How Can I Handle Generic Types for Pointers Implementing Interfaces in Go?

Generic Types for Pointers Implementing Interfaces in Go

In Go, when working with interfaces, it can be useful to create generic functions that operate on data types that implement those interfaces. However, if the interface represents a pointer type, determining the generic type for the pointer can be challenging.

To address this, Go provides a mechanism to declare interfaces with type parameters, which allows the definition of interfaces that expect the implementing type to be a pointer to its type parameter. For instance, consider the following interface:

type A interface {
  SomeMethod()
}

If you have an implementation of this interface via a struct pointer, such as:

type Aimpl struct {}

func (a *Aimpl) SomeMethod() {}

You may encounter issues when attempting to pass a pointer to Aimpl to a generic function that expects a function with an A parameter. The reason for this is that the generic function is expecting a type that implements the A interface, not a pointer to a type that implements it.

To resolve this, you can modify the interface definition to specify a type parameter:

type A[P any] interface {
  SomeMethod()
  *P
}

This indicates that the interface expects a type implementing it to be a pointer to the type parameter P.

Next, you need to adjust the signature of the generic function to accept the modified interface type:

func Handler[P any, T A[P]](callback func(result T)) {
  result := new(P)
  callback(result)
}

The generic function now takes a function with a parameter of type T, which must implement the modified A interface. When you invoke the function with a pointer to Aimpl, it will correctly identify the implementing type and execute as expected.

However, if you are unable to modify the definition of the original A interface, you can create your own wrapper interface that includes the type parameter:

type MyA[P any] interface {
  A
  *P
}

By wrapping the original interface, you can employ the same approach as before:

func Handler[P any, T MyA[P]](callback func(result T)) {
  result := new(P)
  callback(result)
}

This allows you to work with pointer types that implement the original A interface while maintaining the desired generic behavior.

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