Add drop_nulls plugin and bump version

[williamthome]

This PR implements the drop_nulls plugin.
It ignores all keys of maps whose values are included in the 'nulls' option of the encoding.

Example

Maps

1> euneus:encode_to_binary(#{a => 1, b => undefined}, #{plugins => [drop_nulls]}).
{ok,<<"{\"a\":1}">>}

2> euneus:encode_to_binary(#{a => 1, b => undefined, c => nil}, #{nulls => [undefined, nil], plugins => [drop_nulls]}).
{ok,<<"{\"a\":1}">>}

Proplists

1> euneus:encode_to_binary([{a, 1}, {b, undefined}], #{plugins => [proplist, drop_nulls]}).
{ok,<<"{\"a\":1}">>}

2> euneus:encode_to_binary([{a, 1}, {b, undefined}, {c, nil}], #{nulls => [undefined, nil], plugins => [proplist, drop_nulls]}).
{ok,<<"{\"a\":1}">>}

Credits

Thanks both @asabil and @leonardb for the suggestions at the Erlang forums topic.

[leonardb]

Sort of surprising the the drop_nulls plugin does not drop nulls by default but converts the null into a string value "null"

4> {ok, JSON} = euneus:encode_to_binary(#{a => 1, b => undefined, c => null}, #{plugins => [drop_nulls]}).
{ok,<<"{\"a\":1,\"c\":\"null\"}">>}

edit: Should the same behavior apply to empty maps? IE, when the recursed processing of a map results in an empty map, should that be treated as a null/undefined? I'm really not sure, but it's not what I'd 'expect'

7> f(), {ok, JSON} = euneus:encode_to_binary(#{a => 1, b => undefined, c => null, e => #{f => undefined}}, #{plugins => [drop_nulls]}).          
{ok,<<"{\"a\":1,\"c\":\"null\",\"e\":{}}">>}

[williamthome]

@leonardb Yes, that's intentional but sounds strange due to the plugin name. The default for the nulls option is [undefined]. Maybe worth changing it to [null, undefined]? Elixir mostly uses 'nil'. Maybe then the default can be [null, undefined, nil]. What about?

[williamthome]

edit: Should the same behavior apply to empty maps? IE, when the recursed processing of a map results in an empty map, should that be treated as a null/undefined? I'm really not sure, but it's not what I'd 'expect'

@leonardb I'm not aware of this. I think the behavior should be the same of the Javascript:

1> JSON.stringify({})
'{}'

2> JSON.stringify({f: undefined})
'{}'

Edit

The output via this PR is:

1> euneus:encode(#{f => undefined}, #{plugins => [drop_nulls]}).
{ok,<<"{}">>}

[leonardb]

@leonardb Yes, that's intentional but sounds strange due to the plugin name. The default for the nulls option is [undefined]. Maybe worth changing it to [null, undefined]? Elixir mostly uses 'nil'. Maybe then the default can be [null, undefined, nil]. What about?

The Principle of Least Surprise (based on the name of the plugin) would seem to dictate that the default be [null, nil].

Since the initial request that launched the feature was around handling of undefined and matching the JSON.stringify behavior, maybe that should stand alone rather than part of what is really a drop_map_keys_for_matching_values plugin. Or maybe just better renaming it?

[williamthome]

The name is based on the nulls option, so all terms in that list would be ignored, you can define anything there and the default, currently, is [undefined].

The reason for this default is because I mostly see undefined as the "null term" for Erlang.
Javascript has null and undefined built-in, but we do not have them in Erlang.
That's the reason the nulls option exists in Euneus. I do not have another name in my mind for this option.

FWIW there is a null_term option for decoding, it also can be anything and the default is undefined, for the same reason of the nulls option.

1> {ok, JSON} = euneus:encode_to_binary(#{a => undefined, b => null, c => nil}, #{nulls => [undefined, null, nil]}).
{ok,<<"{\"c\":null,\"a\":null,\"b\":null}">>}

2> euneus:decode(JSON, #{null_term => nil}).
{ok,#{<<"a">> => nil,<<"b">> => nil,<<"c">> => nil}}

So we cannot have exactly the same behavior of the Javascript.

Edit

The null does not come from Javascript but from the null literal of JSON.

[williamthome]

It's a bit confusing thinking about null, undefined, nil, etc in Erlang vs JSON.

I agree with you when you say about the Principle of Least Surprise, but I'm to implement the drop_nulls plugin as it is in this PR and change it in the future if needed.

Feel free to open an issue about this. I will be happy to discuss it in a dedicated topic and with more people involved :)