Reader

Since ask and asks are defined for ReaderT instead of Reader, how come they work also for Reader? The reason is that Reader isn't defined as

newtype Reader r a = Reader { runReader :: r -> a }

at all. It only behaves as if it was. The real definition is

type Reader r a = ReaderT r Identity a

runReader :: Reader r a -> r -> a
runReader f = runIdentiry . runReader f

The Reader monad simply stacks the decoration provided by the ReaderT monad transformer on top of the Identity monad. But as we discussed before, the Identity monad doesn't provide any decoration at all. It's merely a wrapper for pure values. Thus, Reader decorates pure values with a context in the same way that ReaderT decorates computations in some monad m with some context.