allow customization of backend port

backend/app/Main.hs

@@ -1,3 +1,6 @@
+{-# LANGUAGE NamedFieldPuns #-}
+{-# LANGUAGE RecordWildCards #-}
+{-# LANGUAGE ApplicativeDo #-}
 module Main where
 
 import AutoTypes qualified as U
@@ -39,6 +42,8 @@ args :: A.Parser Args
 args = Args <$> cmd'
 
 data Cmd = Serve
+  { serverPort :: Int
+  }
 
 cmd' :: A.Parser Cmd
 cmd' =
@@ -48,7 +53,9 @@ cmd' =
     ]
 
 serveCmd :: A.Parser Cmd
-serveCmd = pure Serve
+serveCmd = do
+  serverPort <- A.option A.auto (A.metavar "PORT" <> A.showDefault <> A.value 8081 <> A.long "port" <> A.short 'p' <> A.help "The server port")
+  pure Serve {..}
 
 data Repo = Repo
   { commits :: [Commit]
@@ -195,8 +202,8 @@ main = do
   repoT <- newEmptyTMVarIO
   _ <- forkIO do watch repoT root ref
   A.execParser (A.info (args <**> A.helper) A.idm) >>= \case
-    Args {cmd = Serve} -> do
-      W.runEnv 8081 $ \req respond -> do
+    Args {cmd = Serve {serverPort}} -> do
+      W.runEnv serverPort $ \req respond -> do
         case P.parseOnly R.parser (W.rawPathInfo req) of
           Right (R.SchemaJson path) -> do
             repo <- atomically (readTMVar repoT)