Root node access

[waynevanson]

I would like to have access to the data I've passed in within any part of query.

Currently expressions in official implementations, only access to the current node is allowed.

• Similar in spirit to Allow reference to external parameter #33 but only scoped for the initial data.
• Someone made an implementation like this in https://github.com/daz-is/jmespath.js, where the root node is $. I do not know if it goes against any other part of the spec yet.

[springcomp]

For the record, we have submitted the $ - root-node reference proposal.