Functors, Applicatives, And Monads In Pictures
Written by Eric Normand. Published: May 23, 2013

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Reference: Functors, Applicatives, And Monads In Pictures
This is a nice, short post. It does the job well, though it might need a few readings if you have never seen these concepts before.
Learn these ideas and get on with your life. The sooner we get over this Functor/Applicative/Monad obsession, the better.