fix(lsp): Improve lsp behaviour on goto definition (#1893)

Co-authored-by: Oscar Spencer <oscar.spen@gmail.com>

compiler/src/language_server/definition.re

@@ -53,19 +53,18 @@ let send_definition =
     }),
   );
 };
+type check_position =
+  | Forward
+  | Backward;
+let rec find_definition =
+        (
+          ~check_position=Forward,
+          sourcetree: Sourcetree.sourcetree,
+          position: Protocol.position,
+        ) => {
+  let results = Sourcetree.query(position, sourcetree);
 
-let process =
-    (
-      ~id: Protocol.message_id,
-      ~compiled_code: Hashtbl.t(Protocol.uri, Lsp_types.code),
-      ~documents: Hashtbl.t(Protocol.uri, string),
-      params: RequestParams.t,
-    ) => {
-  switch (Hashtbl.find_opt(compiled_code, params.text_document.uri)) {
-  | None => send_no_result(~id)
-  | Some({program, sourcetree}) =>
-    let results = Sourcetree.query(params.position, sourcetree);
-
+  let result =
     switch (results) {
     | [Value({definition}), ..._]
     | [Pattern({definition}), ..._]
@@ -74,18 +73,50 @@ let process =
     | [Exception({definition}), ..._]
     | [Module({definition}), ..._] =>
       switch (definition) {
-      | None => send_no_result(~id)
+      | None => None
       | Some(loc) =>
         let uri = Utils.filename_to_uri(loc.loc_start.pos_fname);
-
-        send_definition(
-          ~id,
-          ~range=Utils.loc_to_range(loc),
-          ~target_uri=uri,
-          Utils.loc_to_range(loc),
-        );
+        Some((loc, uri));
       }
-    | _ => send_no_result(~id)
+    | _ => None
+    };
+  switch (result) {
+  | None =>
+    if (check_position == Forward && position.character > 0) {
+      // If a user selects from left to right, their pointer ends up after the identifier
+      // this tries to check if the identifier was selected.
+      find_definition(
+        ~check_position=Backward,
+        sourcetree,
+        {line: position.line, character: position.character - 1},
+      );
+    } else {
+      None;
+    }
+  | Some((loc, uri)) => result
+  };
+};
+
+let process =
+    (
+      ~id: Protocol.message_id,
+      ~compiled_code: Hashtbl.t(Protocol.uri, Lsp_types.code),
+      ~documents: Hashtbl.t(Protocol.uri, string),
+      params: RequestParams.t,
+    ) => {
+  switch (Hashtbl.find_opt(compiled_code, params.text_document.uri)) {
+  | None => send_no_result(~id)
+  | Some({program, sourcetree}) =>
+    let result = find_definition(sourcetree, params.position);
+    switch (result) {
+    | None => send_no_result(~id)
+    | Some((loc, uri)) =>
+      send_definition(
+        ~id,
+        ~range=Utils.loc_to_range(loc),
+        ~target_uri=uri,
+        Utils.loc_to_range(loc),
+      )
     };
   };
 };