Getting an "undefined: distance" error from Go compiler

I'm a seasoned "old school" programmer, but a rank beginner with Go. I'm making my way through the "CreateSpace: An Introduction to
Programing in Go" book. On p. 111, the third of Chapter 9's Chapter Problems, the task is to add a new method to the user-defined Shape interface. The interface has been built up over the course of the chapter and here's what I have so far:

package main
import (
"fmt"
"math"
)
type Shape interface {
area() float64
perimeter() float64
}
type Distance struct {
x1, y1, x2, y2 float64
}
func (d *Distance) distance() float64 {
a := d.x2 - d.x1
b := d.y2 - d.y1
return math.Sqrt(a*a + b*b)
}
type Rectangle struct {
x1, y1, x2, y2 float64
}
func (r *Rectangle) area() float64 {
l := distance(r.x1, r.y1, r.x2, r.y1)
w := distance(r.x1, r.y1, r.x1, r.y2)
return l * w
}
type Circle struct {
x, y, r float64
}
func (c *Circle) area() float64 {
return math.Pi * c.r * c.r
}
type Perimeter struct {
x1, y1, x2, y2 float64
}
func (p *Perimeter) perimeter() float64 {
s1 := distance(p.x1, p.y1, p.x1, p.y2)
s2 := distance(p.x1, p.y2, p.x2, p.y2)
s3 := distance(p.x2, p.y2, p.x2, p.y1)
s4 := distance(p.x2, p.y1, p.x1, p.y1)
return s1 + s2 + s3 + s4
}
func main() {
d := new(Distance)
d.x2, d.y2, d.x1, d.y1 = 0, 0, 10, 10
p := new(Perimeter)
p.x1, p.y1 = 0, 0
p.x2, p.y2 = 10, 10
fmt.Println(p.perimeter())
r := new(Rectangle)
r.x1, r.y1 = 0, 0
r.x2, r.y2 = 10, 10
fmt.Println(r.area())
c := Circle{0, 0, 5}
fmt.Println(c.area())
}

The problem is that I am getting the following error (from the compiler?):

user@pc /c/Go/src/golang-book/chapter9/chapterProblems
$ go run interface.go
# command-line-arguments
.\interface.go:25: undefined: distance
.\interface.go:26: undefined: distance
.\interface.go:42: undefined: distance
.\interface.go:43: undefined: distance
.\interface.go:44: undefined: distance
.\interface.go:45: undefined: distance

I've spent a good amount of time doing my "due diligence" hereto pertaining by rereading the chapter text and thinking hard about what this
"undefined: distance" error might mean, but so far to no avail.
As you can see, I have defined a "Distance struct", created a new() instance of
it called d, initialized its fields with the . operator, and created a distance() func, but, clearly, I'm not grokking some relevant piece(s) of information.

You don't have a function called distance. It's a method of the type *Distance. You need to create a *Distance first, and then call the method.

d := &Distance{r.x1, r.y1, r.x2, r.y1}
l := d.distance()

I'd suggest starting with Effective Go. It's a very good introduction into the language for "a seasoned programmer".

Your function defined here:

func (d *Distance) distance() float64 {
a := d.x2 - d.x1
b := d.y2 - d.y1
return math.Sqrt(a*a + b*b)
}

is a method on the Distance object. It looks like you're trying to create a new Distance instance here:

func (r *Rectangle) area() float64 {
l := distance(r.x1, r.y1, r.x2, r.y1)
w := distance(r.x1, r.y1, r.x1, r.y2)
return l.distance() * w.distance()
}

but what you're actually doing is trying to call a function called distance.

You want

func (r *Rectangle) area() float64 {
l := &Distance{r.x1, r.y1, r.x2, r.y1}
w := &Distance{r.x1, r.y1, r.x1, r.y2}
return l.distance() * w.distance()
}

thanks @Ainar-G and @Momer! After I got my "mind right" (had to fix a couple more self-inflicted syntax errors), the following worked:

<pre><code>
package main
import ("fmt"; "math")
type Shape interface {
area() float64
perimeter() float64
}
type Distance struct {
x1, y1, x2, y2 float64
}
func distance(x1, y1, x2, y2 float64) float64 {
a := x2 - x1
b := y2 - y1
return math.Sqrt(a*a + b*b)
}
type Rectangle struct {
x1, y1, x2, y2 float64
}
func (r *Rectangle) area() float64 {
l  := distance(r.x1, r.y1, r.x2, r.y1)
w := distance(r.x1, r.y1, r.x1, r.y2)
return l * w
}
type Circle struct {
x, y, r float64
}
func (c *Circle) area() float64 {
return math.Pi * c.r*c.r
}
type Perimeter struct {
x1, y1, x2, y2 float64
}
func (p *Perimeter) perimeter() float64 {
s1 := distance(p.x1, p.y1, p.x1, p.y2)
s2 := distance(p.x1, p.y2, p.x2, p.y2)
s3 := distance(p.x2, p.y2, p.x2, p.y1)
s4 := distance(p.x2, p.y1, p.x1, p.y1)
return s1 + s2 + s3 + s4
}
func main() {
d := new(Distance)
d.x1, d.y1, d.x2, d.y2 = 0, 0, 10, 10
p := new(Perimeter)
p.x1, p.y1, p.x2, p.y2 = 0, 0, 10, 10
fmt.Println(p.perimeter())
r := new(Rectangle)
r.x1, r.y1 = 0, 0
r.x2, r.y2 = 10, 10
fmt.Println(r.area())
c := new(Circle)
c.x, c.y, c.r = 0, 0, 5
fmt.Println(c.area())
}
<pre><code>

Here's the resulting output:

<pre><code>
David Bailey@DAVIDBAILEY-PC /c/Go/src/golang-book/chapter9/chapterProblems
$ go run interface.go
40
100
78.53981633974483
David Bailey@DAVIDBAILEY-PC /c/Go/src/golang-book/chapter9/chapterProblems
$
<pre><code>

Again, thank you.