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

[Nathan Hüsken]

Hey,

Thanks for the reply. Ah, yes of course. I see the problem.

How exactly would you do the encoding into an ADT?
What I also could do is used nested ifs ...

Is there no easy, compact way to switch over a set of ints in haskell, 
that are defined in variables?

Best Regards,
Nathan

On 09/01/2013 01:12 AM, Edward Z. Yang wrote:
> 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
>>