Phantom types in Go

phantom_types.go

package main

import (
	"fmt"

	"github.com/google/uuid"
)

// with this, the "value" field can't be accessed outside the package with PhantomID,
// which prevents PhantomID[T] from being compared to PhantomID[U]
// makes working with the ID any other way difficult, however
type PhantomID[T any] struct {
	value uuid.UUID
}

type Dog1 struct {
	id    PhantomID[Dog1]
	breed string
}

type Cat1 struct {
	id      PhantomID[Cat1]
	numToes int
}

// embedding allows any caller to access the underlying UUID with id.UUID;
// which might allow incorrect ID comparisons, but allows the UUID to be read for other uses
type EmbeddedPhantomID[T any] struct {
	uuid.UUID
}

type Dog2 struct {
	id    EmbeddedPhantomID[Dog2]
	breed string
}

type Cat2 struct {
	id      EmbeddedPhantomID[Cat2]
	numToes int
}

func main() {
	dog1 := Dog1{
		// have to specify type parameter when instantiating PhantomID
		id: PhantomID[Dog1]{
			value: uuid.New(),
		},
		breed: "Beagle",
	}
	cat1 := Cat1{
		// have to specify type parameter when instantiating PhantomID
		id: PhantomID[Cat1]{
			value: uuid.New(),
		},
		numToes: 6,
	}

	// compiles; isn't rejected by typechecker
	fmt.Println(dog1.id.value == cat1.id.value)

	// is correctly rejected by typechecker; does not compile
	// fmt.Println(dog1.id == cat1.id)

	// compiles (correctly), returns true
	fmt.Println(dog1.id == dog1.id)

	dog2 := Dog2{
		// have to specify type parameter when instantiating EmbeddedPhantomID
		id:    EmbeddedPhantomID[Dog2]{uuid.New()},
		breed: "Golden Shepherd",
	}
	cat2 := Cat2{
		// have to specify type parameter when instantiating EmbeddedPhantomID
		id:      EmbeddedPhantomID[Cat2]{uuid.New()},
		numToes: 5,
	}

	// compiles; isn't rejected by typechecker
	fmt.Println(dog2.id.UUID == cat2.id.UUID)

	// is correctly rejected by typechecker; does not compile
	// fmt.Println(dog2.id == cat2.id)

	// compiles (correctly)
	fmt.Println(dog2.id == dog2.id)
}

Future work:

• Generic type for adding ID to an existing type
• For "private" value, exposing functions to get/set it for use when it's really needed (i.e. reading value from database)