Factorial in Haskell

Welcome to the Factorial in Haskell page! Here, you'll find the source code for this program as well as a description of how the program works.

Current Solution

module Main where

import System.Environment
import Text.Read

factorial :: Integer -> Integer
factorial 0 = 1
factorial n = n * factorial (n - 1)

headMaybe :: [a] -> Maybe a
headMaybe [] = Nothing
headMaybe (x:xs) = Just x

main :: IO ()
main = do
  args <- getArgs
  let n = headMaybe args
  case n >>= readMaybe of
    Nothing -> putStrLn "Usage: please input a non-negative integer"
    Just n
      | n < 0       -> putStrLn "Usage: please input a non-negative integer"
      | n <= 250000 -> putStrLn $ take 10 $ show $ factorial n
      | otherwise   -> putStrLn $ "!" ++ (show n) ++ " is out of the reasonable bounds for calculation"

Factorial in Haskell was written by:

• Parker Johansen

If you see anything you'd like to change or update, please consider contributing.

How to Implement the Solution

No 'How to Implement the Solution' section available. Please consider contributing.

How to Run the Solution

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