> 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?