Mounting Suave in an Owin application - f#

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.1 ofAppFunc 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.