Hide record fields with default values in to_yaml output

[maurobringolf]

This modifies the to_yaml function to take into account the [@default] attribute of record fields. If the field value is equal (=) to the default value, then it is omitted from the YAML object as proposed in #35.

I can see how this is useful for 'a option fields with default None. But I can imagine that for other types/scenarios this behavior is less useful than the current one. Maybe the library could let the user decide by including an optional argument in to_yaml, e.g.

val person_to_yaml : ?include_defaults:bool -> person -> Yaml.value 

Just a thought. If you have any feedback/suggestions on the implementation I am happy to iterate on it.

[sim642]

I can see how this is useful for 'a option fields with default None. But I can imagine that for other types/scenarios this behavior is less useful than the current one.

Indeed, this also went through my mind when I opened the issue, but ppx_deriving_yojson, which is much more widely used, has always done it this way without much complaining.

There does appear to be an old issue about it there though: ocaml-ppx/ppx_deriving_yojson#19. The more flexible approach suggested there is to make it controllable per-field, instead of globally.

[patricoferris]

Thanks @maurobringolf for the PR (and @sim642 for the research into how others do it!). Given this is how yojson does it, which is used by many, I'm inclined to merge this.

There does appear to be an old issue about it there though: ocaml-ppx/ppx_deriving_yojson#19. The more flexible approach suggested there is to make it controllable per-field, instead of globally.

If people complain I think going the per-field approach would be better, but it doesn't need to be a part of this PR :))