# Read an external limit from a snapshot

## Intention

I want to obtain an external indicator from a precisely pinned data set.

An external indicator is read only from a presented snapshot; there is no ambient access.

## Incorrect form and why it stays silent

Declaring a function without a link to a snapshot does not specify a reproducible call.

```law title="Incorrect form"
external snapshot function limit(key: Text) -> Decimal {
    capability "recipe.limit/1";
}
```
Even a complete declaration without a presented snapshot yields EXTERNAL_SNAPSHOT_MISSING.

## Correct form

The package declares the contract; the scene presents the resource `resources/04-limit.json`. Its hash is produced by the neighbouring generator, with no semantics of its own.

```law
language "law.core" version "0.2";
package recipes.n.r04 version "0.1.0";
namespace "urn:recipe:n-package:04";

external snapshot function limit(key: Text) -> Decimal {
    capability "recipe.limit/1";
    snapshot_from context.limits;
}
relation measured(key: Text, amount: Decimal);
relation exceeds(key: Text);
rule Exceeds strict {
    for key: Text;
    for amount: Decimal;
    when measured(key, amount) and amount > limit(key);
    then exceeds(key);
}
```

## Frozen execution scene

| Facts | Question | Answer |
|---|---|---|
| measured("lead",2.0), snapshot limit 1.0 | exceeds("lead") | TRUE_ONLY, COMPUTED |
| Same facts, no snapshot | exceeds("lead") | NEITHER, EXTERNAL_SNAPSHOT_MISSING |

The external snapshot is passed as an evaluation-request input; the command-line test runner has no such argument, so for this scene the test runner checks both evaluators through the same lowered test.

```law
test "limit reads from snapshot" {
    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";
        }
        assert measured("lead", 2.0);
    }
    evaluate truth(exceeds("lead"));
    expect truth_status == TRUE_ONLY;
    expect evaluation_status == COMPUTED;
}
```

```law
test "missing snapshot refuses external call" {
    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";
        }
        assert measured("lead", 2.0);
    }
    evaluate truth(exceeds("lead"));
    expect truth_status == NEITHER;
    expect issue(EXTERNAL_SNAPSHOT_MISSING);
}
```

## Counterfactual

The teaches mutation removes snapshot_from and pins a static rejection. The second scene drops the resource under the same complete declaration and facts: there is no network lookup of a missing indicator.

## Boundary

This is a synthetic limit, not a live rate. Exchange rates, registries, and measurements are pinned by the supplier. Snapshot completeness and the policy for choosing a value belong to the subject-matter package; data is not requested by URL during computation. Request inputs are schema-checked before evaluation: the pinned resource carries a snapshot schema version, and without it the request itself is refused.

## Pitfall

Absence of a set is not a zero rate. The second scene holds EXTERNAL_SNAPSHOT_MISSING; the first applies the pinned value for real.