Birthday paradox calculator

Birthday.hs

-- | Birthday paradox calculator

import Control.Monad
import Data.Ratio
import System.Environment
import Text.Printf

birthday :: Fractional a => Integer -> Integer -> a
birthday days n = realToFrac $ 1 - inverse
  where
    inverse = product $ zipWith (%) [days, days-1 .. days-n+1] (repeat days)

birthday' :: Fractional a => Integer -> a
birthday' = birthday 365

main :: IO ()
main = do
    args <- getArgs
    case args of
        [n'] -> printf "%.3f\n" (birthday' n :: Double)
          where n = read n'

        _ -> epicDisplay

epicDisplay :: IO ()
epicDisplay = forM_ [1 .. 50] $ \n ->
    printf "%d: %.3f\n" n (birthday' n :: Double)