diff --git a/src/Tiny_httpd.ml b/src/Tiny_httpd.ml
index 53891e53..972f3ea2 100644
--- a/src/Tiny_httpd.ml
+++ b/src/Tiny_httpd.ml
@@ -18,61 +18,64 @@ let _debug k =
        Printf.kfprintf (fun oc -> Printf.fprintf oc "\n%!") stdout fmt)
   )
 
-type out = { fd : Unix.file_descr
-              ; buf : Bytes.t
-              ; mutable pos : int }
+module Out = struct
+  type t = { fd : Unix.file_descr
+           ; buf : Bytes.t
+           ; mutable pos : int }
 
-let out_of_descr fd =
-  let size = Unix.(getsockopt_int fd SO_SNDBUF) in
-  { fd; buf = Bytes.create size; pos = 0 }
+  let out_of_descr fd =
+    let size = Unix.(getsockopt_int fd SO_SNDBUF) in
+    { fd; buf = Bytes.create size; pos = 0 }
 
-let rec write_out ({buf;fd;_} as oc) i len =
-  try
-    let _,w,_ = Unix.select [] [fd] [] (-1.0) in
-    if w <> [] then
+  let rec write_out ({buf;fd;_} as oc) i len =
+    try
+      let _,w,_ = Unix.select [] [fd] [] (-1.0) in
+      if w <> [] then
+        begin
+          let written = Unix.single_write fd buf i len in
+          if written < len then
+            write_out oc (i+written) (len-written)
+        end
+      else assert false
+    with Sys_blocked_io
+       | Unix.Unix_error((EAGAIN|EWOULDBLOCK),_,_) ->
+          write_out oc i len
+       | Unix.Unix_error((EPIPE|EBADF),_,_) ->
+          raise (Sys_error "broken pipe")
+       | e ->
+          Printf.eprintf "unexpected exception in write_out: %s\n%!"
+            (Printexc.to_string e);
+          assert false
+
+  let rec output ({buf;pos;_} as oc) s i len =
+    let buf_len = Bytes.length buf in
+    let do_write, to_write =
+      if len >= buf_len - pos then
+        true, buf_len - pos
+      else
+        false, len
+    in
+    Bytes.blit s i buf pos to_write;
+    oc.pos <- pos + to_write;
+    if do_write then
       begin
-        let written = Unix.single_write fd buf i len in
-        if written < len then
-          write_out oc (i+written) (len-written)
+        write_out oc 0 buf_len;
+        oc.pos <- 0;
+        output oc s (i + to_write) (len - to_write)
       end
-    else assert false
-  with Sys_blocked_io
-     | Unix.Unix_error((EAGAIN|EWOULDBLOCK),_,_) ->
-        write_out oc i len
-     | Unix.Unix_error((EPIPE|EBADF),_,_) ->
-        raise (Sys_error "broken pipe")
-     | e ->
-        Printf.eprintf "unexpected exception in write_out: %s\n%!"
-          (Printexc.to_string e);
-        assert false
 
-let rec output ({buf;pos;_} as oc) s i len =
-  let buf_len = Bytes.length buf in
-  let do_write, to_write =
-    if len >= buf_len - pos then
-      true, buf_len - pos
-    else
-      false, len
-  in
-  Bytes.blit s i buf pos to_write;
-  oc.pos <- pos + to_write;
-  if do_write then
-    begin
-      write_out oc 0 buf_len;
-      oc.pos <- 0;
-      output oc s (i + to_write) (len - to_write)
-    end
+  let output_string oc str =
+    let buf = Bytes.unsafe_of_string str in
+    output oc buf 0 (String.length str)
 
-let output_string oc str =
-  let buf = Bytes.unsafe_of_string str in
-  output oc buf 0 (String.length str)
+  let fprintf oc format =
+    Printf.ksprintf (output_string oc) format
 
-let fprintf oc format =
-  Printf.ksprintf (output_string oc) format
+  let flush oc =
+    write_out oc 0 oc.pos;
+    oc.pos <- 0
+end
 
-let flush oc =
-  write_out oc 0 oc.pos;
-  oc.pos <- 0
 
 module Buf_ = struct
   type t = {
@@ -711,23 +714,23 @@ module Response = struct
       self.code Headers.pp self.headers pp_body self.body
 
   (* print a stream as a series of chunks *)
-  let output_stream_chunked_ (oc:out) (str:byte_stream) : unit =
+  let output_stream_chunked_ (oc:Out.t) (str:byte_stream) : unit =
     let continue = ref true in
     while !continue do
       (* next chunk *)
       let s, i, len = str.bs_fill_buf () in
-      fprintf oc "%x\r\n" len;
-      output oc s i len;
+      Out.fprintf oc "%x\r\n" len;
+      Out.output oc s i len;
       str.bs_consume len;
       if len = 0 then (
         continue := false;
       );
-      output_string oc "\r\n";
+      Out.output_string oc "\r\n";
     done;
     ()
 
