Group and sum over partition

[raffo0707]

No description provided.

[raffo0707]

group_by: function for grouping over a given field;

• usage: group_by(array, expression)

sum_by: function for summing grouping values over a field.

• usage: sum_by(array, expression, expression)
  (the first expression is for grouping, the second one is the field to sum)

[mtdowling]

These are great function ideas. Before merging these here, I'd like to make sure they are first added as part of the official JMESPath spec. Could you open a PR on jmespath.site to propose these functions with examples? We'll also need compliance tests added to jmespath.test.

One idea I've had for sum_by and related reducing functions is to just add a reduce function to jmespath. We could also add arithmetic operators like "+", "-", etc, or arithmetic functions. Then things like sum_by could become something like:

reduce(foo[*], &carry + item)

We'd need to figure out how to bind carry and item to the reducing function (e.g., do we bind an object with carry and item keys, or a list with [0] and [1], or something else).