How to Use Go Interfaces

https://blog.chewxy.com/2018/03/18/golang-interfaces/

by boyter • 8 years ago

324 116 8 years ago

blog.chewxy.com

Great article that corroborates what I've also personally seen in many Go applications.

IMHO, if you're coding Java style interfaces up-front ("Repository", "Service", etc) you're already doing it wrong. It's likely that you're either over-engineering your project or prematurely optimizing for flexibility you will not need. You'll likely feel as though you're fighting the language to build these abstractions and layered architectures.

Go is all about minimalism. As the article says, you should be writing the concrete implementations first, then defining interfaces in the client code. The calling code should pick and choose the behaviour it needs from the implementation. As hard as it is for most seasoned Java developers, you have to silence that anxious voice inside that keeps asking "what if I need to swap this out for something else?". Go is about getting things done, not beautiful abstractions.

dionian• 8 years ago

As someone who works in a lot of Java codebases, I would say that good Java code is more similar to how you describe Go and less similar to the bad, over-abstracted Java code.

seanmcdirmid5• 8 years ago

Sometimes you don’t know what the implementation will be yet, and writing a bunch of interfaces out can help you discover what it should be, then you start working it out with interfaces gradually giving way to implementation.

Interfaces are rarely rarely about interoperability even in Java. Most interfaces have one implementation. Instead, they are about abstraction, design, and so on. This article assumes that the end product is what was intended all along.

xienze• 8 years ago

Most interfaces have one implementation.

Close, with the exception of mocking interfaces for unit testing, which is very common in Java.

XR0CSWV3h3kZWg• 8 years ago

I generally start with making something that accomplishes some small part of what I am trying to eventually accomplish, then iteratively add things.

It's awful to do in Java, but I felt fine doing it in go.

catnaroek5• 8 years ago

Most interfaces have one implementation. Instead, they are about abstraction, design, and so on.

Then what you want is an abstract data type. Unlike interfaces, which can be implemented anywhere, an abstract data type is implemented in a single place. This has obvious benefits for performance (clients don't need to dynamically dispatch on operations), control (implementors don't need to worry about malicious clients supplying impostors) and precision (a single module can export several abstract types, whose implementations are mutually dependent).

Go's interfaces are pretty terrible at all of the above. Not that Java provides anything substantially better.

chewxy10• 8 years ago

Ah, that's why you write your abstract implementations in Haskell, and then rewrite them in Go ;P

peterevans2• 8 years ago

Abstractions are not a bad thing, but early abstraction often lacks understanding of the problem to solve. Abstraction is complexity. We often need complexity, but we should not add complexity without cause.

And I don't think this is a Java vs. Go thing. Let the program, or the problem if you will, prove to you where you need abstraction.

frou_dh3• 8 years ago

An interface whose purpose is to reduce known coupling is not really "abstraction". It's a mechanism for indirection.

IIRC Zed Shaw had an essay on this.

Pxtl• 8 years ago

Dirty secret: do that in Java too.

ploxiln• 8 years ago

There's a difference though: in Java, the class declaration must mention the interface, but in Go it does not. In Go, you can declare an interface for a subset of a type's methods, in some other package, and use it there, without any change to the original type. You can create a new interface which a standard-library type satisfies!

twic• 8 years ago

I've ended up doing something a bit like this in Java, but rather than using interfaces, i do it with a suite of lambdas:

    String perform(Supplier<String> speaker) {
       return speaker.get();
    }

This is a poor example, but that's what's in the article. Better might a method which is going to do some message broadcasting or something. With interfaces:

    interface Broadcaster<T> {
      void send(T value);
      void reset();
      void close();
    }
    
    void pump(Source<T> source, Broadcaster<T> broadcaster) {
      // this logic is nonsense but hopefully it makes the point
      T value;
      while ((value = source.poll()) != null) {
        try {
          broadcaster.send(value);
        } catch (Exception e) {
          broadcaster.reset();
        }
      }
      broadcaster.close();
    }

You might write:

    void pump(Source<T> source, Consumer<T> send, Runnable reset, Runnable close) {
      // still nonsense
      T value;
      while ((value = source.poll()) != null) {
        try {
          send.accept(value);
        } catch (Exception e) {
          reset.run();
        }
      }
      close.run();
    }

