--
-- a suitable instance would be
--
--- > instance Traversable Tree
+-- > instance Traversable Tree where
-- > traverse f Empty = pure Empty
-- > traverse f (Leaf x) = Leaf <$> f x
-- > traverse f (Node l k r) = Node <$> traverse f l <*> f k <*> traverse f r
-- instances for Prelude types
instance Traversable Maybe where
- traverse f Nothing = pure Nothing
+ traverse _ Nothing = pure Nothing
traverse f (Just x) = Just <$> f x
instance Traversable [] where
+ {-# INLINE traverse #-} -- so that traverse can fuse
traverse f = Prelude.foldr cons_f (pure [])
where cons_f x ys = (:) <$> f x <*> ys
-- | This function may be used as a value for `fmap` in a `Functor` instance.
fmapDefault :: Traversable t => (a -> b) -> t a -> t b
+{-# INLINE fmapDefault #-}
fmapDefault f = getId . traverse (Id . f)
-- | This function may be used as a value for `Data.Foldable.foldMap`