package main

import (
    "fmt"
)

type vertex struct {
    x, y float64
}

// note that this one can deal fine with a copy:
func (v vertex) abs() float64 {
    return v.x + v.y
}

// but this function needs access to the actual struct members
func (v *vertex) scale(s float64) {
    v.x *= s
    v.y *= s
}

func main() {
    v := vertex{1.1, 2.2}
    fmt.Println(v.abs())
    v.scale(12)
    fmt.Println(v.abs())
}