[dgb23]

Personally I rarely use or even implement interfaces except some other part needs them. My brain thinks in terms of plain data by default.

I appreciate how they compose, for example when I call io.Copy and how things are handled for me. But when I structure my code that way, it’s extra effort that doesn’t come naturally at all.

[lanstin]

I use them for testing, where I can have a client that is called by the code under test and can either just run a test CB, send a REST call to a remote server, send a gRPC call to a remote server, or make a function call to an in-process gRPC server object.

[__turbobrew__]

Yea, mocking is generally the most use I get out of interfaces

[dgb23]

Plain data is really convenient for testing though.

I think the reason that your example is so useful is not generally because of testing, but because the thing you're interacting with has operational semantics. It's a good use case for object orientation, so interfaces and mocking are the natural way of testing your logic.