[Haskell-beginners] Re: deleteM :: (Ord a) => a -> Maybe (Set a) -> Maybe (Set a)

[Heinrich Apfelmus]

Martin Hofmann wrote:
> I often come across the problem to insert into a collection which might
> not exist yet, or delete from it and want the collection to be deleted
> if it is empty afterwards.
> 
> I always end up with these two functions:
> 
> 
> deleteM :: (Ord a) => a -> Maybe (Set a) -> Maybe (Set a)
> deleteM e s = 
>     liftM (S.delete e) s >>=  \s' -> 
>         if S.null s' then return s' else Nothing
> 
> insertM :: (Ord a) => a -> Maybe (Set a) -> Maybe (Set a)
> insertM  e s   = 
>     case s of
>         (Just s') -> return $ S.insert e s'
>         Nothing   -> return $ S.insert S.empty
> 
> Is there a way to express each in a (polymorphic) point-free one-liner?
> If not, why isn't there such a function for the standard collection,
> because IMHO this is what you need when using 'alter'.

Yes, there is a way. :)

  deleteM e = (\s -> if S.null s then Just s else Nothing) . S.delete e
  insertM e = Just . S.insert e . maybe S.empty id

The  maybe  function is from  Data.Maybe .

I did wonder why you want to delete the empty set, but now I see that
you need it for  alter  .


Regards,
apfelmus

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