unsplit is an arrow that takes a pair of values and combines them to return
a single value:
unsplit :: (Arrow a) => (b -> c -> d) -> a (b, c) d unsplit = arr . uncurry -- arr (\op (x,y) -> x `op` y)
(***) combines two arrows into a new arrow by running the two arrows on a
pair of values (one arrow on the first item of the pair and one arrow on the
second item of the pair).
f *** g = first f >>> second g