Functors, Applicatives, and Monads

• Functor

• Functors apply a function to a wrapped value:
• Easy example to remember it by:  getPostTitle <$> (findPost 1)
• A value can be in a context. Now when you apply a function to this value, you’ll get different results depending on the context. 
• A Functor is any data type that defines howfmap applies to it. 
• The Maybe data type defines two related contexts: we’ll see how function application is different when something is a Just a versus a Nothing
• When a value is wrapped in a context, you can’t apply a normal function to it:This is where fmap comes in
• Applicatives
• Applicatives apply a wrapped function to a wrapped value:
• Easy example to remember it by: Just (+5) <*> (Just 3)
• Applicatives allow you to apply a function that takes two arguments to two wrapped values?
• Monads

• Monads apply a function that returns a wrapped value to a wrapped value.
•  > Just 20 >>= half >>= half >>= half

http://adit.io/posts/2013-04-17-functors,_applicatives,_and_monads_in_pictures.html