Expose a push function for streams (rgrinberg#139)

Second attempt at supporting streams. Instead of exposing
the underlying type, expose an api which gives the user
a push function where they can push strings.

examples/hello_world.ml

@@ -11,16 +11,17 @@ let print_param =
       `String ("Hello " ^ param req "name") |> respond')
 
 let streaming =
+  let open Lwt.Infix in
   get "/hello/stream" (fun _req ->
-      let count = ref 0 in
-      let chunk = "00000000000" in
-      `Streaming
-        (Lwt_stream.from_direct (fun () ->
-             if !count < 1000 then (
-               incr count ;
-               Some (chunk ^ "\n") )
-             else None))
-      |> respond')
+      let f, push = App.create_stream () in
+      let timers =
+        List.map
+          (fun t ->
+            Lwt_unix.sleep t
+            >|= fun () -> push (Printf.sprintf "Hello after %f seconds\n" t))
+          [1.; 2.; 3.]
+      in
+      f (Lwt.join timers))
 
 let default =
   not_found (fun _req ->

opium/app.ml

@@ -266,6 +266,16 @@ module Response_helpers = struct
 
   let respond' ?headers ?code s = s |> respond ?headers ?code |> return
 
+  let create_stream () =
+    let open Lwt.Infix in
+    let stream, push = Lwt_stream.create () in
+    let p' w = push (Some w) in
+    let f ?headers ?code p =
+      Lwt.async (fun () -> p >|= fun () -> push None) ;
+      respond' ?headers ?code (`Streaming stream)
+    in
+    (f, p')
+
   let redirect ?headers uri =
     let headers =
       Cohttp.Header.add_opt headers "Location" (Uri.to_string uri)
@@ -302,3 +312,5 @@ let respond' = Response_helpers.respond'
 let redirect = Response_helpers.redirect
 
 let redirect' = Response_helpers.redirect'
+
+let create_stream = Response_helpers.create_stream

opium/app.mli

@@ -108,6 +108,14 @@ val respond' :
   -> body
   -> Response.t Lwt.t
 
+val create_stream :
+     unit
+  -> (   ?headers:Cohttp.Header.t
+      -> ?code:Cohttp.Code.status_code
+      -> unit Lwt.t
+      -> Response.t Lwt.t)
+     * (string -> unit)
+
 val redirect : ?headers:Cohttp.Header.t -> Uri.t -> Response.t
 
 (* Same as return (redirect ...) *)