[Feature Request] Helper function to get the unique values from a list

[milas]

Feature description:
A to_unique() -> list (or equivalent) function to accompany the unique() -> bool CEL helper function.

Supported today:

[1, 1, 2, 3, 3 ].unique()      # false

Proposal:

[1, 1, 2, 3, 3 ].to_unique()   # [1, 2, 3]

.to_unique().unique() is always true.

Problem it solves or use case:
unique() is a helpful protovalidate addition for something you can't do in CEL otherwise. I want the intermediate form, i.e. the set of unique values, exposed to do even more!

Contribution:
I can likely contribute the PR for this - just let me know re: to_unique() naming at least if no other major concerns.

Examples or references:
Example: Ensure all items in a collection are all siblings

this.collection.map(x, x.parent_id).to_unique().size() == 1

Example: Ensure all structpb.Value items are of the same type

google.protobuf.Value value = 1 [
  (buf.validate.field).cel = {
    id: "repeated.same_type",
    message: "all values must be of same type",
    expression: "type(this) != list || this.map(v, type(v)).to_unique().size() <= 1"
                                          //               ^^^^^^^^^^^^^
                                          //             my hopes & dreams
  }
];

(can you guess this is the specific use case I have in mind? 😉)

[rodaine]

I know it's been almost a year, but wanted to offer another option that's valid CEL without creating some intermediate state. The unique function is already pretty expensive to compute, so something CEL native might be better. Instead of building a unique list, and checking its size, you can leverage the all macro:

type(this) != list || this.size() == 0 || this.all(x, type(x) == type(this[0]))
^-- is list           ^-- isn't empty     ^-- everything matches the type of the first elem

[milas]

That's a great suggestion - thank you for the response!