server: more specific types for decorators

Use a TypeVar so that type checkers understand that the signature is
same as the decorated functions, so that these functions can be
typechecked.

fixes #88

pygls/server.py

@@ -21,7 +21,7 @@
 from concurrent.futures import Future, ThreadPoolExecutor
 from multiprocessing.pool import ThreadPool
 from threading import Event
-from typing import Callable, Dict, List
+from typing import Callable, Dict, List, TypeVar
 
 from pygls.types import (ApplyWorkspaceEditResponse, ConfigCallbackType, Diagnostic, MessageType,
                          RegistrationParams, TextDocumentSyncKind, UnregistrationParams,
@@ -34,6 +34,7 @@
 
 logger = logging.getLogger(__name__)
 
+F = TypeVar('F', bound=Callable)
 
 async def aio_readline(loop, executor, stop_event, rfile, proxy):
     """Reads data from stdin in separate thread (asynchronously)."""
@@ -234,7 +235,7 @@ def apply_edit(self, edit: WorkspaceEdit, label: str = None) -> ApplyWorkspaceEd
         """Sends apply edit request to the client."""
         return self.lsp.apply_edit(edit, label)
 
-    def command(self, command_name: str) -> Callable:
+    def command(self, command_name: str) -> Callable[[F], F]:
         """Decorator used to register custom commands.
 
         Example:
@@ -244,7 +245,7 @@ def my_cmd(ls, a, b, c):
         """
         return self.lsp.fm.command(command_name)
 
-    def feature(self, feature_name: str, **options: Dict) -> Callable:
+    def feature(self, feature_name: str, **options: Dict) -> Callable[[F], F]:
         """Decorator used to register LSP features.
 
         Example: