Support for recursive poly struct?

[fengye87]

I bumped into this project while looking for a lib to solve marshaling my poly struct to JSON (and back from it too). But my poly struct is recursive. Say like

type Shape interface {}

type Square struct {
    ...
    Child Shape
}

type Triangle struct {
    ...
    Child Shape
}

So Child would not marshal/unmarshal as expected here. Any way to support it?

[oliverbestmann]

This should still work if you define Child as a poly value, the same as on the top level of the example in the readme.

[fengye87]

Thanks for pointing it out. I managed to make it work. (BTW: The sample code in the README.md is flawed.) The code I'm using:

package main

import (
	"encoding/json"
	"fmt"

	"github.com/flachnetz/poly"
	"github.com/samber/lo"
)

func main() {
	bytes := lo.Must(json.Marshal(PolySpeark{Animal{
		Speaker: PolySpeark{Dog{}},
	}}))

	fmt.Println(string(bytes))

	var speaker PolySpeark
	lo.Must0(json.Unmarshal(bytes, &speaker))
	speaker.Value.Speak()
}

type Speaker interface{ Speak() }

type Dog struct{ IsDog bool }

func (Dog) Speak() {
	fmt.Println("wuff")
}

type Cat struct{ IsCat bool }

func (Cat) Speak() {
	fmt.Println("miau")
}

type Animal struct {
	Speaker PolySpeark
}

func (c Animal) Speak() {
	c.Speaker.Value.Speak()
}

type PolySpeark poly.Poly[Speaker, poly.TypeItem[Animal, poly.TypeItem[Dog, poly.TypeItem[Cat, poly.Nil]]]]

func (p PolySpeark) MarshalJSON() ([]byte, error) {
	return json.Marshal((*poly.Poly[Speaker, poly.TypeItem[Animal, poly.TypeItem[Dog, poly.TypeItem[Cat, poly.Nil]]]])(&p))
}

func (p *PolySpeark) UnmarshalJSON(bytes []byte) error {
	return json.Unmarshal(bytes, (*poly.Poly[Speaker, poly.TypeItem[Animal, poly.TypeItem[Dog, poly.TypeItem[Cat, poly.Nil]]]])(p))
}

Now the problem becomes: I have dozens of implementations of Speaker, and those last several lines would become a nightmare. Any thoughts?

[oliverbestmann]

It works by using a type alias and pointers. I'll need to have another look on why exactly this is the case. I've probably only implemented json.Marshaller for *Poly. I'll have a look later:

Your example with the necessary changes:

package main

import (
	"encoding/json"
	"fmt"

	"github.com/flachnetz/poly"
	"github.com/samber/lo"
)

func main() {
	bytes := lo.Must(json.Marshal(
		&PolySpeark{
			Animal{
				Speaker: &PolySpeark{Dog{}},
			},
		},
		),
	)

	fmt.Println(string(bytes))

	var speaker PolySpeark
	lo.Must0(json.Unmarshal(bytes, &speaker))
	speaker.Value.Speak()
}

type Speaker interface{ Speak() }

type Dog struct{ IsDog bool }

func (Dog) Speak() {
	fmt.Println("wuff")
}

type Cat struct{ IsCat bool }

func (Cat) Speak() {
	fmt.Println("miau")
}

type Animal struct {
	Speaker *PolySpeark
}

func (c Animal) Speak() {
	c.Speaker.Value.Speak()
}

type PolySpeark = poly.Poly[Speaker, poly.TypeItem[Animal, poly.TypeItem[Dog, poly.TypeItem[Cat, poly.Nil]]]]

[fengye87]

Type alias works in the example, but not in my real case. I have multiple implementations that is recursive. Try include a Speaker *PolySpeark field in Dog, the go compiler would complain about "invalid use of type alias" then.

I have a rough thought here. How about pushing reflection further and also taking in implementation types registration? Something like:

type Final struct {
    ...
    Poly Iface
}

var ifacePoly = poly.New[Iface]().Impls(ImplA{}, ImplB{})

func (f Final) MarshalJSON(...) ... {
    return ifacePoly.MarshalJSON(f)
}

type Iface interface {
    ...
}

type ImplA struct {
    ...
}

type ImplB struct {
    ....
    Poly Iface
}

The benefits are:

1. All recursive fields under Final whose type is Iface would get marshaled as expected magically.
2. If the code is in a lib, the lib user can register more impls to ifacePoly and take the magic for free.