If you had an actual Broadcaster, this is simple to call:

    pump(source, broadcaster::send, broadcaster::reset, broadcaster::close);

And if you don't have an actual Broadcaster, or if there is no such interface, it still simple to call, because you write references to whatever methods you do have. They could be on different objects. They could be lambdas. They could be no-op lambdas if you don't care about one of the events. In a unit test they could be:

    List<String> sent = new ArrayList<>();
    pump(source, sent::add, sent::clear, () -> sent.add("CLOSED"));

Etc.

eadmund2• 8 years ago

Premature abstraction is a smell in every language, but it's particularly bad in Go. If you're building out a giant hierarchy of directories & packages at the beginning of your project — you're probably wrong. If you're building out a giant hierarchy at the middle of your project — you're probably wrong. If you're building it out at the end of your project — your project is over now, why are you spending time abstracting stuff when you can deliver already?

It's not that Go is against abstractions, really; it's just that we programmers love, absolutely love to build a system with operators and binary operators and summing binary operators and addition and numbers and real numbers and integers and complex numbers and imaginary numbers — when all we really need to do is add 2 & 2 to get 4. Go says, 'hey there, slow down, hang on a sec.' You're free to abstract if you need to — but there's a good chance you're wrong to do so.

Write the simplest thing that could possibly work. Maybe, maybe once you've written it three or thirty times, you'll have a good sense of the problem and be able to write a sane abstraction for it.

But if you're spending time fleshing out interface or package hierarchies before you even write any code: you're probably wrong.

googlemike• 8 years ago

Spoken like someone who has never worked on a long-lived project. There is no beginning, middle or end. You joined when it has been around for 7 years. It needs to be around 10 more years.

Early and careful abstraction of absolutely every layer of your program is something you will always thank yourself for. If your classes only talk to interfaces at every single layer (no concrete classes talking to concrete classes, except for maybe a factory here or there to vend out said interfaces), your project will be much easier to test, refactor, and dynamically change.

hota_mazi3• 8 years ago

Every single thing you say apply to Java. And C#. And C++.

And pretty much any language in existence. It's just healthy engineering practices.

Whether developers apply them or not has nothing to do with the language.

tepidandroid4• 8 years ago

The difference is that other languages (like the ones you mentioned) are actually quite accommodating when it comes to indulging developer abstraction fetishes, whereas you will be punished for trying to do so in Go.

closeparen• 8 years ago

... until you write unit tests, at which point you need either interfaces or global mutable function pointers to swap out underlying implementations with mocks.

tepidandroid4• 8 years ago

I don't think we're in disagreement here. I have no qualms with abstractions/interfaces. It's simply a matter of putting the horse before the cart. The implementations should not have to tip-toe around pre-conceived abstractions.

flukus• 8 years ago

We really need some better solutions to this. Aside from abstraction it also comes with the performance penalty of virtual functions everywhere.

I think we had some good tools to solve this but either threw them away in newer languages or just forgot they were possible. Working with c recently I wanted to mock something and the language obviously couldn't help me so I came up with something along the lines of:

  #ifdef TEST
    #define foo mock_foo
  #endif

This worked for mocking single functions but would need to evolve for more complex code. There are a number of ways to do this from optional includes to compiling test fixtures to their own binaries with the mock implementation being the actual implementation. Both options should work with just about any language to varying degrees of effort.

The biggest barrier to approaches like this is the reliance on IDE's and what they can do. I've never seen an IDE that can handle rules like this, they want to compile the project and control it's structure. In c# for instance, it would be pretty easy to have a mock library with concrete alternative implementations and compile the tests individually, something like:

  csc.exe MyTest.cs MyClassUnderTest.cs /reference MyMockAssembly.dll

This is perhaps even easier in c/c++ where the includes are more explicit. Once you take back control of the compilation process a lot more options open up though.

guelo• 8 years ago

I don't know much about Go but in Java and C# mocking frameworks use runtime bytecode manipulation to make it really easy to swap out concrete implementations without needing to create separate interfaces.

nostalgeek2• 8 years ago

IMHO, if you're coding Java style interfaces up-front ("Repository", "Service", etc) you're already doing it wrong.

