Watching Mark Seemann smash together two repositories using monoidal mappend: "I don’t know about you but I think that’s marvellous. I just added two objects together just like I can add two numbers together" https://cleancoders.com/episode/humane-code-real-episode-2/show Download for better quality.

1. How do you compose them?
In OOP you now have to create a new composite class, but this is not
necessary in Haskell.
Both of these quadruples are monoids.
All the functions return values that are monoids, and functions that return
monoids are themselves monoids. Furthermore, tuples of monoids are
also monoids, so that’s why these two values are monoids.
Since they are monoids, I can compose them together using the mappend
function like this: bang!
That’s just worked. I don’t know about you but I think that’s marvellous. I
just added two objects together just like I can add two numbers
together, and what is the type of the repo? It’s the same type as the input
values, clearly as part of the contract of a monoid.
…
This monoidal composition of functions behaves just like the C#
Composite that you saw previously.
The type of each of the two repositories azureRepo and fileRepo is a
quadruple of CRUD functions (C,R,U,D).
He composes the two repos to get a new repo whose CRUD methods operate
on both repos. Lazy evaluation ensures that the read method only reads from
the second repo if reading from the first repo returns nothing.
Current status: watching Mark Seemann’s Humane Code episode 2, Composite as Universal Abstraction
https://cleancoders.com/episode/humane-code-real-episode-2/show