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Welcome to MkUnion

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About

Strongly typed union type in golang that supports generics*

  • with exhaustive pattern matching support
  • with json marshalling including generics
  • and as a bonus, can generate compatible typescript types for end-to-end type safety in your application

Why

Historically in languages without union types like golang, unions were solved either by using Visitor pattern, or using iota and switch statement, or other workarounds.

Visitor pattern requires a lot of boiler plate code and hand crafting of the Accept method for each type in the union. iota and switch statement is not type safe and can lead to runtime errors, especially when new type is added and not all case statements are updated.

On top of that, any data marshalling like to/from JSON requires additional, hand crafted code, to make it work.

MkUnion solves all of those problems, by generating opinionated and strongly typed meaningful code for you.

Example

example/vehicle.go
package example

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

func CalculateFuelUsage(v Vehicle) int {
    // example of pattern matching over Vehicle union type
    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
        },
    )
}

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

func ExampleFromJSON() {
    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}
}

Watch for changes in the file and generate code on the fly: 

mkunion watch ./...

# or use -g flag to generate code without watching
mkunion watch -g ./...

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