Then how do you unit test your controllers in a web app? or anything that depends on a database connection without decoupling service and repository?

Go is all about minimalism.

No, it isn't. It's your opinion about Go. Unit testing doesn't become irrelevant because you are using Go.

you should be writing the concrete implementations first, then defining interfaces in the client code

That's not how you do test driven development via unit testing.

Just because Go is relatively new it doesn't mean software engineering best practices don't apply anymore.

tsavola• 8 years ago

The distinction is that you don't first declare a few all-encompassing interfaces and then their concrete implementations, but piecemeal interfaces for each API that takes those implementations - only the bits of the interface that the API in question needs.

That makes it actually even easier to do unit testing, because for a narrow test you don't need to implement irrelevant methods.

DougBTX• 8 years ago

That's not how you do test driven development via unit testing

Unit tests test concrete implementation "units", so this is completely compatible with writing unit tests.

reificator• 8 years ago

Then how do you unit test your controllers in a web app? or anything that depends on a database connection without decoupling service and repository?

From the post you responded to:

you should be writing the concrete implementations first, then defining interfaces in the client code.

When you need to take an argument that you can swap out for another type, that's when you define your interface. Not earlier.

Unit testing doesn't become irrelevant because you are using Go.

Unit testing doesn't become irrelevant because you don't architecture astronaut from the start. When you need to test, define an interface. But only the interface that you need when you need it.

Just because Go is relatively new it doesn't mean software engineering best practices don't apply anymore.

By providing the client the ability to define the interface it removes the need to pretend you can plan for all circumstances upfront.

Instead, at the moment you need to be able to mock things, you make an interface and mock them.

That's not to say that you should never provide interfaces in a library, just that it's no longer the only option available.

I'm not sure why you think that unit testing becomes harder with this approach, because to me it seems far easier.

notheguyouthink• 8 years ago

That's a nice perspective. With that said though, what do you think about centralized data types?

I admit it, I make Repository and Service quite frequently. With that said, I also have data types shared among implementations. Normally the data types are defined within the parent packages to the implementations, the same packages that hold the Service definition.

I actually agree with your assertion that Service is bad form, and think I'll change it. I still have the issue though where a shared data type is commonly used among many implementations. Where do you think that will live? Perhaps in the same location, just without a formal Service interface?

JepZ2• 8 years ago

Btw. Interfaces can also be used to resolve circular dependencies: Just define compatible interfaces in both packages et voilà, no circular dependencies anymore.

I am not sure if I agree with

Defining an interface upfront is usually a code smell for overengineering.

So far I had less problems with overusing interfaces than with not using them enough. So when you write tests for a package, you define the general interface to the package in terms of types and methods. The remaining challenge is to break that general definition into smaller pieces as one big interface for a package is almost certainly a bad idea.

However, in general I follow the authors verdict to not just build 'Java-style' interfaces, as that completely misses the point of interfaces in Go.

fiatjaf4• 8 years ago

If you have circular dependencies you're doing something wrong. By disagreeing with that and accepting circular dependencies as a normal state of the Go code, you put yourself in the position of doing another wrongness: defining a lot of interfaces.

sanderjd• 8 years ago

There is some stuff I really love about Go, but the seemingly pervasive "you're doing that wrong" culture typified by your comment is a really bad look for a language community. It is much better to show people why things are better than to make proclamations. Neither of the things you proclaim to be "wrong" are wrong a priori; show us why your way is better.

JepZ2• 8 years ago

Well, it might be that I was doing something wrong, but in the end I didn't have circular dependencies anymore, so I was doing right again? (just kidding) ;-)

So in fact, I don't like to have circular dependencies in Go either and try to avoid them, so I agree with you on that. But at the same time I like very small and simple packages and sometimes you have to choose:

1. do you build a larger package with everything inside

2. do you build multiple smaller packages with dependencies on each other

As I said, in general, I favor the smaller packages and that can lead to circular dependencies, if you are not using interfaces properly.

tptacek3• 8 years ago

Go's crypto interfaces, in the standard library, don't hew to this "accept interfaces return structs" rule. NewAES, for instance, returns a cipher.Block.

peterevans2• 8 years ago

A lot of advice I've seen (such as this) postdates much of the standard library implementation by quite a bit. I could not, however, suggest that the current design for crypt is good or bad, for I have not used it often enough to offer a worthy opinion.

