Add join function

[oeyh]

Description

Adds a join function that takes a source key to a list or a map where values are all of list type, and joins list to string with the given delimiter (optional).

It can be used in processors that support value_expression such as add_entries:

  processor:
    - add_entries:
        entries:
          - key: result
            value_expression: join(/source)

With input {"source": [1, 2, 3]}, it will get you {"result": "1,2,3"}
With input

{"source": {"key1": [1, 2, 3], "key2": ["a", "b", "c"]}}

, it will get you

{"result": {"key1": "1,2,3", "key2": "a,b,c"}}

Issues Resolved

Contributes to #3965

Check List

• New functionality includes testing.
• New functionality has a documentation issue. Please link to it in this PR.
  • New functionality has javadoc added
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[dlvenable]

This is a good change. Thanks!

I wonder if we should change the join arguments to make it more extensible in the future.

If we put the delimiter first, then we could potentially have a varargs function in the future.

join(',', request_headers, response_headers)

[dlvenable]

...WithComma...

Perhaps rename this to:

...WithUnescapedComma...

[dlvenable]

This test was explicitly named _StringIT. This may be an indication that we should have a different IT for _MultiTypeIT. Would it duplicate a lot of code?

Thoughts?

[oeyh]

Would it duplicate a lot of code?

That's the reason why I renamed the test class and having new tests for Map here as well. They are using the same GenericExpressionEvaluator, the only difference is the type casting after evaluation. My thought is that we can use this MultiTypeIT class to hold all future types returned by functions, Map, List, etc.

[kkondaka]

Does this mean, we allow join(/aaa, /bbb, /ccc, /ddd) and it takes just "/aaa" and ignores rest? If yes, that seems confusing that we silently ignore other parameters.

[oeyh]

Currently it only supports one or two parameters. There's a check on number of arguments at the beginning, so join(/aaa, /bbb, /ccc, /ddd) won't pass that.

[dlvenable]

So we don't currently support join(/aaa, /bbb, /ccc, /ddd), but the method could be extended to support join(/aaa, /bbb, /ccc, /ddd). We would just need to update the code for it.