fast.hs

import Criterion.Main
import Data.List as L (inits, tails)
import qualified Data.Vector as V
import qualified Data.Vector.Generic as VG
import qualified Data.Vector.Unboxed as VU

splits1, splits2, splits3 :: [a] -> [([a],[a])]
splits1 xs = zip (L.inits xs) (L.tails xs)

splits2 [] = [([],[])]
splits2 xs@(x:xs') = ([],xs) : [(x:as,bs) | (as,bs) <- splits2 xs']

splits3 = go id where
  go prefix xs = (prefix [],xs) : case xs of
    x:xs -> go (prefix . (x :)) xs
    [] -> []

splits4 :: VG.Vector v a => v a -> [(v a, v a)]
splits4 v = [VG.splitAt i v | i <- [0..VG.length v]]

main = defaultMain
  [ bench "splits1" $ nf splits1 input
  , bench "splits2" $ nf splits2 input
  , bench "splits3" $ nf splits3 input
  , bench "splits4 (boxed)" $ nf splits4 input2 
  , bench "splits4 (unboxed)" $ nf splits4 input3 
  ]
  where
    input :: [Int]
    input = [1..1000]
    input2 = VG.fromList input :: V.Vector Int
    input3 = VG.fromList input :: VU.Vector Int

Use

seqList (x:xs) = x `seq` xs
seqList [] = ()

main = defaultMain
  [ bench "splits1" $ whnf (seqList . splits1) input
  , bench "splits2" $ whnf (seqList . splits2) input
  , bench "splits3" $ whnf (seqList . splits3) input
  ...