Tour of Go 2.0

Interfaces

An interface type is a set of method signatures. Any type that implements those methods satisfies the interface — automatically.

A variable of interface type can hold any value that implements the required methods.

Note: There’s an intentional error on line 22. Vertex (the value type) doesn’t implement Abser because Abs is defined on *Vertex (the pointer type). Try it: run the code and read the compile error, then fix it by changing a = v to a = &v.

main.go ▶ Run in Playground 
package main

import (
	"fmt"
	"math"
)

type Abser interface {
	Abs() float64
}

func main() {
	var a Abser
	f := MyFloat(-math.Sqrt2)
	v := Vertex{3, 4}

	a = f  // a MyFloat implements Abser
	a = &v // a *Vertex implements Abser

	// In the following line, v is a Vertex (not *Vertex)
	// and does NOT implement Abser.
	a = v

	fmt.Println(a.Abs())
}

type MyFloat float64

func (f MyFloat) Abs() float64 {
	if f < 0 {
		return float64(-f)
	}
	return float64(f)
}

type Vertex struct {
	X, Y float64
}

func (v *Vertex) Abs() float64 {
	return math.Sqrt(v.X*v.X + v.Y*v.Y)
}