Implement support of spread operator

[ericleib]

This Pull Request adds support of the spread operator ... in the Jsonata syntax.

The implementation works the same way as in Javascript:

{
   "a": "hello",
   "b": "world",
   ...foo
}

This copies the key/value pairs from object foo to a new object, in this case potentially overriding properties a and b. Conversely, if a and b were added AFTER foo, then their values would override the a and b entries of foo.

Please let me know if this is something that the community would be interested in. In my case (an avid Jsonata user), this feature would make a big difference.

[andrew-coleman]

Thank you for raising this.

On the face of it, this looks like a great proposal. What's concerning me though is the implied order of execution of the items within the object. Although not a problem for this Javascript implementation, it could be a problem for other implementations. Take a look at this issue for reference. Although that issue was only relating to the test suite, I think this proposal would bring it into the language itself.

I'm not saying that a feature like this is not possible, but we need to be very mindful of this potential problem.

[ericleib]

Thank you for looking into it!

I understand your concern. It does bring a change to the language. In fact when implementing it, I could see the potential inconsistency of behavior with this error "Multiple key definitions evaluate to same key" (which prevents any key overriding mechanism).