fix: improve error messages when merlin is absent

ps-id: 5AE68BEC-4D7B-4289-9D58-C1D993F443F8

CHANGES.md

@@ -9,6 +9,8 @@
 - Fix infer interface code action crash when implementation source does not
   exist (#597)
 
+- Improve error message when the reason plugin for merlin is absent (#608
+
 # 1.9.1
 
 ## Fixes

ocaml-lsp-server/src/dune

@@ -13,6 +13,7 @@
   lsp
   lsp_fiber
   dot-merlin-reader.merlin_dot_protocol
+  merlin.extend
   merlin.analysis
   merlin.kernel
   merlin.merlin_utils

ocaml-lsp-server/src/ocaml_lsp_server.ml

@@ -184,14 +184,19 @@ let set_diagnostics rpc doc =
     async (fun () -> Diagnostics.send state.diagnostics (`One uri))
   | Reason
   | Ocaml ->
-    async (fun () ->
-        let* diagnostics =
-          let command =
-            Query_protocol.Errors
-              { lexing = true; parsing = true; typing = true }
-          in
-          Document.with_pipeline_exn doc (fun pipeline ->
-              let errors = Query_commands.dispatch pipeline command in
+    let send () =
+      let* diagnostics =
+        let command =
+          Query_protocol.Errors { lexing = true; parsing = true; typing = true }
+        in
+        Document.with_pipeline_exn doc (fun pipeline ->
+            match Query_commands.dispatch pipeline command with
+            | exception Extend_main.Handshake.Error error ->
+              let message =
+                sprintf "%s.\nHint: install merlin-extend reason" error
+              in
+              [ create_diagnostic ~range:Range.first_line ~message () ]
+            | errors ->
               let merlin_diagnostics =
                 List.rev_map errors ~f:(fun (error : Loc.error) ->
                     let loc = Loc.loc_of_report error in
@@ -244,9 +249,11 @@ let set_diagnostics rpc doc =
               |> List.sort
                    ~compare:(fun (d1 : Diagnostic.t) (d2 : Diagnostic.t) ->
                      Range.compare d1.range d2.range))
-        in
-        Diagnostics.set state.diagnostics (`Merlin (uri, diagnostics));
-        Diagnostics.send state.diagnostics (`One uri))
+      in
+      Diagnostics.set state.diagnostics (`Merlin (uri, diagnostics));
+      Diagnostics.send state.diagnostics (`One uri)
+    in
+    async send
 
 let on_initialize server (ip : InitializeParams.t) =
   let state : State.t = Server.state server in