[Feature Request] Support JSON Patch, Diff and (especially) Merge Patch

[opelx]

As far I've seen, there is no support for JSON Patch, Diff and Merge Patch. Following JSON Patch and Diff and JSON Merge Patch it is an proposed standard RFC 6902 and RFC 7386. IMO, even the JSON merge patch has a practical use case: using a built-in JSON document for configuration/settings and having the user overwrite a small subset of it with a read-in configuration file from the filesystem. This in turn also allows dumping the default values to the filesystem for the user (or even the complete 'patched' settings).

[grisumbras]

I'm not principally opposed to the idea of supporting JSON Patch, but our current priorities are with different things. So, the quickest way to get this functionality is to write it yourself :wink:

Merge Patch seems like a very simple function. It's more suited for an example rather than a full-blown library function.

[doganulus]

I agree with the need for JSON Merge Patch as described by the original poster. I think it will be better if the library supports commonly used standards in the JSON ecosystem.

Here I have done a little search and found a related discussion about the merge operation:

• https://github.com/boostorg/json/issues/754

There I have tested the implementations given by @vinniefalco and @grisumbras but they deal with a limited case of object mergers. The functionality described by RFC 7386 requires more than that.

What we need is to:

• Insert or assign to keys specified in the patch
• Erase keys having null values specified in the patch
• Apply the algorithm recursively

See the expected behavior from the RFC 7386 document as follows:

SECTION("RFC 7386 Example")
{
  auto target = boost::json::value{
    {"title", "Goodbye!"},
    {"author", {{"givenName", "John"}, {"familyName", "Doe"}}},
    {"tags", {"example", "sample"}},
    {"content", "This will be unchanged"}};

  auto patch = boost::json::value{
    {"title", "Hello!"},
    {"author", {{"familyName", nullptr}}},
    {"phoneNumber", "+01-123-456-7890"},
    {"tags", {"example"}}};

  auto expected = boost::json::value{
    {"title", "Hello!"},
    {"author", {{"givenName", "John"}}},
    {"tags", {"example"}},
    {"phoneNumber", "+01-123-456-7890"},
    {"content", "This will be unchanged"}};

  boost::json::merge_patch(target, patch);

  REQUIRE(target == expected);
}

Please find my candidate implementation below. What do you think? Would you want to add it into the library?

auto merge_patch(value& target, value const& patch) -> value&
{
  if(patch.kind() == kind::object) {
    if(target.kind() != kind::object) {
      target = object();
    }
    for(auto const& p : patch.get_object()) {
      if(p.value().kind() == kind::null) {
        target.get_object().erase(p.key());
      }
      else {
        auto* it = target.get_object().if_contains(p.key());
        if(it != nullptr) {
          merge_patch(*it, p.value());
        }
        else {
          target.get_object().insert(p);
        }
      }
    }
  }
  else {
    target = patch;
  }
  return target;
}

[grisumbras]

Now that I see the semantics of this merge, I am a bit underwhelmed. I use a similar merge function in my personal projects, but mine also appends arrays. This makes me think that this merge is a bit too opinionated. What do you think, @vinniefalco, add this as API or just as an example?

[doganulus]

Following the practice of Boost.JSON, a merge_options struct can be defined to configure the merge operation in the API.

Further I found a more granular control is achieved using mergeStrategy keyword in the JSON Schema in some libraries. See a related issue from a JSON Merge library for Python:

• https://github.com/avian2/jsonmerge/issues/28

This way the schema value defines how the merge operation behaves for user-defined keys.