Factorial: Use `Integer` instead of `Int` for bigger numbers

The `Integer` type has arbitrary precision. This is why the function was
working fine in GCHi but overflowing when I was testing the bot. Use
`Integer` to get the real values we want.

https://stackoverflow.com/questions/36355757/large-numbers-become-negative-in-haskell

Also add a little extra error handling to deal with unexpected numbers.
Now that I think about it, the regex parser isn't going to handle
negative numbers, so we're good on that front. Set an upper bound so we
don't end up spending a lot of time trying to calculate numbers and
potentially segfault if huge numbers are given.

1 files changed, 10 insertions, 3 deletions

diff --git a/src/Plugin/Factorial.hs b/src/Plugin/Factorial.hs
index 692d20f..97730de 100644
--- a/src/Plugin/Factorial.hs
+++ b/src/Plugin/Factorial.hs
@@ -23,7 +23,14 @@ factorialAction message = do
         []    -> return $ Left "I didn't understand"
         (m:_) -> do
             let number = last m
-            return $ Right $ showt $ calculate $ (read number :: Int)
+            return $ Right $ result (read number :: Integer)
+  where
+    result n = case calculate n of
+        Nothing -> "Input is too large."
+        Just n  -> showt n
 
-calculate :: (Integral a) => a -> a
-calculate n = product [1..n]
+calculate :: Integer -> Maybe Integer
+calculate n
+    | n <= 0      = Just 0
+    | n <= 35000  = Just $ product [1..n]
+    | otherwise   = Nothing