Make the code compatible with base==4.9

pokeNeige.cabal

@@ -39,7 +39,7 @@ executable pokeNeige
                      , Pokemon.Type
                      , Tool.Array
   -- other-extensions:
-  build-depends:       base >=4.11 && <4.12
+  build-depends:       base >=4.9 && <4.12
                      , aeson
                      , bytestring
                      , containers

src/Automaton.hs

@@ -29,7 +29,7 @@ edges Empty Client.NewGame = return $ Game {
 edges Empty (Client.Resume {Client.game}) = return game
 
 edges game message = do
-  let errorMessage = "inacceptable message " <> encode message <> " in this context"
+  let errorMessage = "inacceptable message " `mappend` encode message `mappend` " in this context"
   send (Server.Error $ unpack errorMessage)
   log errorMessage
   return game