People seem to really click with the net/http design, and that does seem well done, but I imagine the standard library itself is a bit unevenly architected.

tptacek3• 8 years ago

The general advice I've always gotten is that if you're looking for well-executed Go idiom, the standard library is your first stop.

And while NewAES might predate this advice, NewGCM (which returns an opaque crypto.AEAD) does not.

chewxy10• 8 years ago

It's in there in a footnote (click on one of the *):

Of course there are subtleties that I am eliding. For example, context-sensitive code would benefit very much from taking interfaces and returning interfaces

Or to steal from Haskell's lexicon: when you do something in a monad.

gkya2• 8 years ago

Showerthought: wouldn't it be easier to comprehend if Haskell called monads contexts?

kasey_junk2• 8 years ago

If your package itself defines multiple implementations (or you know of multiple packages that do) feel free to preemptively define/return interfaces.

Other than that interface definition should be done at the call site, not the implementation site. This is true in Java also. Golangs structural typing doesn’t change that, it just removes boilerplate in the most trivial cases.

zzbzq• 8 years ago

This was so absurd to me at first I had to think about it for a while.

I guess you wrote it as a continuation of the opinion from the article that interfaces should only be used as input.

From my perspective, I was still considering all other cases also. For other cases, it's precisely because of Go's structural typing that such a thing can even be considered. Java can't implement an interface for a type it doesn't control. You could create a wrapper class, but not exhaustively wrap all present and future implementations.

kasey_junk2• 8 years ago

The only reason to have an interface is to decouple the use from the implementation (other than some fairly specific reasons that the article mentions like sealed interfaces).

To decouple the use from the implementation we generally are either adapting the implementation or narrowing it. In the latter case, structural typing saves you boilerplate, in the former it likely doesn't.

The OOP consultant set calls the the concept we are dancing around Dependency Inversion. In Java it very routinely results in wrapper classes. In the most basic cases those classes are pass through boilerplate, but those are fairly rare in practice. Golang dismisses with those.

In more cases in practice, you are actually defining the interface that matches the call site demands and modeling different implementations to that. Golang's structural typing doesn't help much with that.

I tried (not terribly successfully) to write up some of this in the DIP section of this http://kc.my-junk.info/di-ioc-dip it uses scala for the code samples, but it should be fairly easy to see that golang doesn't remove much of the wrapper code if I'd used that.

ramenmeal• 8 years ago

Would this lead to the same or very similar interfaces being defined multiple times? Maybe that's not a problem?

cdoxsey• 8 years ago

One of the more surprising things about Go is that you don't need to name interfaces at all. This:

    func f1(r io.Reader) {}

Is the same as:

    func f2(r interface{ Read([]byte) (int, error) }) {}

In fact this is super common, with the empty interface:

    func f3(r interface{})

But somehow the ramifications of that don't sink in and we fail to generalize the principle.

One practical usage:

    li, _ := net.Listen("tcp", ":9000")
    for {
        conn, err := li.Accept()
        if err != nil {
            if err, ok := err.(interface {
                Temporary() bool
            }); ok && err.Temporary() {
                time.Sleep(time.Second)
                continue
            } else {
                panic(err)
            }
        }
    }

This takes a bare interface and checks to see if it supports the "Temporary() bool" interface (which all net errors do). When paired with a type switch this makes for very interesting behavior:

		switch err := err.(type) {
		case nil:
		case interface{ Temporary() bool }:
			if err.Temporary() {
				time.Sleep(time.Second)
				continue
			}
			panic(err)
		default:
			panic(err)
		}

https://play.golang.org/p/J66vaIBGrs2

All that to say, creating duplicate interfaces is perfectly fine.

chewxy10• 8 years ago

Should probably name them - makes it readable. But you are correct - duplicate interfaces are fine

juststeve• 8 years ago

yeah i dont like it

nothrabannosir• 8 years ago

I’m not a big fan of contrived examples with imaginary use cases, because they tend to fall short in actual use.

