How to use type alias in interface

Consider the following program:

package main

import (
	"fmt"
)

type MyMethod func(i int, j int, k int) (string, error)

type MyInterface interface {
  Hello(i int, j int, k int) (string, error)
}

type myMethodImpl struct {}

func (*myMethodImpl) Hello(i int, j int, k int) (string, error) {
   return fmt.Sprintf("%d:%d:%d\n", i, j, k), nil
}

func main() {
	var im MyInterface = &myMethodImpl{}
	im.Hello(0, 1, 2)
}

How do I use MyMethod in the interface declaration instead of repeating the method signature?

You are mixing two different things here. The one is that you define a type which is a function. When you want that type inside another type you need to use a struct.

Changing your code to a possible solution 1 (not so idiomatic go):

type MyMethod func(i int, j int, k int) (string, error)

type myMethodImpl struct {
	Hello MyMethod
}

var hello MyMethod = func(i int, j int, k int) (string, error) {
	return fmt.Sprintf("%d:%d:%d\n", i, j, k), nil
}

func main() {
	im := &myMethodImpl{Hello: hello}
	fmt.Println(im.Hello(0, 1, 2))
}

https://play.golang.org/p/MH-WOnj-Mu

The other solution would be to change it to use an interface. That solution is your code just without the type definition of MyMethod. https://play.golang.org/p/nGctnTSwnC

But what is the difference between?

If you define a func as a type you need to declare it, when you create a function.

var hello MyMethod = func(i int, j int, k int) (string, error) {
    return fmt.Sprintf("%d:%d:%d\n", i, j, k), nil
}

Now hello has just exactly the type MyMethod. If there would be another type for example:

type YourMethod func(i int, j int, k int) (string, error)

hello will still just the type MyMethod.

To define an interface you need a method set. But the type MyMethod is not a method. It is just a function type. So the function type is something different then a method definition. For go it would be the same if you want to define a string as a method.