411 REFERENCE · Wikibooks
Haskell/Solutions/Understanding arrows
Reference material by Wikibooks contributors. This retains its source identity and is not 411 news reporting.
Pocket guide to Arrow
1.
-- Available from Data.Tuple
swap :: (a, b) -> (b, a)
swap (x, y) = (y, x)
second :: Arrow y => y a b -> y (c, a) (c, b)
second f = arr swap >>> first f >>> arr swap
(***) :: Arrow y => y a c -> y b d -> y (a, b) (c, d)
f *** g = first f >>> second g
dup :: a -> (a, a)
dup x = (x, x)
(&&&) :: Arrow y => y a b -> y a c -> y a (b, c)
f &&& g = arr dup >>> f *** g
2.
swapTags :: Either a b -> Either b a
swapTags e = case e of
Left x -> Right x
Right x -> Left x
right :: y a b -> y (Either c a) (Either c b)
right f = arr swapTags >>> left f >>> arr swapTags
3.
liftY2 :: Arrow y => (a -> b -> c) -> y r a -> y r b -> y r c
liftY2 f g h = g &&& h >>> arr (uncurry f)