Project Euler problem 37 in Haskell

pe_37.hs

-- Project Euler Problem 37
-- The number 3797 has an interesting property. Being prime itself, it is
-- possible to continuously remove digits from left to right, and remain prime
-- at each stage: 3797, 797, 97, and 7. Similarly we can work from right to
-- left: 3797, 379, 37, and 3.
--
-- Find the sum of the only eleven primes that are both truncatable from left
-- to right and right to left.
--
-- NOTE: 2, 3, 5, and 7 are not considered to be truncatable primes.

import Math.NumberTheory.Primes
import Data.List

truncations :: Integer -> [Integer]
truncations = map read . truncations_
  where
    truncations_ x = filter (\s -> (not . null $ s) && (show x /= s))
        $ union (inits . show $ x) (tails . show $ x)

isTruncatablePrime :: Integer -> Bool
isTruncatablePrime x = case truncations x of
    [] -> False
    xs -> all isPrime xs

truncatablePrimes :: [Integer]
truncatablePrimes = take 11 . filter isTruncatablePrime $ primes

main = print . sum $ truncatablePrimes