PROPOSAL: Add Data.Set.mapMaybe

[Thomas Schilling]

On Wed, 2007-10-31 at 14:38 +0100, Thomas Schilling wrote:
> On Wed, 2007-10-31 at 14:29 +0100, Thomas Schilling wrote:
> > Hi there.
> > 
> > I recently find myself missing Data.Set.mapMaybe.  This function is
> > implemented for Data.Map.Map, but is strangely missing from
> > Data.Set.mapMaybe.  I believe it could be implemented using
> > Data.Foldable, but not as efficiently.  The obvious implementation would
> > be:
> > 
> > mapMaybe :: (Ord a, Ord b) => (a -> Maybe b) -> Set a -> Set b
> > mapMaybe f Tip = Tip
> > mapMaybe f (Bin _ x l r) = case f x of
> >   Nothing -> merge (mapMaybe f l) (mapMaybe f r)
> >   Just y  -> union y (mapMaybe f l) (mapMaybe f r)
> > 
> > Any comments?
> 
> D'oh!  This is wrong.  Sorry, for the noise.

Should be: 

mapMaybe :: (Ord a, Ord b) => (a -> Maybe b) -> Set a -> Set b
mapMaybe f = foldl' g Set.empty . Set.toList
  where 
    g s a = case f a of
              Nothing -> s
              Just b -> Set.insert b s

testMapMaybe = mapMaybe f (Set.fromList [(-4),(-3)..5])
  where f x | odd x = Nothing
            | otherwise = Just (-(x * x))