{-# LANGUAGE MagicHash #-}
{-# LANGUAGE UnboxedTuples #-}
{-# LANGUAGE NoImplicitPrelude #-}
module Main (main) where
 
import Prelude hiding (length)
import GHC.Exts
 
length :: [a] -> Int
length xs = case newMutVar# 0 realWorld# of
  (# s, var #) -> worker# var s xs
  where
    worker# v s xs = case readMutVar# v s of
      (# s', l@(I# l#) #) -> case xs of
        []     -> l
        (_:xs) -> worker# v (writeMutVar# v (I# (l# +# 1#)) s') xs
 
main :: IO ()
main = print (length [1, 2, 3, 4, 5, 6])

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