Haskell Cookbook

How it works...

When we combine monad transformers, we embed one type into another. For example, for our current recipe, the type is shown here:

    ReaderT Cursor (WriterT Cursor IO) ()

It can be read as follows: 

• The innermost monad is IO.
• WriterT embeds IO into it. The writer state is the Cursor.
• ReaderT embeds WriterT (and IO) monads. The ReaderT has the environment Cursor.
• Essentially, we start from the innermost monad and keep wrapping it with another monad.

To run the preceding monad, we have unwrapped the outer most monad first. Here, is our example:

• Outermost monad is ReaderT. We will unwrap it with runReaderT.
• Then next monad is WriterT. We will unwrap it with runWriterT.
• The IO monad actions are embedded innermost. When we unwrap ...