2 files changed, 32 insertions, 1 deletions
diff --git a/src/Plugin/Factorial.hs b/src/Plugin/Factorial.hs
new file mode 100644
index 0000000..f8cfee2
--- /dev/null
+++ b/src/Plugin/Factorial.hs
@@ -0,0 +1,29 @@
+{-# LANGUAGE OverloadedStrings #-}
+
+module Plugin.Factorial
+    ( factorial
+    ) where
+
+import Text.Regex.TDFA ((=~))
+import TextShow (showt)
+
+import qualified Message as M
+import Plugin.Base
+
+factorial = Plugin
+    { matchRegex  = "^([0-9]+)!$"
+    , perform     = factorialAction
+    , command     = "<integer>!"
+    , description = "Calculate the factorial of <integer>"
+    }
+
+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 $ showt $ calculate $ (read number :: Int)
+
+calculate :: (Enum a, Num a) => a -> a
+calculate n = product [1..n]
diff --git a/src/PluginList.hs b/src/PluginList.hs
index bfcae7e..7c950c0 100644
--- a/src/PluginList.hs
+++ b/src/PluginList.hs
@@ -3,6 +3,7 @@ module PluginList
     ) where
 
 import Plugin.Base (Plugin)
+import Plugin.Factorial
 import Plugin.GitHubCommit
 import Plugin.GitRemoteSetOrigin
 
@@ -10,6 +11,7 @@ import Plugin.GitRemoteSetOrigin
 -- cause a circular import.
 plugins :: [Plugin]
 plugins =
-    [ gitHubCommit
+    [ factorial
+    , gitHubCommit
     , gitRemoteSetOrigin
     ]