[Haskell-cafe] Library function for map+append

[Sean Leather]

> >>>  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 ()
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