[simiones]

Go's JSON decoder only cares if the fields that match have the expected JSON type (as in, list, object, floating point number, integer, or string). Anything else is ignored, and you'll just get bizarre data when you work with it later.

For example, this will parse just fine [0]:

  type myvalue struct {
    First int `json:"first"`
  }

  type myobj struct {
    List []myvalue `json:"list"`
  }
  js := "{\"list\": [{\"second\": \"cde\"}]}"
  var obj myobj
  err := json.Unmarshal([]byte(js), &obj)
  if err != nil {
    return fmt.Errorf("Error unmarshalling: %+v", err)
  }
  fmt.Printf("The expected value was %+v", obj) //prints {List:[{First:0}]}

This is arguably worse than what you'd get in Python if you tried to access the key "first".

[0] https://go.dev/play/p/m0J2wVyMRkd

[marcus_holmes]

It totally makes sense from a Go perspective: You created a struct, tried (but failed) to populate it with some json data, and ended up with a value initialised to its zero-value. This is fine :)

One of the techniques for dealing with JSON in Go is to not try to parse the entire JSON in one go, but to parse it using smaller structs that only partially match the JSON. e.g. if you endpoint returns either an int or a string, depending on the result, a single struct won't match. But two structs, one with an int and one with a string - that will parse the value and then you can work out which one it was.

[lelanthran]

> It totally makes sense from a Go perspective: You created a struct, tried (but failed) to populate it with some json data, and ended up with a value initialised to its zero-value. This is fine :)

To me it looks like a footgun: if the parsing failed then an error should have been signalled. In this case, there is no error and you silently get the wrong value.

[marcus_holmes]

Yeah, I presented that wrong. It's not actually a failure as such.

[simiones]

> It totally makes sense from a Go perspective: You created a struct, tried (but failed) to populate it with some json data, and ended up with a value initialised to its zero-value. This is fine :)

I do agree that there are good reasons on why this behaves the way it does, but I don't think the reason you cite is good. The implementation detail of generating a 0 value is not a good reason for why you'd implement JSON decoding like this.

Instead, the reason this is not a completely inane choice is that it is sometimes useful to simply not include keys that are meant to have a default value. This is a common practice in web APIs, to avoid excessive verbosity; and it is explicitly encoded in standards like OpenAPI (where you can specify whether a field of an object is required or not).

On the implementation side, I can then get away with always decoding to a single struct, I don't have to define specific structs for each field or combination of fields.

Ideally, this would have been an optional feature, where you could specify in the struct definition whether a fields is required or not (e.g. something like `json:"fieldName;required"` or `json:"fieldName;optional"`). Parsing would fail if any required field was not present in the JSON. However, this would have been more work on the Go team, and they generally prefer to implement something that works and be done with it, rather than working for all important cases.

Separately, ignoring extra fields in the JSON that don't match any fields in the struct is pretty useful for maintaining backwards compatibility. Adding extra fields should not generally break backwards compatibility.

> One of the techniques for dealing with JSON in Go is to not try to parse the entire JSON in one go, but to parse it using smaller structs that only partially match the JSON. e.g. if you endpoint returns either an int or a string, depending on the result, a single struct won't match. But two structs, one with an int and one with a string - that will parse the value and then you can work out which one it was.

I have no idea what you mean here. json.Unmarshal() is an all-or-nothing operation. Are you saying it's common practice to use json.Decoder instead?

[marcus_holmes]

> I have no idea what you mean here. json.Unmarshal() is an all-or-nothing operation. Are you saying it's common practice to use json.Decoder instead?

No, I mean you create a struct that deals with only a part of the JSON, and do multiple calls to Unmarshal. Each struct gets either populated or left at its zero-value depending on what the json looks like. It's useful for parsing json data that has a variable schema depending on what the result was.