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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 like Go that lack native union types, developers have resorted to workarounds such as the Visitor pattern or iota with switch statements.

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

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

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

Example

example/shape.go
//go:tag mkunion:"Shape"
type (
    Circle struct {
        Radius float64
    }
    Rectangle struct {
        Width  float64
        Height float64
    }
    Square struct {
        Side float64
    }
)


func CalculateArea(s Shape) float64 {
    return MatchShapeR1(
        s,
        func(x *Circle) float64 {
            return math.Pi * x.Radius * x.Radius
        },
        func(x *Rectangle) float64 {
            return x.Width * x.Height
        },
        func(x *Square) float64 {
            return x.Side * x.Side
        },
    )
}


func TestShapeJSON(t *testing.T) {
    shape := &Rectangle{
        Width:  10,
        Height: 20,
    }
    result, err := shared.JSONMarshal[Shape](shape)
    if err != nil {
        t.Fatal(err)
    }
    fmt.Println(string(result))
    // Output: {"$type":"example.Rectangle","example.Rectangle":{"Width":10,"Height":20}}

    shape2, err := shared.JSONUnmarshal[Shape](result)
    if err != nil {
        t.Fatal(err)
    }
    fmt.Printf("%#v", shape2)
    // Output: &example.Rectangle{Width:10, Height:20}
}

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 ./...

Next steps