length           :: [a] -> Int
length []        =  0
length (_:l)     =  1 + length l

-- | 'length' returns the length of a finite list as an 'Int'.
-- It is an instance of the more general 'Data.List.genericLength',
-- the result type of which may be any kind of number.
length                  :: [a] -> Int
length l                =  len l 0#
  where
    len :: [a] -> Int# -> Int
    len []     a# = I# a#
    len (_:xs) a# = len xs (a# +# 1#)

xs = [1..500000]
ys = [500001..1000000]
zs = xs ++ ys

main = do
    print $ length zs

xs = [1..500000]
ys = [500001..1000000]
zs = xs ++ ys

main = do
    print $ length xs
    print $ length ys
    print $ length zs
    print $ last   xs
    print $ last   ys
    print $ last   zs

(++) :: [a] -> [a] -> [a]
[]     ++ ys = ys
(x:xs) ++ ys = x : (xs ++ ys)

xs = [1..]
ys = [500001..1000000]
zs = xs ++ ys

main = do
    print $ head $ drop 1000000 zs

xs = [1..]
ys = [500001..1000000]
zs = xs ++ ys

main = do
    print $ head $ drop 1000000 zs
    print $ head $ tail $ drop 1000000 zs

xs = [1..10000000] :: [Int]

main = do
    print $ length xs
    print $ last   xs
    c <- getChar
    print c

$ ghc --version
The Glorious Glasgow Haskell Compilation System, version 6.8.2