Why Does Go Generics Throw 'item.SetId undefined' Errors and How Can I Fix Them?

Go with Generics: Understanding Pointer Type Considerations

The Error Conundrum

When working with generics in Go, you may encounter the following error:

item.SetId undefined (type *T is pointer to type parameter, not type parameter)

This error arises when attempting to implement an object store using generics. The issue stems from the fact that the type parameter T is not the pointer to itself, *T. Hence, pointer receiver methods defined on the concrete type T are not implicitly included in the method set of *T.

Solving the Type Parameter Issue

To solve this, you need to explicitly specify the constraint that T should be a pointer type that implements the GS interface and has a pointer receiver for the SetId method:

<code class="go">func Foo[T any, PT interface { SetId(string); *T}](v T) {}</code>

Fixing the Implementation Constraints

Another error you may encounter is:

A does not implement GS (SetId method has pointer receiver)

This error indicates that the SetId method is declared on *A, not A. To fix this, instantiate MyStore with *A instead of A and adjust the type of the struct field and method accordingly:

<code class="go">var storeA = &MyStore[*A]{}

type MyStore[T GS] struct {
    values map[string]T
}

func (s *MyStore[T]) add(item T) {
}</code>

Conclusion

Understanding these type parameter considerations and implementation constraints is crucial when working with generics in Go. By addressing these issues, you can effectively use generics to implement reusable and maintainable code.

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