Add ability to constrain elements to have distinct nested values.

[popematt]

Problem

Sometimes a list of elements need to be distinct by a specific nested value. For example, we might want to ensure that list of customers have different customer IDs.

[
  { customer_id: 101, name: "Alice" },
  { customer_id: 102, name: "Bob" },
  { customer_id: 103, name: "Bob" },
  { customer_id: 104, name: "Cassidy" },
  { customer_id: 104, name: "Dave" },
]

Using the ISL 2.0 element: distinct::customer, the best we can do is ensure that no two customers have the ID and the same name.

Proposal

This is an admittedly incomplete straw-man proposal, intended to start further discussion about the topic.

type::{
  name: passenger_list,
  type: list,
  element: passenger,
  // Each passenger on the flight must have a distinct rewards account.
  // We specify the location using a "path" syntax where an empty sexp indicates the (relative) root of the child element
  // and each symbol indicates a field name that should be traversed. An integer value indicates indexing into a list or sexp.
  element_distinct_by: (rewards_account id)
}

type::{
  name: passenger,
  fields: {
    rewards_account: {
      fields: { 
        id: int
      }
    },
    name: string,
  }
}

Sharp edges

This is not necessarily a complete list.

• What if we want elements to be distinct by field "foo", but the struct does not have a field "foo"?
• What if we want elements to be distinct by field "foo", but the struct has more than one field "foo"?
• What if we want elements to be distinct by field "foo", but one of the container elements is not a struct (and therefore does not have field names)?
• What if our list contains elements that are a union of two types, and we want the elements to all be distinct, but the field in question is foo for variant 1, but field bar in variant 2?