“Accept interfaces, return structs”

Real example from personal (professional) life against that: Amazon AWS SDK defined S3 as a struct. That makes it bloody hard to mock during unit testing. I eventually ended up defining my own “s3iface” interface with corresponding mock to combat this, but now I have to repeat this for every program that uses it. Ok, I’ll separate it into its own package and include that everywhere. Wait, what was the point of the AWS SDK not doing this directly, again? Oh, look, they eventually ended up doing just that: providing an interface version of their S3 struct:

https://godoc.org/github.com/aws/aws-sdk-go/service/s3/s3ifa...

Package s3iface provides an interface to enable mocking the Amazon Simple Storage Service service client for testing your code.

With the exact same name, to boot.

Please consider providing and returning interfaces. Even if your struct is the only type implementing it.

fwip• 8 years ago

Your code should accept interfaces, not the S3 struct.

Interfaces are light-weight, and it's okay to define an interface that is just the subset of functionality your code needs.

burntsushi• 8 years ago

But this advice is kind of ridiculous in its unqualified form as you have presented it. If folks followed your advice, there wouldn't be an "io" package in the standard library. So why does it exist? Because it's damn useful to not define interfaces every single time you need them. It gets old real fast.

This is exactly what the GP is saying.

sethammons• 8 years ago

My real world AWS sdk s3 experience: I have an uploader interface (I only needed to upload). The concrete uploader has the s3 code. The fake uploader has any custom result/err I need to test against.

yup123• 8 years ago

No idea why you’re getting downvotes, seems like a clean way to do this.

buckhx• 8 years ago

Agreed, would like to see a concrete example of why that advice is being given.

balefrost3• 8 years ago

To take your point and run with it: in the face of evolving code, returning an interface from a function is actually more conservative than returning a struct. If you return a concrete type of some sort, then as you evolve your code, you are obligated to continue to return that specific concrete type. As your implementation of the function changes, you might want to return a different concrete type, but you are prevented from doing so. You either have to change the signature, which would break callers, or else you would need to grow that concrete type to handle all the possible ways in which it would want to represent its data.

I realize that this goes against the Go ethos of "don't think about the future; think about the now". Having said that, I don't understand why returning interfaces is seen as premature design while accepting interfaces is seen as reasonable and expected. Surely both can be examples of premature abstraction.

I'm certainly not saying that you should always return interfaces, or that returning interfaces will prevent all future refactoring pain. But working with interfaces instead of concrete types is more likely to allow your API to remain stable even as the implementation evolves.

joncalhoun• 8 years ago

That makes it bloody hard to mock during unit testing.

Can you share why? Not trying to say you are wrong, but I am guessing that part of this stems from some limitations in the way interfaces work in Go, and this could make a good user experience report.

naikrovek• 8 years ago

I tend to read a lot of other peoples’ codes

"codes?"

Plural form of this type of code is "code."

Stopped reading there. This is one thing I just can't get past when I see it.

Yes, I realize that I am not a superhero-level altruist.

Scramblejams• 8 years ago

“Codes” is idiomatic in some disciplines. For example in aerospace engineering when discussing various analysis implementations, everyone calls them codes. “Which vortex lattice method codes do you like?” “Oh I just use panel codes.” And it’s not slang, you see that usage in textbooks and academic papers as well.

walesey• 8 years ago

Great article! Makes a lot of sense.

holografix2• 8 years ago

Golang newbie here. Using a slice to hold different struct types of the same interface, finding it very annoying that every time I want to access a common attribute of the struct types I need to define a getter/setter.

flashmob• 8 years ago

Perhaps you could try creating a struct with just the common properties, then embed the common struct in all your 'extended' structs. Eg. http://www.hydrogen18.com/blog/golang-embedding.html

holografix2• 8 years ago

That sounds like a bit of a hack. I like it.

fiatjaf4• 8 years ago

I wished godoc.org (or similar service) somehow listed which types in a package implemented which interfaces.

They could match with all the interfaces defined at the standard library (io.Writer etc.) plus the interfaces defined in that same package.

chewxy10• 8 years ago

Go Guru. It's an amazing tool

fiatjaf4• 8 years ago

For code I have on my machine, right?

