Startype 🤩

Startype makes it easy to automatically convert (two-way) between Go types and Starlark-Go API types.

Features

• Two-way conversion between Go and Starlark-Go types
• Two-way conversion for primitive types like bool, integer, float, and string types
• Convert Go slice, array, map, and struct types to compatible Starlark types
• Convert Starlark Dict, StringDict, List, Set, and StarlarkStruct to compatible Go types
• Map Starlark keyword args (from built-in functions) to Go struct values
• Support for type pointers and and empty interface (any) types

Examples

Convert Go value to Starlark value

The following converts a Go struct to a (comptible) *starlarkstruct.Struct value:

func main() {
    data := struct {
		Name  string
		Count int
	}{
		Name:  "test",
		Count: 24,
	}

	var star starlarkstruct.Struct
	if err := startype.Go(data).Starlark(&star); err != nil {
		log.Fatal(err)
	}

	nameVal, err := star.Attr("name")
	if err != nil {
		log.Fatal(err)
	}
	if nameVal.String() != `"test"` {
		log.Fatal("unexpected attr name value: %s", nameVal.String())
	}
}

Convert Starlark value to Go value

Startype can easily convert a Starlark-API value into a standard Go value. The following converts a starlark.Dict dictionary value to a Go map[string]string value.

func main() {
    dict := starlark.NewDict(2)
    dict.SetKey(starlark.String("msg0"), starlark.String("Hello"))
    dict.SetKey(starlark.String("msg1"), starlark.String("World!"))

    gomap := make(map[string]string)
    if err := startype.Starlark(val).Go(&gomap); err != nil {
        log.Fatalf("failed to convert starlark to go value: %s", err)
    }
	
    if gomap["msg0"] != "Hello" {
        log.Fatalf("unexpected map[msg] value: %v", gomap["msg"])
    }
    if gomap["msg1"] != "World!" {
        log.Fatalf("unexpected map[msg] value: %v", gomap["msg"])
    }
}

Use struct annotations to control conversion

Startype supports struct annotations to describe field names to target during conversion. For instance, the following example uses the provided struct tags when creating Starlark-Go values.

func main() {
    data := struct {
		Name  string `name:"msg0"`
		Count int    `name:"msg1"`
	}{
		Name:  "test",
		Count: 24,
	}

	star := make.Struct
	if err := startype.Go(data).Starlark(&star); err != nil {
		t.Fatal(err)
	}

    // starlark struct field created with annotated name
    nameVal, err := star.Attr("msg0")
	if err != nil {
		t.Fatal(err)
	}
	if nameVal.String() != `"test"` {
		t.Errorf("unexpected attr name value: %s", nameVal.String())
	}
}

Similarly, Startype can use struct tags to copy Starlark-API Go values during conversion to Go values, as shown below:

func main() {
    dict := starlark.StringDict{
        "mymsg0": starlark.String("Hello"),
        "mymsg1": starlark.String("World!"),
    }
    star := starlarkstruct.FromStringDict(starlark.String("struct"), dict)
    
    var godata struct {
        Salutation string   `name:"mymsg0"`
        Message string      `name:"mymsg1"`
	}

	if err := Starlark(&star).Go(&goData); err != nil {
		t.Errorf("conversion failed: %s", err)
	}

	if godata.Message != "World!" {
		log.Errorf("unexpected go struct field value: %s", godata.Message)
	}
}

Starlark keyword argument processing

Startype makes it easy to capture and process Starlark keyword arguments (passed as tuples in built-in functions) by automatically map the provided arguments to a Go struct value. For instance, the following maps kwargs (a stand in for a actual keyword arguments) to Go struct args:

func main() {
    kwargs := []starlark.Tuple{
        {starlark.String("msg"), starlark.String("hello")},
        {starlark.String("cnt"), starlark.MakeInt(32)},
    }
    var args struct {
        Message string   `name:"msg"`
        Count int64      `name:"cnt"`
    }
	if err := startype.Kwargs(kwargs).Go(&val); err != nil {
		t.Fatal(err)
	}

    fmt.Printl(args.Message) // prints hello
}

An argument can be marked as optional to avoid error if it is not provided. For instance, if argument cnt is not provided in the kwargs tuple, function KwargsToGo will not report an error.

func main() {
    kwargs := []starlark.Tuple{
        {starlark.String("msg"), starlark.String("hello")},
    }
    var args struct {
        Message string   `name:"msg"`
        Count int64      `name:"cnt optional:true"`
    }
	if err := startype.Kwargs(kwargs).Go(&val); err != nil {
		t.Fatal(err)
	}

    fmt.Printl(args.Message) // prints hello
}

For additional conversion examples, see the test functions in the test files.