diff --git a/src/virtual/make_config_lib.ml b/src/virtual/make_config_lib.ml
index e350078..498579a 100644
--- a/src/virtual/make_config_lib.ml
+++ b/src/virtual/make_config_lib.ml
@@ -1,217 +1,197 @@
-let ( let* ) o f =
-  match o with
-  | Ok v -> f v
-  | Error (`Msg e) ->
-    Fmt.epr "config error, %s" e;
-    exit 1
+module Schema = struct
+  type nil = N
 
-let unwrap param_name opt =
-  match opt with
-  | None -> Fmt.error_msg "parameter `%s` not found" param_name
-  | Some v -> Ok v
+  type empty = E
 
-let print k v =
-  Fmt.pr {|let %s = "%s"@.|} k v;
-  ()
+  (*type dir = D*)
 
-let print_b k v =
-  Fmt.pr "let %s = %b@." k v;
-  ()
+  type _ kind =
+    | Bool : bool kind
+    | Int : int kind
+    | Float : float kind
+    | String : string kind
 
-let print_int k v =
-  Fmt.pr "let %s = %#d@." k v;
-  ()
+  (* todo: one general gadt for params and schema? *)
+  type _ params =
+    | Nil : nil params
+    | Cons : ('a kind * 'b params) -> ('a * 'b) params
 
-let print_int_opt k v =
-  let s = match v with None -> "None" | Some v -> Fmt.str "Some %#d" v in
-  Fmt.pr "let %s = %s@." k s;
-  ()
+  type _ schema =
+    | Empty : empty schema
+    | Dir : (string * 'a params * 'b schema) -> ('a * 'b) schema
 
-let print_l k l =
-  let l = List.map (Fmt.str {|"%s"|}) l in
-  Fmt.pr "let %s = [%s]@." k (String.concat "; " l);
-  ()
+  let v = Dir ("void", Nil, Empty)
 
-let print_arr k arr =
-  let l = Array.map (Fmt.str {|"%s"|}) arr |> Array.to_list in
-  Fmt.pr "let %s = [|%s|]@." k (String.concat "; " l);
-  ()
+  let v1 = Dir ("cc", Cons (String, Nil), v)
 
-let print_fpath k v =
-  Fmt.pr {|let %s = Result.get_ok (Fpath.of_string "%s")@.|} k
-    (Fpath.to_string v);
-  ()
+  let v2 =
+    Dir
+      ( "salut"
+      , Cons (String, Cons (Bool, Cons (Float, Nil)))
+      , Dir ("euj", Cons (Int, Nil), v1) )
+end
 
-let print_uri k v =
-  Fmt.pr {|let %s = Uri.of_string "%s"@.|} k (Uri.to_string v);
-  ()
+(* ------ *)
+
+type _ tag =
+  | String : string tag
+  | Bool : bool tag
+  | Int : int tag
+  | Option : 'a tag -> 'a option tag
+  | List : 'a tag -> 'a list tag
+  | Array : 'a tag -> 'a array tag
+  | Fpath : Fpath.t tag
+  | Uri : Uri.t tag
+
+type w = W : 'a tag -> w
+
+let rec str_aux : type a. a tag -> a -> string =
+ fun tag v ->
+  match tag with
+  | String -> Fmt.str {|"%s"|} v
+  | Bool -> Fmt.str {|%b|} v
+  | Int -> Fmt.str {|%d|} v
+  | Option subtag -> (
+    match v with
+    | None -> "None"
+    | Some v -> Fmt.str "Some ( %s )" (str_aux subtag v) )
+  | List subtag -> (
+    match v with
+    | [] -> "[]"
+    | l ->
+      let l = List.map (fun v -> str_aux subtag v) l in
+      Fmt.str "[ %s ]" (String.concat "; " l) )
+  | Array subtag -> (
+    match v with
+    | [||] -> "[||]"
+    | l ->
+      let l = Array.map (fun v -> str_aux subtag v) l |> Array.to_list in
+      Fmt.str "[| %s |]" (String.concat "; " l) )
+  | Fpath ->
+    Fmt.str {|Result.get_ok (Fpath.of_string "%s")|} (Fpath.to_string v)
+  | Uri -> Fmt.str {|Uri.of_string "%s"|} (Uri.to_string v)
+
+let str : type a. a tag -> string -> a -> string =
+ fun tag k v ->
+  match tag with
+  | List _ -> Fmt.str {|let %s_l = %s@.|} k (str_aux tag v)
+  | _ -> Fmt.str {|let %s = %s@.|} k (str_aux tag v)
 
