Tagged unions in Go

tagged.go

package main

import (
	"fmt"
	"unsafe"
)

// data Shape a = Circle { radius :: a } | Rect { base :: a, height :: a} | Together { left :: Shape a, right :: Shape a }
type ID_shape_float64 struct {
	TAG     int      // Can be 0 (circle), 1 (rect), or 2 (together)
	PAYLOAD [16]byte // sizeof(*ANON_shape_float64_together), the biggest one
}

type ANON_shape_float64_circle struct {
	ID_radius float64
}

type ANON_shape_float64_rect struct {
	ID_base   float64
	ID_height float64
}

type ANON_shape_float64_together struct {
	ID_left  *ID_shape_float64
	ID_right *ID_shape_float64
}

// area :: Num a => Shape a -> a
func (shape ID_shape_float64) area() float64 {

	// case shape of
	switch shape.TAG {

	// v@(Circle _) -> radius v * 3.14159 * 3.14159
	case 0:
		p := *(*ANON_shape_float64_circle)(unsafe.Pointer(&shape.PAYLOAD))
		return p.ID_radius * 3.14159 * 3.14159

	// v@(Rect _ _) -> base v * height v
	case 1:
		p := *(*ANON_shape_float64_rect)(unsafe.Pointer(&shape.PAYLOAD))
		return p.ID_base * p.ID_height

	// v@(Together _ _) -> shape v + shape v
	case 2:
		p := *(*ANON_shape_float64_together)(unsafe.Pointer(&shape.PAYLOAD))
		return p.ID_left.area() + p.ID_right.area()

	}

	panic("non-exhaustive pattern blablablah")
}

func main() {
	// let shape = Circle{radius = 4}
	shape := ID_shape_float64{
		TAG: 0,
	}
	p := ANON_shape_float64_circle{4}
	copy(shape.PAYLOAD[:], (*(*[unsafe.Sizeof(p)]byte)(unsafe.Pointer(&p)))[:])

	fmt.Println("Circle{radius = 4} area:", shape.area())

	// let shape2 = Rect{base = 2, height = 3}
	shape2 := ID_shape_float64{
		TAG: 1,
	}
	p2 := ANON_shape_float64_rect{2, 3}
	copy(shape2.PAYLOAD[:], (*(*[unsafe.Sizeof(p2)]byte)(unsafe.Pointer(&p2)))[:])

	fmt.Println("Rect{base = 2, height = 3} area:", shape2.area())
	
	// let shape3 = Together{left = shape, right = shape2}
	shape3 := ID_shape_float64{
		TAG: 2,
	}
	p3 := ANON_shape_float64_together{&shape, &shape2}
	copy(shape3.PAYLOAD[:], (*(*[unsafe.Sizeof(p3)]byte)(unsafe.Pointer(&p3)))[:])

	fmt.Println("Together{left = shape, right = shape2} area:", shape3.area())
}