[#50] Use maps in the generated Erlang code to handle optional parameters

[robertoaloi]

Currently, the generated Erlang code requires the user to pass all parameters (both the required and optional ones) to the respective functions, eventually using the undefined atom. As you can image, this makes the library useless for Erlang users when it comes to function which accept a significant number of arguments.

What I propose here is to use maps to pass the optional parameters, instead. Before investing too much time on it, I wanted to hear your opinion.

This PR allows to go from the current:

get_object(Client, Bucket, Key, PartNumber, ResponseCacheControl, ResponseContentDisposition, ResponseContentEncoding, ResponseContentLanguage, ResponseContentType, ResponseExpires, VersionId, ExpectedBucketOwner, IfMatch, IfModifiedSince, IfNoneMatch, IfUnmodifiedSince, Range, RequestPayer, SSECustomerAlgorithm, SSECustomerKey, SSECustomerKeyMD5)
  when is_map(Client) ->
    get_object(Client, Bucket, Key, PartNumber, ResponseCacheControl, ResponseContentDisposition, ResponseContentEncoding, ResponseContentLanguage, ResponseContentType, ResponseExpires, VersionId, ExpectedBucketOwner, IfMatch, IfModifiedSince, IfNoneMatch, IfUnmodifiedSince, Range, RequestPayer, SSECustomerAlgorithm, SSECustomerKey, SSECustomerKeyMD5, []).
get_object(Client, Bucket, Key, PartNumber, ResponseCacheControl, ResponseContentDisposition, ResponseContentEncoding, ResponseContentLanguage, ResponseContentType, ResponseExpires, VersionId, ExpectedBucketOwner, IfMatch, IfModifiedSince, IfNoneMatch, IfUnmodifiedSince, Range, RequestPayer, SSECustomerAlgorithm, SSECustomerKey, SSECustomerKeyMD5, Options)
  when is_map(Client), is_list(Options) ->
    Path = ["/", aws_util:encode_uri(Bucket), "/", aws_util:encode_multi_segment_uri(Key), ""],
    SuccessStatusCode = undefined,

    Headers0 =
      [
        {<<"x-amz-expected-bucket-owner">>, ExpectedBucketOwner},
        [...]
      ],
    Headers = [H || {_, V} = H <- Headers0, V =/= undefined],

    Query0_ =
      [
        {<<"partNumber">>, PartNumber},
        [...]
      ],
    Query_ = [H || {_, V} = H <- Query0_, V =/= undefined],

    [...]

To a slightly more user friendly:

 get_object(Client, Bucket, Key)
  when is_map(Client) ->
    get_object(Client, Bucket, Key, #{}, #{}).

get_object(Client, Bucket, Key, QueryMap, HeadersMap)
  when is_map(Client), is_map(QueryMap), is_map(HeadersMap) ->
    get_object(Client, Bucket, Key, QueryMap, HeadersMap, []).

get_object(Client, Bucket, Key, QueryMap, HeadersMap, Options)
  when is_map(Client), is_map(QueryMap), is_map(HeadersMap), is_list(Options) ->
    Path = ["/", aws_util:encode_uri(Bucket), "/", aws_util:encode_multi_segment_uri(Key), ""],
    SuccessStatusCode = undefined,

    Headers0 =
      [
        {<<"x-amz-expected-bucket-owner">>, maps:get(<<"x-amz-expected-bucket-owner">>, HeadersMap, undefined)},
        [...]
      ],
    Headers = [H || {_, V} = H <- Headers0, V =/= undefined],

    Query0_ =
      [
        {<<"partNumber">>, maps:get(<<"partNumber">>, QueryMap, undefined)},
        [...]
      ],
    Query_ = [H || {_, V} = H <- Query0_, V =/= undefined],

    [...]

A few considerations:

• This is my first contribution to the project and my first Elixir code, so there's a high chance that my code is not very idiomatic, if that's the case please advise
• The generated code won't be backward compatible with previous versions of the library
• This is totally untested. Is there any way for me to run some tests?
• I'm not sure whether header parameters can even be required. If they're not, the code can be further simplified.
• I'm only changing the rest module for now.

[robertoaloi]

One of the jobs timed out. Re-running for now to see if it's a deterministic error. Anyone who experienced that before?

[jfacorro]

I think this is one of the things the will make the library a lot more usable.

Regarding testing, sadly the way to test this currently means:

1. Running the generation of the files.
2. Replacing the modules in a local copy of the aws-erlang repository.
3. Run the aws-erlang tests and checks to make sure everything compiles and there are no warnings.
4. Start a rebar3 shell in aws-erlang, create a client and use it to perform some operation in the modified generated modules.

It's manual and it sucks, but it also provides a level of confidence that the code looks and behaves as expected.

[robertoaloi]

I successfully run the Kinesis test from the README, an S3 put and an SSM get and all seem to be fine. I found an encoding issue with S3 GET, but I believe that is not related to this PR, so I will proceed and merge this one.

We really need to get some automation and tests around this.