I've been taking a kind of middle ground lately: build functional APIs. Move your side effects as far outward as possible and make your library's interface take everything it needs and return everything it computes. No HTTP calls, no database access, no random number generation. For example, don't have your library build the query and then run it, have it built the query and return it so it can be run at a higher-level.
I know this seems obvious, but it was a breakthrough I had with organizing the way I built things a few years back and it has been serving me amazingly well. I think I originally stumbled onto the idea through a talk on domain-driven development.
The result is that you get a lot of the benefits of functional (easy testing, portability, etc) but you don't have to deal with all the oddball patterns of recursion or currying or things that, yeah, sure, makes sense on some theoretical level but makes me want to gouge my eyes out when I try to read it. I've tried fully-fledged functional programming in a few cases and it doesn't click with me. I appreciate and understand that some love it, but in the end if your exposed public API is functional, the internals are less important.
To me the important thing is the mindset: move your side effects as high up the chain as you can.
I know this seems obvious, but it was a breakthrough I had with organizing the way I built things a few years back and it has been serving me amazingly well. I think I originally stumbled onto the idea through a talk on domain-driven development.
The result is that you get a lot of the benefits of functional (easy testing, portability, etc) but you don't have to deal with all the oddball patterns of recursion or currying or things that, yeah, sure, makes sense on some theoretical level but makes me want to gouge my eyes out when I try to read it. I've tried fully-fledged functional programming in a few cases and it doesn't click with me. I appreciate and understand that some love it, but in the end if your exposed public API is functional, the internals are less important.
To me the important thing is the mindset: move your side effects as high up the chain as you can.