Config: make allowCustomUnicodeWithTables more usable in Semigroup instance

src/Graphics/Vty.hs

@@ -114,7 +114,7 @@ mkVty :: Config -> IO Vty
 mkVty appConfig = do
     config <- (<> appConfig) <$> userConfig
 
-    when (allowCustomUnicodeWithTables config) $
+    when (allowCustomUnicodeWithTables config /= Just False) $
         installWidthTable config
 
     input  <- inputForConfig config

src/Graphics/Vty/Config.hs

@@ -149,8 +149,10 @@ data Config =
            -- The default is the value of the TERM environment variable.
            , termWidthMaps :: [(String, FilePath)]
            -- ^ Terminal width map files.
-           , allowCustomUnicodeWithTables :: Bool
+           , allowCustomUnicodeWithTables :: Maybe Bool
            -- ^ Whether to permit custom Unicode width table loading.
+           -- 'Just False' indicates that table loading should not be
+           -- performed. Other values permit table loading.
            }
            deriving (Show, Eq)
 
@@ -171,7 +173,7 @@ instance Semigroup Config where
                , termName = termName c1 <|> termName c0
                , termWidthMaps = termWidthMaps c1 <|> termWidthMaps c0
                , allowCustomUnicodeWithTables =
-                   allowCustomUnicodeWithTables c0 || allowCustomUnicodeWithTables c1
+                   allowCustomUnicodeWithTables c1 <|> allowCustomUnicodeWithTables c0
                }
 
 instance Monoid Config where
@@ -186,7 +188,7 @@ instance Monoid Config where
                , outputFd = Nothing
                , termName = Nothing
                , termWidthMaps = []
-               , allowCustomUnicodeWithTables = True
+               , allowCustomUnicodeWithTables = Nothing
                }
 #if !(MIN_VERSION_base(4,11,0))
     mappend = (<>)