err0r500 · September 5, 2018 12:51 · Sep 5, 2018 · Sep 5, 2018 · Sep 5, 2018
diff --git a/main.hs b/main.hs
@@ -5,31 +5,29 @@ module Main where
 
 import Control.Monad.Identity
 import Control.Monad.Trans.Either
+import Control.Monad.Trans.Except
 
 data User = User
   { name :: String
   , firstName :: String
   } deriving (Show)
 
-class Monad m => UserGetter m where
+class Monad m =>
+      UserGetter m
+  where
   getName :: Int -> m (Either String String)
   getFirstname :: Int -> m (Either String String)
 
-class Monad m => BusinessLogic m where
+class Monad m =>
+      BusinessLogic m
+  where
   getCompleteUser :: Int -> m (Either String User)
 
 instance (UserGetter m, Monad m) => BusinessLogic m where
-  getCompleteUser id =do
-          n <- getName id
-          case n of
-            Left err -> return (Left err)
-            Right name -> do
-              fn <- getFirstname id
-              case fn of
-                Left err -> return (Left err)
-                Right firstName -> return $ Right (User name firstName)
-
-
+  getCompleteUser id = runExceptT $ do
+      name <- ExceptT $ getName id
+      firstName <- ExceptT $ getFirstname id
+      return (User name firstName)
 
 instance UserGetter IO where
   getName _ = do

diff --git a/main.hs b/main.hs
@@ -27,4 +27,23 @@ instance (UserGetter m, Monad m) => BusinessLogic m where
               fn <- getFirstname id
               case fn of
                 Left err -> return (Left err)
-                Right firstName -> return $ Right (User name firstName)
+                Right firstName -> return $ Right (User name firstName)
+
+
+
+instance UserGetter IO where
+  getName _ = do
+    n <- getLine
+    return (Right n)
+  getFirstname _ = return (Right "ioUserFirstname")
+
+instance (Monad m) => UserGetter (IdentityT m) where
+  getName _ = return (Right "identityUserName")
+  getFirstname _ = return (Right "identityUserFirstname")
+
+main :: IO ()
+main = do
+  user1 <- runIdentityT (getCompleteUser 12)
+  user2 <- getCompleteUser 13
+  print user1
+  print user2
diff --git a/main.hs b/main.hs
@@ -0,0 +1,30 @@
+{-# LANGUAGE FlexibleInstances #-}
+{-# LANGUAGE UndecidableInstances #-}
+
+module Main where
+
+import Control.Monad.Identity
+import Control.Monad.Trans.Either
+
+data User = User
+  { name :: String
+  , firstName :: String
+  } deriving (Show)
+
+class Monad m => UserGetter m where
+  getName :: Int -> m (Either String String)
+  getFirstname :: Int -> m (Either String String)
+
+class Monad m => BusinessLogic m where
+  getCompleteUser :: Int -> m (Either String User)
+
+instance (UserGetter m, Monad m) => BusinessLogic m where
+  getCompleteUser id =do
+          n <- getName id
+          case n of
+            Left err -> return (Left err)
+            Right name -> do
+              fn <- getFirstname id
+              case fn of
+                Left err -> return (Left err)
+                Right firstName -> return $ Right (User name firstName)