Improve usability on 'ghcup config add-release-channel'

Fixes #751 (or so I hope).

app/ghcup/GHCup/OptParse/Config.hs

@@ -51,7 +51,7 @@ data ConfigCommand
   = ShowConfig
   | SetConfig String (Maybe String)
   | InitConfig
-  | AddReleaseChannel URI
+  | AddReleaseChannel Bool URI
 
 
 
@@ -74,7 +74,7 @@ configP = subparser
   showP = info (pure ShowConfig) (progDesc "Show current config (default)")
   setP  = info argsP (progDesc "Set config KEY to VALUE (or specify as single json value)" <> footerDoc (Just $ text configSetFooter))
   argsP = SetConfig <$> argument str (metavar "<JSON_VALUE | YAML_KEY>") <*> optional (argument str (metavar "YAML_VALUE"))
-  addP  = info (AddReleaseChannel <$> argument (eitherReader uriParser) (metavar "URI" <> completer fileUri))
+  addP  = info (AddReleaseChannel <$> switch (long "force" <> help "Delete existing entry (if any) and append instead of failing") <*> argument (eitherReader uriParser) (metavar "URI" <> completer fileUri))
     (progDesc "Add a release channel from a URI")
 
 
@@ -203,22 +203,40 @@ config configCommand settings userConf keybindings runLogger = case configComman
         pure $ ExitFailure 65
       VLeft _ -> pure $ ExitFailure 65
 
-  AddReleaseChannel uri -> do
-    case urlSource settings of
-      AddSource xs -> do
-        doConfig (defaultUserSettings { uUrlSource = Just $ AddSource (xs <> [Right uri]) })
-        pure ExitSuccess
-      GHCupURL -> do
-        doConfig (defaultUserSettings { uUrlSource = Just $ AddSource [Right uri] })
-        pure ExitSuccess
-      OwnSource xs -> do
-        doConfig (defaultUserSettings { uUrlSource = Just $ OwnSource (xs <> [Right uri]) })
-        pure ExitSuccess
-      OwnSpec spec -> do
-        doConfig (defaultUserSettings { uUrlSource = Just $ OwnSource ([Left spec, Right uri]) })
+  AddReleaseChannel force uri -> do
+    r <- runE @'[DuplicateReleaseChannel] $ do
+      case urlSource settings of
+        AddSource xs -> do
+          when (not force && Right uri `elem` xs) $ throwE (DuplicateReleaseChannel uri)
+          lift $ doConfig (defaultUserSettings { uUrlSource = Just $ AddSource (appendUnique xs (Right uri)) })
+          pure ()
+        GHCupURL -> do
+          lift $ doConfig (defaultUserSettings { uUrlSource = Just $ AddSource [Right uri] })
+          pure ()
+        OwnSource xs -> do
+          when (not force && Right uri `elem` xs) $ throwE (DuplicateReleaseChannel uri)
+          lift $ doConfig (defaultUserSettings { uUrlSource = Just $ OwnSource (appendUnique xs (Right uri)) })
+          pure ()
+        OwnSpec spec -> do
+          lift $ doConfig (defaultUserSettings { uUrlSource = Just $ OwnSource [Left spec, Right uri] })
+          pure ()
+    case r of
+      VRight _ -> do
         pure ExitSuccess
+      VLeft e -> do
+        runLogger $ logError $ T.pack $ prettyHFError e
+        pure $ ExitFailure 15
 
  where
+  -- appends the element to the end of the list, but also removes it from the existing list
+  appendUnique :: Eq a => [a] -> a -> [a]
+  appendUnique xs' e = go xs'
+   where
+    go [] = [e]
+    go (x:xs)
+      | x == e    =     go xs -- skip
+      | otherwise = x : go xs
+
   doConfig :: MonadIO m => UserSettings -> m ()
   doConfig usersettings = do
     let settings' = updateSettings usersettings userConf

lib/GHCup/Errors.hs

@@ -35,6 +35,8 @@ import           URI.ByteString
 
 import qualified Data.Map.Strict               as M
 import qualified Data.Text                     as T
+import qualified Data.Text.Encoding            as E
+import qualified Data.Text.Encoding.Error      as E
 import           Data.Data (Proxy(..))
 
 
@@ -82,6 +84,7 @@ allHFError = unlines allErrors
     , let proxy = Proxy :: Proxy HadrianNotFound in format proxy
     , let proxy = Proxy :: Proxy ToolShadowed in format proxy
     , let proxy = Proxy :: Proxy ContentLengthError in format proxy
+    , let proxy = Proxy :: Proxy DuplicateReleaseChannel in format proxy
     , ""
     , "# high level errors (4000+)"
     , let proxy = Proxy :: Proxy DownloadFailed in format proxy
@@ -640,6 +643,19 @@ instance HFErrorProject ContentLengthError where
   eBase _ = 340
   eDesc _ = "File content length verification failed"
 
+data DuplicateReleaseChannel = DuplicateReleaseChannel URI
+  deriving Show
+
+instance HFErrorProject DuplicateReleaseChannel where
+  eBase _ = 350
+  eDesc _ = "Duplicate release channel detected when adding URI.\nGiving up. You can use '--force' to remove and append the duplicate URI (this may change order/semantics)."
+
+instance Pretty DuplicateReleaseChannel where
+  pPrint (DuplicateReleaseChannel uri) =
+    text $ "Duplicate release channel detected when adding: \n  "
+      <> (T.unpack . E.decodeUtf8With E.lenientDecode . serializeURIRef') uri
+      <> "\nGiving up. You can use '--force' to remove and append the duplicate URI (this may change order/semantics)."
+
     -------------------------
     --[ High-level errors ]--
     -------------------------

lib/GHCup/Types.hs

@@ -66,7 +66,7 @@ data GHCupInfo = GHCupInfo
   , _ghcupDownloads   :: GHCupDownloads
   , _globalTools      :: Map GlobalTool DownloadInfo
   }
-  deriving (Show, GHC.Generic)
+  deriving (Show, GHC.Generic, Eq)
 
 instance NFData GHCupInfo
 
@@ -87,7 +87,7 @@ data Requirements = Requirements
   { _distroPKGs :: [Text]
   , _notes      :: Text
   }
-  deriving (Show, GHC.Generic)
+  deriving (Show, GHC.Generic, Eq)
 
 instance NFData Requirements