prevent Tiny_httpd_dir.config from being built by hand; provide builder

src/Tiny_httpd_dir.ml

@@ -5,22 +5,36 @@ module Pf = Printf
 type dir_behavior =
   | Index | Lists | Index_or_lists | Forbidden
 
+type hidden = unit
 type config = {
   mutable download: bool;
   mutable dir_behavior: dir_behavior;
   mutable delete: bool;
   mutable upload: bool;
   mutable max_upload_size: int;
+  _rest: hidden
 }
 
-let default_config () : config =
+let default_config_ : config =
   { download=true;
     dir_behavior=Forbidden;
     delete=false;
     upload=false;
     max_upload_size = 10 * 1024 * 1024;
+    _rest=();
   }
 
+let default_config () = default_config_
+let config
+    ?(download=default_config_.download)
+    ?(dir_behavior=default_config_.dir_behavior)
+    ?(delete=default_config_.delete)
+    ?(upload=default_config_.upload)
+    ?(max_upload_size=default_config_.max_upload_size)
+    () : config =
+  { download; dir_behavior; delete; upload; max_upload_size;
+    _rest=()}
+
 let contains_dot_dot s =
   try
     String.iteri

src/Tiny_httpd_dir.mli

@@ -23,7 +23,12 @@ type dir_behavior =
   | Forbidden
   (** Forbid access to directory. This is suited for serving assets, for example. *)
 
-(** configuration for static file handlers *)
+type hidden
+(** Type used to prevent users from building a config directly.
+    Use {!default_config} or {!config} instead. *)
+
+(** configuration for static file handlers. This might get
+    more fields over time. *)
 type config = {
   mutable download: bool;
   (** Is downloading files allowed? *)
@@ -40,6 +45,9 @@ type config = {
   mutable max_upload_size: int;
   (** If {!upload} is true, this is the maximum size in bytes for
       uploaded files. *)
+
+  _rest: hidden;
+  (** Just ignore this field. *)
 }
 
 (** default configuration: [
@@ -51,6 +59,17 @@ type config = {
   }] *)
 val default_config : unit -> config
 
+val config :
+  ?download:bool ->
+  ?dir_behavior:dir_behavior ->
+  ?delete:bool ->
+  ?upload:bool ->
+  ?max_upload_size:int ->
+  unit ->
+  config
+(** Build a config from {!default_config}.
+    @since NEXT_RELEASE *)
+
 (** [add_dirpath ~config ~dir ~prefix server] adds route handle to the
     [server] to serve static files in [dir] when url starts with [prefix],
     using the given configuration [config]. *)

src/bin/http_of_dir.ml

@@ -20,8 +20,9 @@ let parse_size s : int =
   with _ -> raise (Arg.Bad "invalid size (expected <int>[kM]?)")
 
 let main () =
-  let config = D.default_config() in
-  config.dir_behavior <- Index_or_lists; (* keep old behavior *)
+  let config =
+    D.config ~dir_behavior:Index_or_lists ()
+  in
   let dir_ = ref "." in
   let addr = ref "127.0.0.1" in
   let port = ref 8080 in