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LDC-E2113 — An external snapshot function missing required items

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# LDC-E2113 — An external snapshot function missing required items

## What it means

An external snapshot function is a typed boundary to outside data,
and the boundary is only complete with all its pieces: a result type,
the capability it runs under, and a binding path saying where the
snapshot comes from. A missing piece — or a binding that is not a
plain dotted path — leaves the boundary underspecified, and the
compiler names exactly which item is absent.

Supplying each required item, with the binding as a dotted path,
completes the declaration and clears the diagnostic.

## Example

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

entity Currency;
external snapshot function rate(from: Currency) -> Decimal {
  capability "nbk.exchange-rates";
  snapshot_from resolve_snapshot();
}
```

## Fix

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

entity Currency;
external snapshot function rate(from: Currency) -> Decimal {
  capability "nbk.exchange-rates";
  snapshot_from context.exchange_rate_snapshot;
}
```

## Engine message

The engine reports this in its own wording:

```text
example.law:8:17: error LDC-E2113: external snapshot function "rate": `snapshot_from` must be a dotted binding path, because ExternalDecl.snapshotBinding is a string (§47.2, legal-ir.schema.json)
```

## Related

- [Sources](/constructs/sources/) — where outside data enters a package.