That's great (really, thank you for such a fun example of Arrow programming), but isn't the (*) on line two of mapPair supposed to be a "point"? How would you make a point-less version of mapPair that actually had the type signature (a->a->a)->[a]->[a]*? (For that matter, /would/ you?) Devin * Grr, you're right. Were it not for that odd requirement, the type could be loosened to (a->a->b)->[a]->[b]. Maybe mapPairs should take a monadic (that is, one-arg) function to handle the dangling oddies. Dan Weston wrote:
import Control.Arrow((&&&),(>>>)) import Data.Maybe(catMaybes,maybeToList)
mapPair = (id &&& tail >>> -- offset list by one uncurry (zipWith (*)) >>> -- multiply adjacent alternate >>> -- mark even elements catMaybes) -- delete even elements
&&& -- Tuple this up with...
(alternate >>> -- keep odd indices (Nothing:) >>> -- make sure there is a last last >>> -- get last maybeToList) -- keep if it had odd index
>>> -- and then...
uncurry (++) -- append pair of lists
where alternate = zipWith ($) (cycle [Just,const Nothing]) -- Mark even-indexed elements for deletion -- cycle goes on forever, but zipWith stops at -- the end of the shorter list, so no worries.