Tour of Go 2.0

Interface values

Under the hood, an interface value is a pair: a concrete value and its type.

(value, type)

When you call a method on an interface, Go dispatches to the method on the underlying concrete type.

Try it: the describe helper prints both the value and the type for each assignment. Watch how they change as i holds first a *T, then an F.

main.go ▶ Run in Playground 
package main

import (
	"fmt"
	"math"
)

type I interface {
	M()
}

type T struct {
	S string
}

func (t *T) M() {
	fmt.Println(t.S)
}

type F float64

func (f F) M() {
	fmt.Println(f)
}

func main() {
	var i I

	i = &T{"Hello"}
	describe(i)
	i.M()

	i = F(math.Pi)
	describe(i)
	i.M()
}

func describe(i I) {
	fmt.Printf("(%v, %T)\n", i, i)
}