LDC-E2119 — An empty-collection fallback of the wrong kind

What it means

An aggregate over a collection declares its element kind, and the empty-collection fallback must be of that same kind: the fallback becomes the aggregate value when the collection is empty, so a money collection needs a money fallback, not an integer. The usual scalar-widening rule does not apply here — the fallback is the aggregate’s own value, not an arithmetic operand.

Rewriting the fallback in the element kind clears it. The message names both kinds found, which shows which side to adjust.

Example

Arxo Law

language "law.core" version "0.2";
package demo.diagnostics version "0.1.0";
namespace "urn:law:demo:diagnostics";

entity Order;
relation amount(x: Order, a: Money) kind empirical;
relation total(x: Order, a: Money) kind institutional;
rule R strict {
  for o: Order; for t: Money;
  when amount(o, t);
  then total(o, sum(collect all a: Money, x: Order where amount(x, a), empty: 0));
}

Fix

Arxo Law

language "law.core" version "0.2";
package demo.diagnostics version "0.1.0";
namespace "urn:law:demo:diagnostics";

entity Order;
relation amount(x: Order, a: Money) kind empirical;
relation total(x: Order, a: Money) kind institutional;
rule R strict {
  for o: Order; for t: Money;
  when amount(o, t);
  then total(o, sum(collect all a: Money, x: Order where amount(x, a), empty: 0 KZT));
}

Engine message

The engine reports this in its own wording:

Output

example.law:11:79: error LDC-E2119: sum: empty-collection policy kind Integer does not match its element kind Money (§59). The §49 scalar rule does not apply here: the policy is not an arithmetic operand but the aggregate's OWN VALUE — before errata E-0195 such an entry silently changed the answer kind

Related

• Expressions and quantities — how aggregate calls are written.