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Getting started

Install mkunion

Run in your terminal 

go install github.com/widmogrod/mkunion/cmd/mkunion@latest

Define your first union type

Create your first union. In our simple example, we will represent different types of vehicles. But in a more complex example, you may want to represent different states of your application, model domain aggregates, or create your own DSL.

example/vehicle.go
package example

//go:tag mkunion:"Vehicle"
type (
    Car struct {
        Color  string
        Wheels int
    }
    Plane struct {
        Color   string
        Engines int
    }
    Boat struct {
        Color      string
        Propellers int
    }
)

In the above example, you can see a few important concepts:

//go:tag mkunion:"Vehicle"

Tags are a powerful and flexible way to add metadata to your code. You may be familiar with tags when you work with JSON in Go:

type User struct {
    Name string `json:"name"`
}

Unfortunately, Go doesn't extend this feature to other parts of the language.

MkUnion defines the //go:tag comment, following other idiomatic definitions like go:generate and go:embed, to allow adding metadata to struct types. And MkUnion uses it heavily to offer a way of adding new behavior to Go types.

Tags supported by MkUnion
  • go:tag mkunion:"Vehicle" - defines a union type.
  • go:tag serde:"json" - enables serialization type (currently only JSON is supported), enabled by default.
  • go:tag shape:"-" - disables shape generation for this type, useful in cases where an x/shared package cannot depend on other x packages, to avoid circular dependencies.
  • go:tag mkunion:",no-type-registry" - if you want to disable generation of the type registry in a package, define this tag in one of the Go files above the package declaration: 
    //go:tag mkunion:",no-type-registry"
package example
  • go:tag mkmatch:"" - generate custom pattern matching function from interface definition
    example/vehicle.go
    //go:tag mkmatch:"MatchPairs"
type MatchPairs[A, B Vehicle] interface {
    MatchCars(x, y *Car)
    MatchBoatAny(x *Boat, y any)
    Finally(x, y any)
}

type (...) convention

A union type is defined as a set of types in a single type declaration. You can think of it as a "one of" type. To make it more readable, as a convention, I decided to use the type (...) declaration block instead of individual type declarations.

Generate code and watch for changes

Run in your terminal to generate union types for your code and watch for changes: 

mkunion watch ./...

This command will: 1. Generate union types and shapes from your code 2. Automatically run go generate ./... to trigger any other code generators 3. Continue watching for file changes and repeat the process

To generate unions without watching for changes (one-time generation): 

mkunion watch -g ./...

If you want to skip the automatic go generate step: 

mkunion watch -G ./...
# or
mkunion watch --dont-run-go-generate ./...

Alternatively, you can run the mkunion command directly on specific files: 

mkunion -i example/vehicle.go

Automatic go generate execution

As of the latest version, mkunion watch automatically runs go generate ./... after generating union types and shapes. This eliminates the need to run two separate commands.

If you need to skip the automatic go generate step (for example, if you want to run it manually with specific flags), use the --dont-run-go-generate flag: 

mkunion watch --dont-run-go-generate ./...
# or use the short form
mkunion watch -G ./...

This automatic execution works well with extensions like moq that depend on union types being defined first.

Match over union type

When you run the mkunion command, it will generate a file alongside your original file with the union_gen.go suffix (example vehicle_union_gen.go).

You can use these functions to do exhaustive matching on your union type.

For example, you can calculate fuel usage for different types of vehicles with a function that looks like this:

example/vehicle.go
func CalculateFuelUsage(v Vehicle) int {
    return MatchVehicleR1(
        v,
        func(x *Car) int {
            return x.Wheels * 2
        },
        func(x *Plane) int {
            return x.Engines * 10
        },
        func(x *Boat) int {
            return x.Propellers * 5
        },
    )
}

And as you can see, it leverages generics to make it easy to write. No need to cast, check types, or use switch statements.

matching functions Match{Name}R1

Where Name is the name of your union type. Where R0, R1, R2, R3 stand for the number of return values.

Example of MatchVehicleR1 function signature: 

func MatchVehicleR1[T0 any](
    x Vehicle,
    f1 func(x *Car) T0,
    f2 func(x *Plane) T0,
    f3 func(x *Boat) T0,
) T0 {
    /* ... */
}

JSON marshalling

MkUnion also generates JSON marshalling functions for you. You just need to use the shared.JSONMarshal and shared.JSONUnmarshal functions to marshal and unmarshal your union type.

Example:

func ExampleVehicleFromJSON() {
    vehicle := &Car{
        Color:  "black",
        Wheels: 4,
    }
    result, _ := shared.JSONMarshal[Vehicle](vehicle)
    fmt.Println(string(result))
    // Output: {"$type":"example.Car","example.Car":{"Color":"black","Wheels":4}}
}

func ExampleVehicleToJSON() {
    input := []byte(`{"$type":"example.Car","example.Car":{"Color":"black","Wheels":4}}`)
    vehicle, _ := shared.JSONUnmarshal[Vehicle](input)
    fmt.Printf("%#v", vehicle)
    // Output: &example.Car{Color:"black", Wheels:4}
}

You can notice that it has an opinionated way of marshalling and unmarshalling your union type. It uses the $type field to store type information, and then stores the actual data in a separate field with the corresponding name.

You can read more about it in the Marshaling union in JSON section.