Comment by kbolino
20 hours ago
There's some other issues, too.
For JSON, you can't encode Optional[T] as nothing at all. It has to encode to something, which usually means null. But when you decode, the absence of the field means UnmarshalJSON doesn't get called at all. This typically results in the default value, which of course you would then re-encode as null. So if you round-trip your JSON, you get a materially different output than input (this matters for some other languages/libraries). Maybe the new encoding/json/v2 library fixes this, I haven't looked yet.
Also, I would usually want Optional[T]{value:nil,exists:true} to be impossible regardless of T. But Go's type system is too limited to express this restriction, or even to express a way for a function to enforce this restriction, without resorting to reflection, and reflection has a type erasure problem making it hard to get right even then! So you'd have to write a bunch of different constructors: one for all primitive types and strings; one each for pointers, maps, and slices; three for channels (chan T, <-chan T, chan<- T); and finally one for interfaces, which has to use reflection.
For JSON I just marshall to/from:
For nil, that's interesting. I've never ran into issues there, so I never considered it.
Ideally, I would want Optional[T] to encode the same as T when a value is present, and to encode in a configurable way when the value is absent. Admittedly, the nothing to null problem exists with *T too, and even with *T and `json:",omitempty"`, you get the opposite problem (null turns to nothing). I didn't think about that at the time, so it's really more of an issue with encoding/json rather than Optional[T] per se. However, you can't implement MarshalJSON and output nothing as far as I know.