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Generic Methods and Interfaces

Our repeater repeats strings, so you'd expect it to satisfy this interface:

type stringRepeater interface {
	Repeat(string) []string
}

Let's try it:

r := repeater{count: 3}
fmt.Println(r.Repeat("Go"))
// [Go Go Go]

var words stringRepeater = r
// doesn't compile

The call works, but the assignment doesn't! Repeat[T any](T) []T doesn't match Repeat(string) []string. Go infers type arguments for calls, not for implementing interfaces.

Fix the Type on the Struct

Move the type parameter to the struct:

type typedRepeater[T any] struct {
	count int
}

func (r typedRepeater[T]) Repeat(value T) []T {
	result := make([]T, r.count)
	for i := range result {
		result[i] = value
	}
	return result
}

The receiver's [T] brings the struct's type parameter into scope. The method declares none of its own, so typedRepeater[string] has the required Repeat(string) []string method:

var words stringRepeater = typedRepeater[string]{count: 3}
fmt.Println(words.Repeat("Go"))
// [Go Go Go]

An interface can have type parameters too, but its methods can't declare their own.