I'm trying to use the built-in map type as a set for a type of my own (Point, in this case). The problem is, when I assign a Point to the map, and then later create a new, but equal point and use it as a key, the map behaves as though that key is not in the map. Is this not possible to do?

// maptest.go

package main

import "fmt"

func main() {
    set := make(map[*Point]bool)

    printSet(set)
    set[NewPoint(0, 0)] = true
    printSet(set)
    set[NewPoint(0, 2)] = true
    printSet(set)

    _, ok := set[NewPoint(3, 3)] // not in map
    if !ok {
        fmt.Print("correct error code for non existent element\n")
    } else {
        fmt.Print("incorrect error code for non existent element\n")
    }

    c, ok := set[NewPoint(0, 2)] // another one just like it already in map
    if ok {
        fmt.Print("correct error code for existent element\n") // should get this
    } else {
        fmt.Print("incorrect error code for existent element\n") // get this
    }

    fmt.Printf("c: %t\n", c)
}

func printSet(stuff map[*Point]bool) {
    fmt.Print("Set:\n")
    for k, v := range stuff {
        fmt.Printf("%s: %t\n", k, v)
    }
}

type Point struct {
    row int
    col int
}

func NewPoint(r, c int) *Point {
    return &Point{r, c}
}

func (p *Point) String() string {
    return fmt.Sprintf("{%d, %d}", p.row, p.col)
}

func (p *Point) Eq(o *Point) bool {
    return p.row == o.row && p.col == o.col
}