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

[Edward Z. Yang]

Hello Nathan,

The problem is that because these "numbers" are actually
variable names, Haskell is not pattern matching against the
number; it is actually binding the result type to the variable.
You might as well have written:

>      case res of
>        x -> return False
>        x -> return True
>        x -> do

which of course is overlapping.

There are a few ways to rewrite your program, but my recommendation
would be to define an abstract data type which encodes the three
choices, and pattern match against that.

Edward

Excerpts from Nathan Hüsken's message of Sat Aug 31 15:46:01 -0700 2013:
> 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
>