Project Euler Problem 017

-- 英単語を記述していくのもよいが数値のまま計算するのが楽
dig :: Num a => Int -> a
dig i = [3,3,5,4,4,3,5,5,4] !! (i-1)   -- 1-9
teen :: Num a => Int -> a
teen i = [6,6,8,8,7,7,9,8,8] !! (i-11)  -- 11-19
tenth :: Num a => Int -> a
tenth i = [3,6,6,5,5,5,7,6,6] !! (i-1) -- 10,20,..,90
hundred :: Integer
hundred = 7
thousand :: Integer
thousand = 11 --onethousand
len :: Int -> Integer
len 0 = 0
len n | n<10 = dig n
len n | n==10 = tenth 1
len n | n<20 = teen n
len n | n<100 = tenth(n`quot`10) + len (n`rem`10)
len n | n==1000 = thousand
len n = len (n`quot`100) + hundred + r where
        left = len (n`rem`100)
        r = if left/=0 then 3+left else 0 -- andをつけるかつけないか
ans1 :: Integer
ans1 = sum [len n|n<-[1..1000]]
--21124
main :: IO ()
main = print ans1