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One of the most powerful things I like about Go, is the ability to put methods on any type.

Type definitions

…

Type definitions may be used to define different boolean, numeric, or string types and associate methods with them:

type TimeZone int

const (
	EST TimeZone = -(5 + iota)
	CST
	MST
	PST
)

func (tz TimeZone) String() string {
	return fmt.Sprintf("GMT%+dh", tz)
}

In many other languages, if you want to associated a method with, say, an integer, you have to wrap that integer in an Object. Not so in Go. With Go, you can just define a custom type, with an underlying type of int, and put methods on it!

You can even put methods on function types, which seems a bit strange at first, but is also very powerful. There’s actually a good chance you’ve already used this pattern, perhaps without realizing it, if you use the net/http package. The type HandlerFunc is such an example:

type HandlerFunc func(ResponseWriter, *Request)

This type, which is a function type, has a single method defined on it:

func (f HandlerFunc) ServeHTTP(w ResponseWriter, r *Request)

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