10 Golang Interfaces Best Practices

In Golang, an interface is a set of methods that a type must implement to be considered as that interface type. Interfaces are a powerful tool in Golang that allows for code reuse and decoupling of dependencies.

In this article, we will discuss 10 best practices for working with Golang interfaces. By following these best practices, you can write more maintainable and testable code.

1. Use interfaces to define the behavior of objects

When you use an interface to define the behavior of an object, it makes your code more flexible and easier to change in the future. For example, let’s say you have a struct that represents a user. You might have an interface that defines the methods that can be called on a user. Then, you can have different implementations of that interface for different types of users.

For example, you might have a “normal” user and an “admin” user. Both of these types of users would implement the same interface, but they would have different behavior. This is much more flexible than having a single struct that has all of the fields and methods for both types of users.

It also makes your code easier to test. When you have different implementations of an interface, you can easily mock those objects in your tests. This way, you don’t have to actually create real objects when you’re testing your code.

2. Use interfaces as parameters

When you use an interface as a parameter, it allows for greater flexibility in the code. For example, let’s say you have a function that takes an io.Reader as a parameter. This means that the function can accept any type that implements the io.Reader interface, such as a file, a buffer, or even a network connection.

This also makes the code easier to test because you can mock out the dependencies that the code relies on. For instance, if your code uses an HTTP client to make a request, you can create a mock HTTP client that returns canned responses. This way, you don’t have to actually make a real HTTP request when testing your code.

Overall, using interfaces as parameters is a best practice because it leads to more flexible and testable code.

3. Return interface types from functions

When you return an interface type from a function, it allows the caller to decide what concrete type to use. This is useful when the caller knows more about the context in which the returned value will be used. For example, if the caller is going to marshal the returned value to JSON, they can use a json.Marshaler interface.

Returning an interface type also makes your code more flexible and easier to change in the future. If you need to change the underlying implementation of the returned type, you can do so without breaking the API for the caller.

Of course, there are times when it doesn’t make sense to return an interface type. For example, if the caller isn’t going to do anything with the returned value other than pass it to another function, there’s no point in returning an interface type. In this case, it would be more efficient to return the concrete type directly.

4. Don’t use an interface when a concrete type will do

When you use an interface, you are essentially saying that any type that implements that interface can be used in its place. This is great for code reuse and flexibility, but it comes at a cost.

Using an interface adds an extra level of indirection to your code. This can make your code more difficult to understand and maintain. It can also lead to performance issues as the Go runtime has to do more work to figure out which concrete type to use.

So, only use interfaces when they are absolutely necessary. If you can use a concrete type instead, then do so.

5. Avoid large numbers of tiny interfaces

When you have a large number of small interfaces, it’s difficult to maintain them and keep them consistent with each other. This can lead to errors and inconsistencies in your code.

It’s much better to have a smaller number of larger interfaces. This way, you can more easily see how they relate to each other and spot any errors or inconsistencies. Plus, it’s easier to maintain and update a smaller number of interfaces than a large number of interfaces.

6. Declare variables with the smallest possible interface

When you declare a variable with a large interface, you’re essentially saying that your code is willing to accept any type of data that satisfies that interface. This might not be a problem if the interface is well-defined and you have complete control over all the types of data that might be passed into your code.

However, in many cases, you won’t have complete control over the data that’s passed into your code. For example, if you’re working with an API that returns JSON data, you won’t know ahead of time what the structure of that JSON data will be.

If you declare a variable with a large interface, you run the risk of accepting data that doesn’t conform to the expectations of your code. This can lead to runtime errors that are difficult to debug.

On the other hand, if you declare a variable with a small interface, you can be confident that any data that is accepted by your code will conform to the expectations of your code. This can save you a lot of time and effort in debugging errors.

7. Prefer smaller, more specific interfaces over larger ones

Larger interfaces tend to be more difficult to implement and maintain because they can change over time as the codebase evolves. This can lead to breaking changes that need to be propagated throughout the code, which can be error-prone and time-consuming.

Smaller, more specific interfaces are easier to understand and use because they are focused on a single task. They are also less likely to change over time, so you can be confident that your code will continue to work as expected.

8. Always implement all methods in an interface

When you create an interface, you are defining a set of methods that must be implemented by any type that wants to implement the interface. This means that if your type only implements some of the methods in the interface, it will not compile.

This may seem like a pain at first, but it’s actually a good thing. It forces you to think about all the methods in an interface and how your type should behave when those methods are called. It also makes it easy for other developers to see which methods have been implemented and which have not.

If you find yourself in a situation where you need to implement an interface but don’t want to implement all the methods, you can use a blank identifier (_). This will cause the compiler to ignore the fact that the method is not being implemented.

9. Avoid pointer receivers on value types

When you have a pointer receiver on a value type, the method can modify the underlying data. This can lead to unexpected side effects if the caller is not expecting the data to be modified.

It is generally better to use value receivers on value types and pointer receivers on pointer types. This way, it is clear to the caller what methods are safe to call and which ones might modify the data.

10. Use nil to represent absent values

When you use nil to represent an absent value, it makes your code more expressive. For example, let’s say you have a function that returns an error if one occurs. If you use nil to represent the absence of an error, it’s clear to anyone reading your code that there was no error.

On the other hand, if you were to return a zero-value instead of nil, it wouldn’t be as clear. The reader would have to know that the zero-value for errors is nil, which isn’t always obvious.

Additionally, using nil to represent absent values can help you avoid bugs. For example, if you forget to check for a nil value before dereferencing it, you’ll get a runtime panic. This is much better than silently getting the wrong answer because you forgot to check for a nil value.

Overall, using nil to represent absent values is a best practice because it makes your code more expressive and can help you avoid bugs.