I mean `class Database implements IPersist`, `class Redis implements ICache`, `class MyLogger implements ILogger`, etc.

So classes/interfaces to each service your app uses. I just use very basic autofac, declaring which implementations to use in code. (Really nothing I couldn't wire up by hand; autofac is just a tad less cumbersome).

I've heard about OCaml's module system and am intrigued by it but never used it.