import Data.Foldable (Foldable())
import Data.Monoid (Monoid)
+#if defined(__GLASGOW_HASKELL__)
+import GHC.Arr
+#elif defined(__HUGS__)
+import Hugs.Array
+#elif defined(__NHC__)
+import Array
+#endif
+
-- | Functors representing data structures that can be traversed from
-- left to right.
--
--
-- 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
mapM = Prelude.mapM
+instance Ix i => Traversable (Array i) where
+ traverse f arr = listArray (bounds arr) `fmap` traverse f (elems arr)
+
-- general functions
-- | 'for' is 'traverse' with its arguments flipped.