Project Euler Problem 071

import Data.List
import Data.Ratio
-- a/b c/dの隣接項は(a+c)/(b+d)
anr :: (Num t, Num t1) => (t, t1) -> (t, t1) -> (t, t1)
anr (a,b) (c,d) = (a+c,b+d)
an :: (Integer,Integer) -> (Integer,Integer) -> Ratio Integer -> Integer -> (Integer,Integer)
an ll rr dest limit= imp ll rr ll where
  imp l r ans =
    if y>limit then ans
    else if (x%y)< dest then
           imp n r n
         else
           imp l n l
    where
      n@(x,y) = anr l r

ans1 :: Integer
ans1 = fst $ an (0,1) (1,1) (3%7) (10^(6::Integer))
-- 428570
main :: IO ()
main = print ans1