Get rid of exception in control flow

Signed-off-by: Etienne Millon <me@emillon.org>

src/format_config.ml

@@ -53,11 +53,13 @@ let enabled_for_to_dyn =
   | Only l -> constr "only" (List.map ~f:language_to_dyn l)
   | All -> string "all"
 
-type t =
+type 'enabled_for generic_t =
   { loc : Loc.t
-  ; enabled_for : enabled_for
+  ; enabled_for : 'enabled_for
   }
 
+type t = enabled_for generic_t
+
 let to_dyn {enabled_for; loc = _} =
   let open Dyn.Encoder in
   record ["enabled_for", enabled_for_to_dyn enabled_for]
@@ -106,43 +108,38 @@ let field =
 
 let loc t = t.loc
 
-exception Delete
-
-module Encoder = struct
-  open Dune_lang.Encoder
-
-  let encode_language = function
-    | Dune -> string "dune"
-    | Dialect d -> string d
-
-  let explicit_langs =
-    function
-    | Only l -> l
-    | All -> Exn.raise_notrace Delete
-
-  let encode_enabled_for ef =
-    explicit_langs ef
-    |> List.map ~f:encode_language
-
-  let encode_formatting {loc=_; enabled_for}=
-    record_fields
-    [ field_i "enabled_for" encode_enabled_for enabled_for
-    ]
-
-  let conf conf =
-    [field_i "formatting" encode_formatting conf]
-    |> record_fields
-    |> List.hd
-end
+let encode_language =
+  let open Dune_lang.Encoder in
+  function
+  | Dune -> string "dune"
+  | Dialect d -> string d
+
+let encode_formatting {loc=_; enabled_for}=
+  let open Dune_lang.Encoder in
+  record_fields
+  [ field_i "enabled_for" (List.map ~f:encode_language) enabled_for
+  ]
+
+let encode_explicit conf =
+  let open Dune_lang.Encoder in
+  [field_i "formatting" encode_formatting conf]
+  |> record_fields
+  |> List.hd
+
+let to_explicit {loc;enabled_for} =
+  match enabled_for with
+  | All -> None
+  | Only l -> Some {loc; enabled_for = l}
 
 let compat_error_message ext =
   let suggestion =
-    match Encoder.conf ext with
-    | x ->
+    match to_explicit ext with
+    | Some explicit ->
+      let dlang = encode_explicit explicit in
       [ Pp.textf "To port it to the new syntax, you can replace this part by:"
-      ; Pp.tag ~tag:User_message.Style.Details (Dune_lang.pp x)
+      ; Pp.tag ~tag:User_message.Style.Details (Dune_lang.pp dlang)
       ]
-    | exception Delete -> [ Pp.textf "To port it to the new syntax, you can delete this part."]
+    | None -> [ Pp.textf "To port it to the new syntax, you can delete this part."]
   in
   User_error.make
     ~loc:ext.loc