stack IO with WriterT

src/Command.hs

@@ -2,31 +2,32 @@ module Command where
 import           Control.Monad.Writer
 
 data Light = Light {
-      turnOn  :: String
-    , turnOff :: String     
+      turnOn  :: IO String
+    , turnOff :: IO String     
 }
 
 simpleLamp = Light { 
-      turnOn  = "The Light is on"
-    , turnOff = "The Light is off"
+      turnOn  = putStrLn "The Light is on"  >> return "on"
+    , turnOff = putStrLn "The Light is off" >> return "off"
 }
 
-flipUpCommand :: Light -> String
+flipUpCommand :: Light -> IO String
 flipUpCommand = turnOn
 
-flipDownCommand :: Light -> String
+flipDownCommand :: Light -> IO String
 flipDownCommand = turnOff
 
-storeAndExecute :: String -> Writer[String] ()
+storeAndExecute :: IO String -> WriterT[String] IO ()
 storeAndExecute command = do
-    let logEntry = command
+    logEntry <- liftIO command
     tell [logEntry]
-  
+ 
+
 commandDemo :: IO ()
 commandDemo = do
     let lamp = simpleLamp
-    print $ execWriter $ 
+    result <- execWriterT $ 
         storeAndExecute (flipUpCommand lamp)   >>
         storeAndExecute (flipDownCommand lamp) >>
         storeAndExecute (flipUpCommand lamp)
-    
+    putStrLn $ "switch history: " ++ show result