Lets be honest, most dependency injection frameworks and techniques are about hiding junk under the rug. But they fix the symptoms, not the disease. You see, if you find yourself having components with too many dependencies, feeling pain on initialization, the problem is that you have too many dependencies, which actually means you have too much tight coupling and not that it is hard to initialize them. At this point you should embrace that pain and treat the actual disease.
Also, functional programming naturally leads to building descriptions of what you want, in a declarative way. So instead of depending directly on services that trigger side-effects directly, like a DB component that does inserts or something doing HTTP requests or whatever, instead you build your application to trigger events that will eventually be linked to those side-effects triggering services.
There are multiple ways of doing this. For example you could have a channel / queue of messages, with listeners waiting for events on that queue. And TFA actually speaks about the Free monad. Well the Free monad is about separating the business logic from the needed side-effects, the idea being to describe your business logic in a pure way and then build an interpreter that will go over the resulting signals and trigger whatever effects you want. There's no dependency injection needed anymore, because you achieve decoupling.
> And because of that, functional code is often very tedious to test.
That hasn't been my experience at all, quite the contrary, we've had really good results and we're doing such a good job of pushing the side-effects at the edge that we no longer care to unit-test side-effecting code. And yes, I believe you've had that experience, but I think it happens often with people new to FP that try and shoehorn their experience into the new paradigm.
E.g. do you need a component that needs to take input from a database? No, it doesn't have to depend on your database component at all. Do you need a component that has to insert stuff into the database? No, it doesn't have to depend on your database component at all. Etc.
