[Haskell-beginners] Pattern match(es) are overlapped ... but I do not see that they do

[Nathan Hüsken]

Hey,

I have the following code (which uses wxHaskell):

import Graphics.UI.WX
import Graphics.UI.WX.Controls
import Graphics.UI.WXCore
import Graphics.UI.WXCore.Events

(...)

dealWithUnsavedChanges :: Var ProgramState -> TextCtrl a -> IO Bool
dealWithUnsavedChanges state tc = do
   ProgramState unsavedChanges _ <- varGet state
   if not unsavedChanges then return True else do
     res <- messageDialog tc "Unsaved changes ..." "You have unsaved 
changes, do you want to save them?" (wxYES_NO .+. wxCANCEL .+. 
wxICON_EXCLAMATION)
     case res of
       wxID_CANCEL -> return False
       wxID_NO     -> return True
       wxID_YES    -> do
         onSave state tc
         -- check if saving worked
         ProgramState newUnsavedChanges _ <- varGet state
         return (not newUnsavedChanges)

When I compile it, I get:

Main.hs:130:5: Warning:
     Pattern match(es) are overlapped
     In a case alternative:
         wxID_NO -> ...
         wxID_YES -> ...

which is strange. I checked, wxID_NO is defined as 5104 and wxID_YES as 
5103. So they are not overlapping. On the other hand, when I replace the 
wxID_NO/YES/CANCEL with the actual values, the warning vanishes.

Any Idea what this could be?

Thanks!
Nathan