Rounding required by the rule

Intention

I want to set the scale and rounding mode of a money amount explicitly.

The policy must be explicit: the scenes below check the positive and negative half-steps and the HALF_UP/HALF_EVEN difference.

Correct formFrozen execution sceneBoundary

Incorrect form and why it stays silent

Incorrect form

pure function Pay(x: Money) -> Money = round(x);

Rounding has no hidden scale or mode. Trailing zeros on a literal do not set legal precision: the arguments of round do.

Correct form

Arxo Law

language "law.core" version "0.2";
package recipes.l.r03 version "0.1.0";
namespace "urn:recipe:l-calc:03";

pure function Pay(x: Money) -> Money = round(x, 2, "HALF_UP");

Frozen execution scene

Facts and choice	Question	Answer
1. positive half-step	Pay(1.005 KZT)	1.01 KZT / COMPUTED
2. negative half-step	Pay(-1.005 KZT)	-1.01 KZT / COMPUTED
3. banker’s rounding of a half	round(1.005 KZT, 2, "HALF_EVEN")	1 KZT / COMPUTED
4. literal zeros do not change the amount	Pay(1.5000 KZT)	1.5 KZT / COMPUTED

positive half step up

Arxo Law

test "positive half step up" {
    given {
        context {
            legal_time @2026-09-13;
            decision_time @2026-09-13T09:00:00+05:00;
            knowledge_time @2026-09-13T09:00:00+05:00;
            timezone "Asia/Almaty";
        }

    }
    evaluate Pay(1.005 KZT);
    expect value == 1.01 KZT;
    expect evaluation_status == COMPUTED;

}

negative half step

Arxo Law

test "negative half step" {
    given {
        context {
            legal_time @2026-09-13;
            decision_time @2026-09-13T09:00:00+05:00;
            knowledge_time @2026-09-13T09:00:00+05:00;
            timezone "Asia/Almaty";
        }

    }
    evaluate Pay(-1.005 KZT);
    expect value == -1.01 KZT;
    expect evaluation_status == COMPUTED;

}

banker's rounding of half

Arxo Law

test "banker's rounding of half" {
    given {
        context {
            legal_time @2026-09-13;
            decision_time @2026-09-13T09:00:00+05:00;
            knowledge_time @2026-09-13T09:00:00+05:00;
            timezone "Asia/Almaty";
        }

    }
    evaluate round(1.005 KZT, 2, "HALF_EVEN");
    expect value == 1 KZT;
    expect evaluation_status == COMPUTED;

}

literal zeros keep sum

Arxo Law

test "literal zeros keep sum" {
    given {
        context {
            legal_time @2026-09-13;
            decision_time @2026-09-13T09:00:00+05:00;
            knowledge_time @2026-09-13T09:00:00+05:00;
            timezone "Asia/Almaty";
        }

    }
    evaluate Pay(1.5000 KZT);
    expect value == 1.5 KZT;
    expect evaluation_status == COMPUTED;

}

Counterfactual

Mutation: round(x, 2, "HALF_UP") → round(x); expected LDC-E2116. Additional counterfactuals are shown as separate table rows.

Boundary

Round at the stage the rule names. For a quotient, first compute it exactly or use div_round — see Division under an explicit policy. Canonical value notation may drop trailing zeros; that is not a change of rounding mode.

Pitfall

Rounding with an unstated scale or mode has no default; the scenes pin both half-steps and both modes.