RosettaCodeData/Task/Modular-exponentiation/Haskell/modular-exponentiation.hs

powm :: Integer -> Integer -> Integer -> Integer -> Integer
powm b 0 m r = r
powm b e m r
  | e `mod` 2 == 1 = powm (b * b `mod` m) (e `div` 2) m (r * b `mod` m)
powm b e m r = powm (b * b `mod` m) (e `div` 2) m r

main :: IO ()
main =
  print $
  powm
    2988348162058574136915891421498819466320163312926952423791023078876139
    2351399303373464486466122544523690094744975233415544072992656881240319
    (10 ^ 40)
    1