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