During a small project I'm trying to develop a small application. It becomes quite often that I need a function mapapp:
mapapp _ [] ap = ap mapapp f (a:as) ap = f a : map f as ap
Of course,
(map f list) ++ append would do the same as
mapapp f list append
but with less efficiency. Or am I wrong?
I timed each of the following five operations with ...
ghc -O2 --make MapApp.hs time ./MapApp
... and they produced no statistically significant differences. Each ran for about 3.8 seconds. Perhaps you can try it to convince yourself? Sean -- module Main where mapapp0 :: (a -> b) -> [b] -> [a] -> [b] mapapp0 f tail xs = map f xs ++ tail mapapp1 :: (a -> b) -> [b] -> [a] -> [b] mapapp1 _ tail [] = tail mapapp1 f tail (a:as) = f a : mapapp1 f tail as mapapp2 :: (a -> b) -> [b] -> [a] -> [b] mapapp2 f tail = go where go [] = tail go (x:xs) = f x : go xs mapapp3 :: (a -> b) -> [b] -> [a] -> [b] mapapp3 f tail = foldr ((:) . f) tail main = do writeFile "/dev/null" $ show $ [1 .. 10001000] -- writeFile "/dev/null" $ show $ mapapp0 (+3) [1 .. 10000000] [1 .. 1000] -- writeFile "/dev/null" $ show $ mapapp1 (+3) [1 .. 10000000] [1 .. 1000] -- writeFile "/dev/null" $ show $ mapapp2 (+3) [1 .. 10000000] [1 .. 1000] -- writeFile "/dev/null" $ show $ mapapp3 (+3) [1 .. 10000000] [1 .. 1000] return ()