README.md

$ ghc -O2 a.hs
$ time ./a
./a  1.39s user 1.70s system 69% cpu 4.479 total
$ ghc -O2 b.hs
$ time ./b
./b  1.52s user 1.73s system 73% cpu 4.448 total

a.hs

{-# LANGUAGE NoImplicitPrelude  #-}
{-# LANGUAGE ExplicitForAll     #-}

import Prelude         hiding (length)
import Control.Monad

incr :: Int -> Int
{-# INLINE incr #-}
incr = (+1)

lengthCPS :: forall a. (Int -> Int) -> [a] -> Int
lengthCPS cont []       = cont 0
lengthCPS cont (_:xs)   = lengthCPS (cont . incr) xs

length :: forall a. [a] -> Int
length = lengthCPS id

main :: IO ()
main = do
  forM_ [0..1000000] $ const $ do
    print $ length [0..10000]

b.hs

{-# LANGUAGE NoImplicitPrelude  #-}
{-# LANGUAGE ExplicitForAll     #-}

import Prelude         hiding (length)
import Control.Monad

length  :: forall a. [a] -> Int
length []       = 0
length (_:xs)   = 1 + length xs

main :: IO ()
main = do
  forM_ [0..1000000] $ const $ do
    print $ length [0..10000]