 let config_path =
-  let* config_dir =
-    Dir.config
-    |> Option.to_result ~none:(`Msg "can't compute configuration directory")
+  let config_dir =
+    match Dir.config with
+    | None -> Fmt.failwith "can't compute configuration directory"
+    | Some config_dir -> config_dir
   in
   let config_path = Fpath.(config_dir / "config.scfg") in
   config_path
 
 let config =
-  let* exists = Bos.OS.File.exists config_path in
-  let* config =
+  let exists = Bos.OS.File.exists config_path in
+  let config =
     match exists with
-    | false ->
-      Fmt.error_msg "configuration file `%a` does not exist, please create it"
+    | Error (`Msg s) -> Fmt.failwith "%s" s
+    | Ok false ->
+      Fmt.failwith "configuration file `%a` does not exist, please create it"
         Fpath.pp config_path
-    | true -> Scfg.Parse.from_file config_path
+    | Ok true -> Scfg.Parse.from_file config_path
   in
-  config
+  config |> Result.get_ok
 
-let get_dir param_name =
-  let* dir = unwrap param_name @@ Scfg.Query.get_dir param_name config in
-  dir
-
-let mk_str param_name =
-  let* v = Scfg.Query.get_param 0 (get_dir param_name) in
-  print param_name v;
-  ()
-
-let mk_bool param_name =
-  let* v = Scfg.Query.get_param_bool 0 (get_dir param_name) in
-  print_b param_name v;
-  ()
-
-let mk_int ?min param_name =
-  let min = Option.value ~default:0 min in
-  let* n = Scfg.Query.get_param_int 0 (get_dir param_name) in
-  let* () =
+let get : type a. a tag -> string -> a =
+ fun tag k ->
+  let open Scfg.Query in
+  let dir = get_dir_exn k config in
+  match tag with
+  | String -> get_param_exn 0 dir
+  | Bool -> get_param_bool_exn 0 dir
+  | Int -> get_param_int_exn 0 dir
+  | Option Int -> (
+    let n = Scfg.Query.get_param_int_exn 0 dir in
     match n with
-    | _ when n < 0 -> Fmt.error_msg "negative `%s` value" param_name
-    | _ when n < min ->
-      Fmt.error_msg "less than minimum (`%d`) `%s` value" min param_name
-    | _ -> Ok ()
-  in
-  print_int param_name n;
-  ()
-
-(* we use -1 in config file for None *)
-let mk_int_opt param_name =
-  let* opt =
-    match Scfg.Query.get_dir param_name config with
-    | None -> Ok None
-    | Some dir -> (
-      let* n = Scfg.Query.get_param_int 0 dir in
-      match n with
-      | -1 -> Ok None
-      | n when n < 0 -> Fmt.error_msg "negative `%s` value" param_name
-      | n -> Ok (Some n) )
-  in
-  print_int_opt param_name opt;
-  ()
-
-let mk_str_l param_name =
-  let dirs = Scfg.Query.get_dirs param_name config in
-  let l =
+    | -1 -> None
+    | n when n < 0 -> Fmt.failwith "negative `%s` value" k
+    | n -> Some n )
+  | List String ->
+    let dirs = Scfg.Query.get_dirs k config in
     List.map
       (fun dir ->
-        let* v = Scfg.Query.get_param 0 dir in
+        let v = Scfg.Query.get_param_exn 0 dir in
         v )
       dirs
-  in
-  print_l (Fmt.str "%s_l" param_name) l;
-  ()
+  | Option _ | List _ | Array _ | Fpath | Uri -> Fmt.failwith "unimplemented"
+
+let mk : type a. a tag -> string -> unit =
+ fun tag k ->
+  let v = get tag k in
+  let str = str tag k v in
+  Fmt.pr "%s" str
 
