private type with exported fields

But you could have a public constructor, right?

So if you had a NewGeometryPoint func defined, then you maybe could do (haven't tested against the compiler)

p := NewGeometryPoint(640,480);
fmt.Println("X:",p.X, "Y:",p.Y);

An abstract base type ?

package geometry

type point struct {
    X, Y int;
}

type Point struct {
    point;
    name string;
}

type Rect struct {
    P1, P2 point;
    name string;
}

This same question is presented in this Go course as:

[...]You may even have a private type with exported fields. Exercise: when is that useful?

As presented here you can access externally an element defined as internal to a package, you just can't access it directly. In the case of the structure "point" in your example, it means you CANNOT access elements of point directly, as in

---

// geometry.go

package geometry

type point struct {
    X, Y int
}

---

// main.go

package main

import (
    "fmt"    
    "./geometry"
)

func main() {
    point := geometry.point{
        X: 10,
        Y: 20
    }

    fmt.Printf("Point: %#v\n", point)
}

---

But you CAN use the defined point to export elements that use its exported internal elements, as in

---

// geometry.go

package geometry

type point struct {
    X, Y int
}

//Vector ...
type Vector struct {
    Start point
    End   point
}

---

// main.go

package main

import (
    "fmt"

    "./geometry"
)

func main() {
    vector := geometry.Vector{}
    vector.Start.X = 10
    vector.Start.Y = 10
    vector.End.X = 10
    vector.End.Y = 10

    fmt.Printf("Vector: %#v\n", vector)
}

---

Result -> Vector: geometry.Vector{Start:geometry.point{X:10, Y:10}, End:geometry.point{X:10, Y:10}}

---

So, in my view, this mechanism is meant to give you flexibility in declaring internal data structures.

When using the JSON package (http://golang.org/pkg/json/). You need to have exported fields, to pass a type to json.Marshal(), but you might not want to have that type publicly available to other external packages.