add MyRW monad (#27)

Author: d1skort

MonadsAndEffects/3.3/myrw.hs

@@ -0,0 +1,29 @@
+module MyRw where
+
+import Control.Monad.Trans.Reader
+import Control.Monad.Trans.Writer
+import Control.Monad.Trans (lift)
+import Data.Char (toUpper)
+
+
+type MyRW = ReaderT [String] (Writer String)
+
+
+myAsks :: ([String] -> a) -> MyRW a
+myAsks = asks
+
+
+myTell :: String -> MyRW ()
+myTell = lift . tell
+
+
+logFirstAndRetSecond :: MyRW String
+logFirstAndRetSecond = do
+  el1 <- myAsks head
+  el2 <- myAsks (map toUpper . head . tail)
+  myTell el1
+  return el2
+
+
+runMyRW :: MyRW a -> [String] -> (a, String)
+runMyRW rw e = runWriter (runReaderT rw e)