ELI5: free monad

What is a free monad?

A free monad is like a recipe that lets you write out a set of instructions step-by-step, but you don't actually follow them right away [2][3]. Instead, you write them down as a data structure that describes what you want to do [1].

Why do we use it?

Free monads let you separate the "what you want to do" from the "how you actually do it" [1][3]. This is useful because you can write your program once, and then let different people decide how to follow those instructions in different ways.

What does it look like?

Instead of running code immediately, a free monad creates a tree-like structure that shows all your steps and decisions [1]. Later, you can "fold" or walk through this tree to actually execute the instructions [1].

How do you use it?

You give a free monad any functor (a special kind of container), and it automatically becomes a monad that you can use with all the monad tools [2][5]. Then you can interpret or run that monad however you want [1].