {
            appBuilder.UseSuave();

            var config = new HttpConfiguration();
            config.MapHttpAttributeRoutes();
            appBuilder.UseWebApi(config);
        }
    }
}

given

namespace Main.Example
{
    using System.Web.Http;

    [RoutePrefix("api")]
    public class ExampleController : ApiController
    {
        [HttpGet, Route("")]
        public string Index()
        {
            return "Hello World";
        }
    }
}

And both URLs work:

http://localhost:9000/hello

Hello GET

http://localhost:9000/api

   <string xmlns="http://schemas.microsoft.com/2003/10/Serialization/">Hello World</string>

_{¹ At least none I know of. I'm happy to be proven wrong.}