proposal: encoding/json: "nonil" struct tag to marshal nil slices and maps as non-null

[pjebs]

There have been many reports about encoding a nil slice or map as [ ] or { } respectively.

See: #2278

Currently it is encoded as null. This is a source of bugs in many non-theoretical cases (eg consumers of JSON APIs). There are many issues on Google about others falling into this pitfall so it is at least not uncommon.

It would be nice if a struct tag can be used to signify that we intend the nil slice or map to be treated as an empty slice or map respectively for the purposes of marshalling.

Current solutions include: https://github.com/helloeave/json. This is not a nice solution because it requires the assistance of a third-party package.

This is how it's implemented in third party lib: homelight/json@7b6e442

My PR can be found here: #27813 (subject to an agreed struct tag name, which when decided, can be updated in the PR)

[mvdan]

I've retitled this to be a proposal.

I'm not sure if it's a good idea, but it's certainly feasible and wouldn't add much new API. /cc @bradfitz @dsnet

[pjebs]

For slices and maps, it should be noted that the proposed struct tag is mutually exclusive to omitempty. For backwards compatibility, if both omitempty and the new struct tag is present, then omitempty takes precedence.

[pjebs]

Here is my pull request. I hope it gets accepted because I believe it adds value to the "Go" programming language.

#27813

[pjebs]

I called the struct tag: nonil

[bradfitz]

"heed" is a unique word, at least in terms of Go's existing API.

If this is accepted (once it's reviewed together with all the existing JSON proposals), I suspect we'd want to change "heed" to something else.

[pjebs]

I scoured the dictionary and thesaurus for appropriate words that were short and exemplified the intention but couldn't find anything. Hopefully a better word can be found because I was initially hesitant of the word too - although admittedly it is growing on me.

[pjebs]

@bradfitz Any idea of the timeframe to when all the JSON proposals will be examined? Some of them are now 2+ years. The pivotal proposal is 7+ years.

[QtRoS]

@bradfitz @pjebs what about nullsafe or nullfree or nonull?
+1 that heednull is unacceptable.