-- Some examples of using anonymous functions, currying, and composition
-- by Scot Drysdale on 10/11/07

-- flip      :: (a -> b -> c) -> b -> a -> c
-- flip f x y = f y x

--from Standard Prelude:

myreverse = foldl (flip (:)) []

pairwiseAverageLists = zipWith (\ x y -> (x + y)/2)

notSame :: (Eq a) => [(a,a)] -> [(a,a)]
notSame = filter (\(x,y) -> x /= y)

reversePairs = map (\(x,y) -> (y,x))

-- Functional composition + currying is powerful

-- Version we wrote for SA 1
distance :: [Double] -> [Double] -> Double
distance p1 p2 = sqrt (foldl (+) 0.0 (map (^2) (zipWith (-) p1 p2)))

-- Uses functional composition, except for the last.  Can't use it because
-- zipWith is 2-parameter function.
distanceC :: [Double] -> [Double] -> Double
distanceC p1 p2 = sqrt . foldl (+) 0.0 . map (^2) $ zipWith (-) p1 p2


-- Uses functional application operator $ to apply each function 
-- to what follows
distanceA :: [Double] -> [Double] -> Double
distanceA p1 p2 = sqrt $ foldl (+) 0.0 $ map (^2) $ zipWith (-) p1 p2