Use only the error monad

Signed-off-by: Rudi Grinberg <rudi.grinberg@gmail.com>

src/lib.ml

@@ -887,32 +887,39 @@ let rec instantiate db name info ~stack ~hidden =
     resolve_dep db (name : Lib_name.t) ~allow_private_deps ~loc ~stack in
 
   let implements =
-    Lib_info.implements info
-    |> Option.map ~f:(fun ((loc,  _) as name) ->
-      let* vlib = resolve name in
-      let virtual_ = Lib_info.virtual_ vlib.info in
-      match virtual_ with
-      | None ->
-        Error.not_virtual_lib ~loc ~impl:info ~not_vlib:vlib.info
-      | Some _ ->
-        let variant = Lib_info.variant info in
-        match variant with
-        | None -> Ok vlib
-        | Some variant ->
-          (* If the library is an implementation tagged with a
-             variant, we must make sure that that it's correctly part of the
-             virtual library's known implementations. *)
-          let vlib_impls = Lib_info.known_implementations vlib.info in
-          Variant.Map.find vlib_impls variant
-          |> Option.bind
-            ~f:(fun (_, impl_name) ->
-              let name = Lib_info.name info in
-              Option.some_if (Lib_name.equal impl_name name) (Ok vlib))
-          |> Option.value
-          ~default:(
-            let vlib_name = Lib_info.name vlib.info in
-            let name = Lib_info.name info in
-            Error.vlib_known_implementation_mismatch ~loc ~name ~variant ~vlib_name))
+    let open Option.O in
+    let+ ((loc, _) as name) = Lib_info.implements info in
+    let open Result.O in
+    let* vlib = resolve name in
+    let virtual_ = Lib_info.virtual_ vlib.info in
+    match virtual_ with
+    | None ->
+      Error.not_virtual_lib ~loc ~impl:info ~not_vlib:vlib.info
+    | Some _ ->
+      let variant = Lib_info.variant info in
+      begin match variant with
+      | None -> Ok vlib
+      | Some variant ->
+        (* If the library is an implementation tagged with a variant, we must
+           make sure that that it's correctly part of the virtual library's
+           known implementations. *)
+        let name = Lib_info.name info in
+        let error () =
+          let vlib_name = Lib_info.name vlib.info in
+          Error.vlib_known_implementation_mismatch
+            ~loc ~name ~variant ~vlib_name
+        in
+        let vlib_impls = Lib_info.known_implementations vlib.info in
+        let* (_, impl_name) =
+          match Variant.Map.find vlib_impls variant with
+          | None -> error ()
+          | Some impl_name -> Ok impl_name
+        in
+        if Lib_name.equal impl_name name then
+          Ok vlib
+        else
+          error ()
+      end
   in
   let resolve_impl impl_name =
     let* impl = resolve impl_name in