-  let output_ (oc:out) (self:t) : unit =
-    fprintf oc "HTTP/1.1 %d %s\r\n" self.code (Response_code.descr self.code);
+  let output_ (oc:Out.t) (self:t) : unit =
+    Out.fprintf oc "HTTP/1.1 %d %s\r\n" self.code (Response_code.descr self.code);
     let body, is_chunked = match self.body with
       | `String s when String.length s > 1024 * 500 ->
         (* chunk-encode large bodies *)
@@ -746,14 +749,14 @@ module Response = struct
     let self = {self with headers; body} in
     _debug (fun k->k "output response: %s"
                (Format.asprintf "%a" pp {self with body=`String "<…>"}));
-    List.iter (fun (k,v) -> fprintf oc "%s: %s\r\n" k v) headers;
-    output_string oc "\r\n";
+    List.iter (fun (k,v) -> Out.fprintf oc "%s: %s\r\n" k v) headers;
+    Out.output_string oc "\r\n";
     begin match body with
       | `String "" | `Void -> ()
-      | `String s -> output_string oc s;
+      | `String s -> Out.output_string oc s;
       | `Stream str -> output_stream_chunked_ oc str;
     end;
-    flush oc
+    Out.flush oc
 end
 
 (* semaphore, for limiting concurrency. *)
@@ -872,7 +875,7 @@ end
 
 (* a request handler. handles a single request. *)
 type cb_path_handler =
-  out ->
+  Out.t ->
   byte_stream Request.t ->
   resp:(Response.t -> unit) ->
   unit
@@ -940,7 +943,7 @@ let add_path_handler_
         | handler ->
           (* we have a handler, do we accept the request based on its headers? *)
           begin match accept req with
-            | Ok () -> Some (Ok (fun (_oc:out) req ~resp -> resp (handler (tr_req req))))
+            | Ok () -> Some (Ok (fun (_oc:Out.t) req ~resp -> resp (handler (tr_req req))))
             | Error _ as e -> Some e
           end
         | exception _ ->
@@ -971,7 +974,7 @@ let add_route_handler_
         | Some handler ->
           (* we have a handler, do we accept the request based on its headers? *)
           begin match accept req with
-            | Ok () -> Some (Ok (fun (oc:out) req ~resp -> tr_req oc req ~resp handler))
+            | Ok () -> Some (Ok (fun (oc:Out.t) req ~resp -> tr_req oc req ~resp handler))
             | Error _ as e -> Some e
           end
         | None ->
@@ -993,7 +996,7 @@ let[@inline] _opt_iter ~f o = match o with
   | Some x -> f x
 
 let add_route_server_sent_handler ?accept self route f =
-  let tr_req (oc:out) req ~resp f =
+  let tr_req (oc:Out.t) req ~resp f =
     let req = Request.read_body_full req in
     let headers = ref Headers.(empty |> set "content-type" "text/event-stream") in
 
@@ -1010,13 +1013,13 @@ let add_route_server_sent_handler ?accept self route f =
 
     let send_event ?event ?id ?retry ~data () : unit =
       send_response_idempotent_();
-      _opt_iter event ~f:(fun e -> fprintf oc "data: %s\n" e);
-      _opt_iter id ~f:(fun e -> fprintf oc "id: %s\n" e);
-      _opt_iter retry ~f:(fun e -> fprintf oc "retry: %s\n" e);
+      _opt_iter event ~f:(fun e -> Out.fprintf oc "data: %s\n" e);
+      _opt_iter id ~f:(fun e -> Out.fprintf oc "id: %s\n" e);
+      _opt_iter retry ~f:(fun e -> Out.fprintf oc "retry: %s\n" e);
       let l = String.split_on_char '\n' data in
-      List.iter (fun s -> fprintf oc "data: %s\n" s) l;
-      output_string oc "\n"; (* finish group *)
-      flush oc
+      List.iter (fun s -> Out.fprintf oc "data: %s\n" s) l;
+      Out.output_string oc "\n"; (* finish group *)
+      Out.flush oc
     in
     let module SSG = struct
       let set_headers h =
@@ -1058,7 +1061,7 @@ let find_map f l =
 
 let handle_client_ (self:t) (client_sock:Unix.file_descr) : unit =
   let _ = Unix.set_nonblock client_sock in
-  let oc = out_of_descr client_sock in
+  let oc = Out.out_of_descr client_sock in
   let buf = Buf_.create() in
   let is = Byte_stream.of_descr client_sock in
   let continue = ref true in