Well, I guess it's better than nothing.

keypusher2• 8 years ago

I don't see what the benefit is of switching it around like this. The author says the problems with the original implementation are:

The most obvious of which is that it has only one type that fulfills the interface with no obvious means of extension. (sic)

Seems obvious to me.

type Cat struct{}

func (a Cat) Speaks() string { return "meow" }

Does this not work?

Functions typically take concrete types instead of interface types.

Ok...? This seems like some arbitrary rule that the creators of Go claimed was a good idea, I guess. The author has offered no justification for it. In general I guess I fail to see most of the proclaimed benefits of Go, every time I read articles like this I come away favoring something like Java for anything in the application layer. I can see preferring Go to C for low-level and systems programming, but I shudder to think of what a mess the large Go codebases must look like.

chewxy10• 8 years ago

Author here.

Does this not work?

It works

However, this is what I have recently found quite a lot. An interface is defined:

    type Foo interface {
        Method1()
        unexportedMethod()
    }

And in the same package, only one type (usually unexported) implements the interface. And the functions declared in the package typically take the concrete type instead of the interface type.

This is typical when developers come from Java and they do `abstract class Fooer` and immediately do `class Foo implements Fooer` within the same package. This is an anti pattern in Go.

This seems like some arbitrary rule that the creators of Go claimed was a good idea, I guess

No. I think I was unclear. I specifically mention that it's not a great idea. It's a code smell. Instead, interfaces should be defined at point-of-use.

freyir• 8 years ago

What's the point of exporting the Foo interface if other packages cannot create or use Foo values?

keypusher2• 8 years ago

I specifically mention that it's not a great idea. It's a code smell. Instead, interfaces should be defined at point-of-use.

You still have not said WHY it's not a great idea, or what the benefits of defining the interfaces at point-of-use are.

hota_mazi3• 8 years ago

Accept interfaces, not structs

Besides the fact that there is zero justification from this claim, the simple fact that Go think it's important to distinguish the two is another illustration of late 90s programming language design.

There is really no good reason to separate these concepts today, it only causes confusion and fragments the code base in arbitrary chunks that accumulate smelly code over the years.

But I'm happy to hear a good justification of why it's important to have both structs and interfaces, if there is one.

chewxy10• 8 years ago

But I'm happy to hear a good justification of why it's important to have both structs and interfaces, if there is one.

Performance is a good reason. If you are familiar with Java or C#, Go interfaces can be seen from one point of view as boxing (i.e. making a primitive value an object). Think what that would require: vtables for function dispatch.

A bunch of inlining opportunities cannot happen with that (well, it can, if you allow for name mangling during your compile phase, but that's very gross). JIT is the second fix to that, but with JIT, you're bringing in a lot of unpredictability - useful in some contexts, not so in others.

Perhaps you are fine with the "Everything Is An Object/Function" paradigm, but I'm not entirely fine with that.

yegle• 8 years ago

Why would you think struct and interface should be treated the same? To me the fact a struct can implement multiple interfaces means they are not on the same "layer" of abstraction.

hota_mazi3• 8 years ago

Imagine a struct with functions that don't have an implementation: that's an interface.

If it has some implementations and some without, it's an interface with default implementations.

See how Kotlin does it, it's really nice and consistent, and you never need to agonize over the decision of picking a class or a struct.

tepidandroid4• 8 years ago

I don't think interface segregation can be simply dismissed as just 'another illustration of late 90s programming language design' (implying there's something wrong with late 90s language design...).

The clearest benefit is the decoupling of dependencies which leads to more testable, extensible and maintainable code. That any struct can simply implement the desired behaviour to implicitly satisfy externally defined interfaces is an extremely powerful notion.

Of all the things to complain about in Go, interfaces seem like a strange bone to pick.

klmr• 8 years ago

I don’t think the reference to Postel’s law is correct here: Postel’s law is about robustness, not about abstractness.

Furthermore, taken at face value, Postel’s law was a phenomenal failure in principle and in practice (see e.g. [1]). In hindsight it’s probably a bad design principle.

[1] https://tools.ietf.org/html/draft-thomson-postel-was-wrong-0...

brightball• 8 years ago

This is really excellent advice. It was the first thing that jumped out at me when I read the language spec.

It's one of the reason why I think Go seems to draw more people from dynamic languages than static because the static folks have a tendency to shoehorn some old habits into it.

How to Use Go Interfaces

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