How to Handle Implementing Interfaces with Identical Signatures in Different Packages in Go?

Implementing Two Interfaces with Identical Signatures in Different Packages

In Go, interfaces provide a way to define a contract for types that implement them, ensuring that any type implementing a given interface must provide implementations for all its methods. However, implementing interfaces with identical method signatures in different packages can lead to confusion and inconsistencies.

Consider the following scenario:

<code class="go">// Package A
package A

type Doer interface {
    Do() string
}

func FuncA(doer Doer) {
    // Logic using doer.Do()
}

// Package B
package B

type Doer interface {
    Do() string
}

func FuncB(doer Doer) {
    // Logic using doer.Do()
}</code>

Now, suppose you have a type C in your main package that implements both A.Doer and B.Doer with different implementations for Do(). When passing an instance of C to FuncA(), it behaves as expected, utilizing the implementation defined in A.Doer. However, when passing C to FuncB(), the logic triggers an unexpected behavior due to the different implementation of Do().

To address this issue, there are two recommended approaches:

Method Type Assertions:

According to the Go FAQ, "matching only by name and requiring consistency in the types was a major simplifying decision in Go's type system." Consequently, Go does not support overloading methods based on different signatures. However, you can perform method type assertions to verify if an object satisfies a specific interface type. For example:

<code class="go">if _, ok := obj.(A.Doer); ok {
    // obj conforms to A.Doer
    // Use obj.Do() as desired
}</code>

Wrapper Types:

Alternatively, you can create wrapper types around your object that implement the necessary interfaces with the desired logic. For example, you could have DoerA that wraps C and provides a specific implementation of A.Do(), and a DoerB that wraps C and provides an implementation of B.Do(). This approach allows you to pass the appropriate wrapper type to the desired function, ensuring the correct logic is executed.

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