PROPOSAL: Add Data.Set.mapMaybe
Don Stewart
dons at galois.com
Wed Oct 31 14:01:18 EDT 2007
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
More information about the Libraries
mailing list