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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