Fusion.hs

{-# LANGUAGE NumDecimals, BlockArguments #-}

import System.Environment
import Data.List

f 0 = (length .) . filter
f 1 = count
f 2 = count' 
f 3 = count''

a = length $ filter odd [1..1e7]
b = count'' odd [1..1e7]

count p [] = 0
count p (x:xs) = (if p x then 1 else 0) + count p xs

count' p = foldr (\x acc -> if p x then acc + 1 else acc) 0

count'' p = foldl' (\acc x -> if p x then acc + 1 else acc) 0


main = do
  [i] <- map read <$> getArgs
  print . ($[1..1e8]) . ($odd) . f $ i

{-

$ ghc Fusion.hs -O2

(length .) . filter
$ time ./Fusion 0
50000000

real    0m3.052s
user    0m3.013s
sys     0m0.010s

count
$ time ./Fusion 1
50000000

real    0m3.888s
user    0m3.658s
sys     0m0.202s

count'
$ time ./Fusion 2
50000000

real    0m3.906s
user    0m3.654s
sys     0m0.222s


count''
$  time ./Fusion 3
50000000

real    0m3.059s
user    0m3.024s
sys     0m0.011s

-}