PROPOSAL: Add Data.Set.mapMaybe

[Don Stewart]

nominolo:
> 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))
> 

Looks good. Did you open up a libraries ticket?

-- Don