I would say that extension functions is just another tool in dealing with complexity, and especially the “dependency” part of complexity. Like modules, but on a finer grained level.

Consider you have something like http-fetch library, and you have a

Client

and it has

connect

method.

What would it accept as an end point? IP? domain name? url?

If it accepts domain name, entire

Client

has dependency on DNS subsystem.

What if I want then some

WINS

? Or some externally provided json for custom hosts file?

I don’t want to put it all there. So I make it accept IP address, and the obviously useful

connect(hostName: String)

as an extension function.

Then I know how to test

Client

itself, and testing surface is not very big.

Excellent example @orangy.

I obviously need to test DNS too, if I package it in my library, but that’s testing only the resolve part, I don’t need to test the whole Client with domain, because when resolve converted hostName to IP correctly, the rest is already tested.

Yep, that's a good example about extending already existing logic 😛

He is saying you start out like this.

but would you do it that way if you are going to build "Client" from scratch?

That’s my example.

Keep the

Client

class as simple as possible, put the "extras" in extension methods.

I mean that I’m building Client from scratch.

Even if you plan on supporting DNS from the start, you don't include it in your

Client

class.

But the problem I see with that is precisely dependency inversion

That’s basically SRP, and then instead of creating

DNSAwareClient : Client

you just use extension functions.

Stuff like that should be documented somewhere. Maybe in a "How to design your kotlin projects"? 😄

why a "Client" extension function needs to know about another subsystem?

@Andreas Sinz: I think with many languages supporting extension functions these days, it would be better in a generalized "design patterns" book/blog/wiki.

And how do you include a dependency in that?

Need the dependency to be hardcoded? As a new parameter?

@kmruiz: I’m not sure you read it attentively simple smile

If I understood well, you put a new extension function on Client

Client.connect(dns: String)

now, that extension function depends on an external service, say DNSSystemService

how do you pass that dependency to the extension function?

as a new parameter?

it's hardcoded on the extension function?