Reversible numbers (Euler, 145)

-- ### Project Euler, problem 145 ### --

euler145 :: Int -> Int
euler145 = length . filter (and . map odd . digits . f) . filter (not . leading0) . list 1
  where
    rev :: Int -> Int
    rev = read . reverse . show

    digits :: Int -> [Int]
    digits = map (read . return) . show  -- in this case, return v = [v] because the Monad is actually a List
                                         -- "return" = "(:[])"

    list a b = [a..b]
    f x = x + rev x
    leading0 = (== 0) . (`mod` 10)
    str ch = [ch]

main :: IO()
main = do
  solveit 1000    -- control
  solveit (10^9)  -- problem
  where
    solveit = putStrLn . show . euler145