Generic Methods

Go 1.27 introduced methods with their own type parameters. This extends earlier support for methods on generic receiver types.

Since Go 1.18, a method of a generic type may use its receiver’s parameters, as in func (b Box[T]) Get() T. Beginning with Go 1.27 (August 2026), a method declaration can introduce an additional type parameter: func (b Box[T]) Convert[U any](f func(T) U) U.

Generic methods cannot implement interface methods, and interface method declarations themselves cannot declare type parameters. The primary runnable example below demonstrates receiver type parameters and is compatible with Go 1.23; the Go 1.27 example below demonstrates the new syntax.

Follow this diagram before executing the example. Check where a value or decision changes what runs next.

Go 1.27+: a true generic method

The following complete program requires the Go 1.27 language version. It is provided separately because the validation environment uses Go 1.23.

package main

import "fmt"

type Box[T any] struct { Value T }

func (b Box[T]) Convert[U any](f func(T) U) U {
    return f(b.Value)
}

func main() {
    box := Box[int]{Value: 12}
    text := box.Convert(func(n int) string {
        return fmt.Sprintf("ID-%d", n)
    })
    fmt.Println(text)
}

Expected output: ID-12. See Go 1.27 release notes and Generic Methods for the language rules. Ordinary receiver-parameter methods still work with older Go releases.

This example is a complete program. Save it as main.go or paste it into the Go Playground.

Go code

package main

import "fmt"

type Box[T any] struct{ Value T }

func (b Box[T]) Get() T   { return b.Value }
func (b *Box[T]) Set(v T) { b.Value = v }
func main() {
	number := Box[int]{Value: 4}
	number.Set(9)
	fmt.Println("Value:", number.Get())
}

Output

Value: 9

The pointer-receiver Set updates the Box[int] value from 4 to 9, and Get returns an int. This is an established generic receiver method, not the new Go 1.27 method-level parameter feature shown below.

• A method-level type parameter needs Go 1.27 or newer, not Go 1.23.
• A generic method is not a way to satisfy an interface method with its own type parameters.

With Go 1.27, add a Convert[U any] method that maps Box[int] into a string. Compare it with a standalone generic Convert function.

Tip: Edit the example above, predict the output, and run the modified code to test your reasoning.

You have practiced Generic Methods in a small, self-contained program. The important habit is to predict what runs, inspect the actual output and distinguish the language rule from the particular example.

Before continuing, make sure you can explain the program to someone who has not seen it, including the line that changes its result.