-(* server config *)
 let print_config_serv () =
+  Fmt.pr "%s" (str Fpath "config_path" config_path);
+  let data_dir = Dir.data |> Option.get in
+  (* create data_dir *)
   let () =
-    print_fpath "config_path" config_path;
-    ()
+    match Bos.OS.Dir.create data_dir with
+    | Ok true -> ()
+    | Ok false -> ()
+    | Error (`Msg _) -> Fmt.failwith "error when creating %a" Fpath.pp data_dir
   in
-
-  let () =
-    let* data_dir =
-      Dir.data |> Option.to_result ~none:(`Msg "can't compute data directory")
-    in
-    (* create data_dir *)
-    let* () =
-      match Bos.OS.Dir.create data_dir with
-      | Ok true -> Ok ()
-      | Ok false -> Ok ()
-      | Error (`Msg _) ->
-        Fmt.error_msg "error when creating %a" Fpath.pp data_dir
-    in
-    let db_path = Fpath.(data_dir / "geochan.db") in
-    let s = Uri.of_string @@ Fmt.str "sqlite3://%a" Fpath.pp db_path in
-    print_uri "db_uri" s;
-    ()
+  let db_path = Fpath.(data_dir / "geochan.db") in
+  let uri = Uri.of_string @@ Fmt.str "sqlite3://%a" Fpath.pp db_path in
+  Fmt.pr "%s" (str Uri "db_uri" uri);
+  let l =
+    [ (W Bool, "default_logger")
+    ; (W Bool, "custom_logger")
+    ; (* TODO add temporary password hash + email for first connection? *)
+      (W (List String), "admin")
+    ]
   in
-
-  let () = mk_bool "default_logger" in
-
-  let () = mk_bool "custom_logger" in
-
-  (* TODO add temporary password hash + email for first connection? *)
-  let () = mk_str_l "admin" in
-
+  List.iter (fun (W tag, k) -> mk tag k) l;
   ()
 
 (* config (server & client) *)
 let print_config () =
-  let () = mk_str "source_code_url" in
-
-  let () = mk_str "hostname" in
-
-  let () = mk_int "port" in
-
-  let () = mk_int "csrf_lifetime" in
-
-  let () = mk_int "session_lifetime" in
-
-  let () = mk_bool "open_registration" in
-
-  let () = mk_int "thread_max_count" in
-
-  let () = mk_int "thread_alive_max_count" in
-
-  let () = mk_int "thread_replies_max_count" in
-
-  let () = mk_int "subject_max_length" in
-
-  let () = mk_int_opt "subject_min_length" in
-
-  let () = mk_int "comment_max_length" in
-
-  let () = mk_int_opt "comment_min_length" in
-
-  let () = mk_int "report_max_length" in
-
-  let () = mk_int "nick_max_length" in
-
-  let () = mk_int ~min:1 "nick_min_length" in
-
-  let () = mk_int "biography_max_length" in
-
-  let () = mk_int "password_max_length" in
-
-  let () = mk_int ~min:1 "password_min_length" in
-
-  let () = mk_int "image_name_max_length" in
-
-  let () = mk_int "image_description_max_length" in
-
-  let () = mk_int "image_max_size" in
-  print_arr "supported_mime_type"
-    [| "image/jpeg"; "image/png"; "image/webp"; "image/gif" |];
-
+  let l =
+    [ (W String, "source_code_url")
+    ; (W String, "hostname")
+    ; (W Int, "port")
+    ; (W Int, "csrf_lifetime")
+    ; (W Int, "session_lifetime")
+    ; (W Bool, "open_registration")
+    ; (W Int, "thread_max_count")
+    ; (W Int, "thread_alive_max_count")
+    ; (W Int, "thread_replies_max_count")
+    ; (W Int, "subject_max_length")
+    ; (W (Option Int), "subject_min_length")
+    ; (W Int, "comment_max_length")
+    ; (W (Option Int), "comment_min_length")
+    ; (W Int, "report_max_length")
+    ; (W Int, "nick_max_length")
+    ; (W Int, "nick_min_length")
+    ; (W Int, "biography_max_length")
+    ; (W Int, "password_max_length")
+    ; (W Int, "password_min_length")
+    ; (W Int, "image_name_max_length")
+    ; (W Int, "image_description_max_length")
+    ; (W Int, "image_max_size")
+    ]
+  in
+  List.iter (fun (W tag, k) -> mk tag k) l;
+  Fmt.pr "%s"
+    (str (Array String) "supported_mime_type"
+       [| "image/jpeg"; "image/png"; "image/webp"; "image/gif" |] );
   ()
 
 let () =
-  let* arg =
-    if Array.length Sys.argv > 1 then Ok Sys.argv.(1)
-    else Fmt.error_msg "make config failure, no argument provided"
+  let arg =
+    if Array.length Sys.argv > 1 then Sys.argv.(1)
+    else Fmt.failwith "make config failure, no argument provided"
   in
-  let* () =
+  let () =
     match arg with
-    | "--config-serv" -> Ok (print_config_serv ())
-    | "--config" -> Ok (print_config ())
-    | _ -> Fmt.error_msg "make config failure, invalid argument `%s`" arg
+    | "--config-serv" -> print_config_serv ()
+    | "--config" -> print_config ()
+    | _ -> Fmt.failwith "make config failure, invalid argument `%s`" arg
   in
   ()