format the signature help to improve the readability

Author: Lucccyo

ocaml-lsp-server/src/signature_help.ml

@@ -63,12 +63,19 @@ let run (state : State.t) { SignatureHelpParams.textDocument = { uri }; position
          sprintf "%s : " fun_name
        in
        let offset = String.length prefix in
-       let+ doc =
+       let* doc =
          Document.Merlin.doc_comment
            ~name:"signature help-position"
            merlin
            application_signature.function_position
        in
+       let+ formatted_signature =
+         let typ = application_signature.signature in
+         let* result = Ocamlformat_rpc.format_type state.ocamlformat_rpc ~typ in
+         match result with
+         | Ok v -> Fiber.return (String.trim v)
+         | Error _ -> Fiber.return typ
+       in
        let info =
          let parameters =
            List.map
@@ -90,7 +97,7 @@ let run (state : State.t) { SignatureHelpParams.textDocument = { uri }; position
            in
            format_doc ~markdown ~doc
          in
-         let label = prefix ^ application_signature.signature in
+         let label = prefix ^ formatted_signature in
          SignatureInformation.create ~label ?documentation ~parameters ()
        in
        SignatureHelp.create

ocaml-lsp-server/test/e2e-new/signature_help.ml

@@ -37,7 +37,7 @@ let _ : int = f
       "activeSignature": 0,
       "signatures": [
         {
-          "label": "f : int -> int -> int -> int -> int -> int -> int -> int -> int -> int",
+          "label": "f : int ->\nint ->\nint ->\nint ->\nint ->\nint ->\nint ->\nint ->\nint ->\nint",
           "parameters": [
             { "label": [ 4, 7 ] },
             { "label": [ 11, 14 ] },