Comment by closeparen | Hacker News Reader

Comment by closeparen | Hacker News Reader

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Take a look at the major DI frameworks for Go, Wire [0] and FX [1]. They just plumb the gaps between methods that return types and methods that want them. Although technically they work for concrete types, 99% of the time it's going to be an interface, so that you can substitute a mock for testing.

Prior to DI frameworks we used global variables, often initialized in a package's init() method.

>The desire to do this reflects a misunderstanding of interfaces.

Generally you have a few layers: gRPC/HTTP/Kafka handler, business logic, and database or external service access. Layers are unit tested individually against mocks of the layers below. Because you're going to inject a mock, you can't depend on a concrete type, so you depend on an interface. Often when you're developing you want to know what the concrete implementation of a lower layer does, so it's useful to have "go-to-definition" see through the interface declaration to its implementations.

I think the implicit satisfaction of interfaces is very cool and I wish I had it in every language. I wouldn't give it up just to simplify the IDE's job. But the IDE having this functionality does matter.

[0] https://github.com/google/wire

[1] https://github.com/uber-go/fx

> Take a look at the major DI frameworks for Go, Wire [0] and FX [1]

I'm well aware of them. (There are several more, too, which see at least as much use as these two, unfortunately.) They also reflect a misunderstanding of the language. Wire less so than FX. (I'm also aware Wire is written by Googlers, no need to point that out.)

> Prior to DI frameworks we used global variables, often initialized in a package's init() method.

I don't understand how these things are related. DI frameworks aren't necessary for Go-idiomatic dependency injection, and global variables were never an appropriate way to manage dependencies.

> Generally you have a few layers: gRPC/HTTP/Kafka handler, business logic, and database or external service access.

Yes!

> Layers are unit tested individually against mocks of the layers below.

Yes!

> Because you're going to inject a mock, you can't depend on a concrete type, so you depend on an interface.

Yes!

> Often when you're developing you want to know what the concrete implementation of a lower layer does, so it's useful to have "go-to-definition" see through the interface declaration to its implementations.

...no?

There are basically two situations where you use an interface in this way. One, most often, in an application where the "production" code uses one implementation, which you know a priori and therefore have no need to go spelunking. Or, two, in a library where you by definition cannot know what implementation your users will provide to you. In the first case, your mock would model the implementation, which I guess is trivial. In the second case, your mock would reflect your expectations of the interface's implementations, explicitly without knowledge of any implementation.

> which you know a priori and therefore have no need to go spelunking.

Except when you're working on a codebase that's new to you, and you're trying to navigate it to make a change somewhere, so you don't have a priori knowledge about what implements which interface.

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