add MonadTrans overview and MaybeT example

Author: levinotik

content/monad-transformers.md

@@ -15,21 +15,65 @@ The following is a list of some basic transformers:
 
 ### MaybeT
 
-A `Maybe a` wrapped in any other monad, i.e. `m (Maybe a)
+A `Maybe a` wrapped in any other monad, i.e. `m (Maybe a)`
 
 ### ReaderT
 
 A `Reader r a` in which the resulting `a` is wrapped in any other monad, i.e. `r -> m a`
 
 ### StateT
 
-A `State s a` in which the return value `a` and state `s` are wrapped in any other monad, i.e. `s -> m (a, s)`
+A `State s a` in which the return value and state, namely `(a, s)`, are wrapped in any other monad, i.e. `s -> m (a, s)`
 
 ### EitherT
 
 An `Either e a` wrapped in any other monad, i.e. `m (Either e a)`
 
-* Simple examples of usage
+## Simple examples of usage
+
+### MonadTrans
+
+[transformers](https://www.stackage.org/lts-3.20/package/transformers-0.4.2.0) is a widely used package which provides transformer versions of various monads. 
+
+It also provides a `MonadTrans` class which makes it easy to embed one monad into another. All of the transformers defined in the `transformers` package are instances `MonadTrans`.  
+
+`MonadTrans` defines one method, `lift`, the signature of which is 
+
+```haskell
+lift :: Monad m => m a -> t m a
+```
+
+Given a monad `m`, we can "lift" into a constructed monad transformer `t` so long as `t` is an instance of `MonadTrans`
+
+Examples:
+
+### MaybeT 
+
+```haskell
+import Control.Monad 
+import Control.Monad.Trans.Maybe 
+import Control.Monad.Trans.Class 
+
+main = do 
+  password <- runMaybeT getPassword
+  case password of 
+    Just p  -> putStrLn "valid password!"
+    Nothing -> putStrLn "invalid password!"
+
+isValid :: String -> Bool
+isValid = (>= 10) . length
+
+getPassword :: MaybeT IO String 
+getPassword = do 
+  password <- lift getLine
+  guard (isValid password)
+  return password 
+```
+
+In this example, we combine the `IO` and `Maybe` monads. `lift getLine` allows us to embed the `IO` action into the `MaybeT` transformer, yielding a value of type `MaybeT IO String`.  
+
+
+* More transformer usage examples
 * Pitfalls of Writer laziness
 * Dealing with exceptions and control structures (monad-control and exceptions packages), and losing state
 * Monad transformers: [EitherT vs IO](http://stackoverflow.com/questions/25752900/exceptions-and-monad-transformers/25753497#25753497)