Computes the product of the first n elements of the Wallis succession

wallis.hs

import Data.Ratio
import Text.Read
import System.Environment

wallis_succession :: [Rational]
wallis_succession = merge odds evens
  where
    odds = [n % (n - 1) | n <- [2, 4..]]
    evens = [n % (n + 1) | n <- [2, 4..]]
    merge (x:xs) (y:ys) = x : y : merge xs ys

wallisProduct :: Int -> Rational
wallisProduct n = product $ take n wallis_succession

main = do
    args <- getArgs
    let n = do
        (n:_) <- Just args
        readMaybe n
    case n of
        Nothing -> putStrLn "Invalid input"
        Just n -> do
            putStrLn $ "Wallis product for " ++ (show n) ++ " elements"
            putStrLn $ "Fraction: " ++ (show . wallisProduct $ n)
            putStrLn $ "Value: " ++ (show . fromRational . wallisProduct $ n)