Implement with/2 and without/2

[nalundgaard]

Resolves #35.

See README updates in the commit for implementation notes.

[nalundgaard]

@mrallen1 just a quick WIP implementation that I think does all the things for with, based on checking:

6> Object = #{<<"foo">> => #{<<"bar">> => 1, <<"baz">> => 1, <<"fum">> => [1,2,3]}}.
#{<<"foo">> =>
      #{<<"bar">> => 1,<<"baz">> => 1,<<"fum">> => [1,2,3]}}
7> jsn:with([<<"foo.bar">>, <<"foo.baz">>, <<"foo.fum.nope">>, {<<"foo">>, <<"fum">>, last},{<<"foo">>, <<"fum">>, first}], jsn:as_proplist(Object)).
[{<<"foo">>,
  [{<<"bar">>,1},{<<"baz">>,1},{<<"fum">>,[1,3]}]}]
8> jsn:with([<<"foo.bar">>, <<"foo.baz">>, <<"foo.fum.nope">>, {<<"foo">>, <<"fum">>, 2},{<<"foo">>, <<"fum">>, first}], jsn:as_proplist(Object)).
[{<<"foo">>,
  [{<<"bar">>,1},{<<"baz">>,1},{<<"fum">>,[1,2]}]}]
9> jsn:with([<<"foo.bar">>, <<"foo.baz">>, <<"foo.fum.nope">>, {<<"foo">>, <<"fum">>, 2},{<<"foo">>, <<"fum">>, first}], Object).
#{<<"foo">> =>
      #{<<"bar">> => 1,<<"baz">> => 1,<<"fum">> => [1,2]}}

Before I write unit tests, I am curious if you agree with this approach. Note that it takes any JSON value, and if you ask for paths in something that doesn't have paths, you get that thing back. So like, if an intermediate element of your path is an array (or value), you just get that thing back at that layer, and it doesn't dig further or raise an exception.

I'm not sure what the right approach is. Raising exceptions sounds annoying as heck, but also it sounds tedious to take a set of deep paths and give back a shallow object with only top-level matches without telling the user, "OK, I did what you asked, but you might not like it—maybe you should validate your inputs better!"

[nalundgaard]

... Obviously, code organization, documentation, unit tests are all TODO.

[jadeallenx]

I think the behavior of with/2 should mimic the behavior of maps:with/2 which is to say if a specified (possibly deep) key doesn't exist, it's ignored in the o/p.

[va17]

First take LGTM. I also think we should go down the path of mimicking maps:with/2 and not return errors, just ignore missing paths.

[nalundgaard]

OK, sounds like my implementation is on the right track. I added without/2 and fixed a little bit of missing functionality, including array index handling, support for atom keys in path elements, and support for passing a pre-built path element map into the function (which will definitely be needed for use on lists of things).

[nalundgaard]

OK, implementation and unit tests done, along with basic function documentation. Will add a README section for the 2 functions as well, along with the fun caveats about optimization (pass a pre-computed path elements map when applying repeatedly) and atom keys (only supports atom keys that are in the atom table when the path elements map is constructed, and won't work if applied to structures with atom keys that are added to the atom table later (TL;DR: don't use atom keys in JSON representation).

Curious what you think of the implementation.

[nalundgaard]

Alright—I've written the README. I think this is ready to be reviewed as soon as you have the time, @mrallen1.

[jadeallenx]

Great.

[rmpalomino]

Just some pedantic comments.

[rmpalomino]

Is it actually feasible to check if an atom is in the atom table? I feel like like the recommendation here is better left out.

[nalundgaard]

Yes, binary_to_existing_atom/2 is how we check. As for ensuring that they're present, you just have to reference them in compiled code. This is not better left out IMO.