Cool but all you really needed to do was fix the contract between NewActionDeleteObject’s struct creation and the switch statements result = print. What’s really crazy is you can create anonymous structs to unmarshal json without having to fully flesh out data models thanks to its “we’ll only map the fields we see in the struct” approach. Mapstructure takes this even further with squash. In the end, the type checker missed the error because it was user error by contract and that, due to lack of validation that the actions were constructed properly, resulted in a rabbit hole debugging session that ended with an excellent blog post about the gotchas of unmarshaling.

> human error

Humans suck at formalisms.

When XML was the 'sane choice' I grew to hate it, until I met a person who introduced me to XMLSpy. It has a mode where instead of trying to define the schema you offer it example documents and it offers you a schema that seems to satisfy your examples, at which point you can either switch to manual or provide more examples. He was right, it did make me hate XML a lot less.

At least until the next job when they wouldn't pay for it.

IME this is a longstanding pain point with Go. There's an attempt to propose an encoding/json/v2 package [1] being kicked around at the moment [2], spawned from a discussion [3].

This at least seems to improve the situation of marshalling to/from an interface directly slightly by providing the ability to pass custom Unmarshalers for a specific type (via json.WithUnmarshalers and json.UnmarshalFunc) to the Unmarshal functions, but it appears to still have the inefficient double-decode problem. Or I just haven't found a decent way around it yet.

Looks like they're intentionally punting on a first class solution until (if) the language gets some sort of sum type, but I still think the second-class solution could do a bit more to make this extremely common use-case more convenient. Pretty much every serious production Go app I've worked on in the last 10 years or so has had some horrible coping strategy for the "map a field-discriminated object to/from implementations of an interface" gap.

Quote from the proposal [1]:

> First-class support for union types: It is common for the type of a particular JSON value to be dynamically changed based on context. This is difficult to support in Go as the equivalent of a dynamic value is a Go interface. When unmarshaling, there is no way in Go reflection to enumerate the set of possible Go types that can be stored in a Go interface in order to choose the right type to automatically unmarshal a dynamic JSON value. Support for such use cases is deferred until better Go language support exists.

  [1]: https://pkg.go.dev/github.com/go-json-experiment/json
  [2]: https://github.com/golang/go/issues/71497
  [3]: https://github.com/golang/go/discussions/63397

BTW double unmarshalling (and double marshalling) can be quite slow, so to speed up determining the object type you can extract the type field e.g. by using gjson (https://github.com/tidwall/gjson). It can be easily 10x faster for this kind of scenario

Pulled my hair out about doing this all over the place when integrating with a node api, ended up writing https://github.com/byrnedo/pjson. Feels like this should be covered as a more first class thing in go.