// signatures.

​

package main

​

import "fmt"

import "math"

​

// Here's a basic interface for geometric shapes.

type geometry interface {

    area() float64

    perim() float64

}

​

// For our example we'll implement this interface on

// `square` and `circle` types.

type square struct {

    width, height float64

}

type circle struct {

    radius float64

}

​

// To implement an interface in Go, we just need to

// implement all the methods in the interface. Here we

// implement `geometry` on `square`s.

func (s square) area() float64 {

    return s.width * s.height

}

func (s square) perim() float64 {

    return 2*s.width + 2*s.height

}

​

// The implementation for `circle`s.

func (c circle) area() float64 {

    return math.Pi * c.radius * c.radius

}

func (c circle) perim() float64 {

    return 2 * math.Pi * c.radius

}

​

// If a variable has an interface type, then we can call

// methods that are in the named interface. Here's a

// generic `measure` function taking advantage of this

// to work on any `geometry`.

func measure(g geometry) {

    fmt.Println(g)

    fmt.Println(g.area())

    fmt.Println(g.perim())

}

​

func main() {

    s := square{width: 3, height: 4}

    c := circle{radius: 5}

​

    // The `circle` and `square` struct types both

    // implement the `geometry` interface so we can use

    // instances of

    // these structs as arguments to `measure`.

    measure(s)

    measure(c)

}

​