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|
{-# LANGUAGE OverloadedStrings #-}
module Plugin.Factorial
( factorial
) where
import Text.Regex.TDFA ((=~))
import TextShow (showt)
import qualified Message as M
import Plugin.Base
factorial = defaultPlugin
{ matchRegex = "^([0-9]+)!$"
, perform = factorialAction
, command = "<integer>!"
, description = "Calculate the factorial of <integer> for whole numbers \
\up to 35000."
}
factorialAction :: PluginAction
factorialAction message = do
case M.textStr message =~ matchRegex factorial :: [[String]] of
[] -> return $ Left "I didn't understand"
(m:_) -> do
let number = last m
return $ Right $ result (read number :: Integer)
where
result n = case calculate n of
Nothing -> "Input is too large."
Just n -> showt n
calculate :: Integer -> Maybe Integer
calculate n
| n <= 0 = Just 0
| n <= 35000 = Just $ product [1..n]
| otherwise = Nothing
|
