Type definitions

…

A generic type may also have methods associated with it. In this case, the method receivers must declare the same number of type parameters as present in the generic type definition.

// The method Len returns the number of elements in the linked list l.
func (l *List[T]) Len() int  { … }

This probably needs further explanation. Or an example. It did for me.

Let’s look at a simple generic type:

type Thing[T any] struct {
	value T
}

Now you might think you could define a method on this type as so:

func (t *Thing) Foo() { ... } // compilation fails: cannot use generic type Thing[T any] without instantiation

But what we’ve just read says this doesn’t work. You must inclue the correct number of type parameters (in this case one) in the method definition:

func (t *Thing[T]) Type() string { // valid
	return fmt.Sprintf("%T", t.value)
}

But why? Perhaps to distinguish between a non-generic type of the same name? Nope, that’s invalid…

type Thing[T any] struct {
	value T
}

type Thing struct { // compilation fails: Thing redeclared in this block
	value any
}

Well, you can actually use the type parameter within the method. Here’s a re-implementation of my earlier Type() method:

func (t *Thing[T]) Type2() string {
	var x T
	return fmt.Sprintf("%T", x)
}

See this example on the playground

Quotes from The Go Programming Language Specification Version of December 15, 2022