interfaces.go

package main

import "fmt"

// Shape of thing (e.g. square)
type Shape interface {
    // Signature of functions that implement this interface
    Area() float64
    Perimeter() float64
}

// Different types of shapes:
type square struct {
    X float64
}

// Area() implementation (see the name of the function) for the Shape interface of the square type (as a receiver)
// receiver.field accesses the fields of the struct
func (s square) Area() float64 {
    return s.X * s.X
}

// Perimeter() implementation (see the name) for the Shape interface of the square type (as a receiver)
// receiver.field accesses the fields of the struct
func (s square) Perimeter() float64 {
    return 4 * s.X
}

// Calculate the shape of a thing (area and perimeter)
// Function using the interface Shape (see the input) -- has no output
func Calculate(x Shape) {
    v, ok := x.(square)
    if ok {
        fmt.Println("Is a square:", v)
    }

    fmt.Println(x.Area())
    fmt.Println(x.Perimeter())
}

func main() {
    x := square{X: 10}
    fmt.Println("Perimeter:", x.Perimeter())
    Calculate(x)
}