# Module Contract Internals

This document describes the module contract in `internal/module`.

## Purpose

`internal/module` defines the shared identifiers and data envelopes used for
prompt-facing modules. Report definitions use module IDs for composition,
module builders produce outputs with stanza names, prompt input packages consume
snapshots, and Recent Changes compares snapshot stanzas.

## Inputs And Outputs

Inputs:

- ordered `module.ConfigItem` values from report definitions or config
  overrides
- `module.Output` values produced by module builders

Outputs:

- stable `module.ID` constants
- typed option structs for registered modules
- `module.Snapshot` with schema version `weatherreporter.modules.v1`
- ordered snapshot outputs with module ID, stanza name, and typed value
- typed stanza lookup through `module.StanzaValue`

## Registered Module IDs

The registry recognizes these IDs:

- `metadata`
- `current_conditions`
- `derived_daily_summary`
- `derived_daypart_summaries`
- `hourly_table`
- `precip_timing`
- `alert_digest`
- `area_forecast_discussion`
- `weather_story`
- `forecast_delta`
- `outdoor_windows`
- `tomorrow_planning`
- `weekend_planning`
- `storm_window_summary`

Modules with builders emit stanzas into module snapshots. Registered modules
without builders are valid composition entries but do not emit snapshot stanzas.
That keeps report composition declarations centralized while limiting prompt
packages to data the application builds.

## Options

Most modules use an empty options struct. `area_forecast_discussion` accepts:

```yaml
sections:
  - product
  - key_messages
  - short_term
  - long_term
```

An omitted or empty `sections` list includes all available discussion sections.
Invalid option shapes fail during config normalization or composition
validation.

## Boundaries

- This package owns module identifiers, config item envelopes, output
  envelopes, snapshot validation, and typed stanza lookup.
- It does not define report IDs, execute builders, fetch weather data, derive
  forecast facts, write state, or invoke Scriptorium.

## State Or Manifest Behavior

`module.Snapshot` values are persisted by `internal/state` as JSON. Snapshot
validation rejects missing schema version, missing module IDs, missing stanza
names, duplicate module outputs, and duplicate stanza names while preserving
output order.

## Failure Behavior

- Snapshot construction fails for duplicate module outputs or duplicate stanza
  names.
- Typed stanza lookup returns `found=false` for missing stanzas.
- Typed stanza lookup wraps JSON marshal/decode failures with stanza context.

## Tests

Inspect:

- `internal/module/module_test.go`
- `internal/briefing/modules_test.go`
- `internal/report/period_test.go`

## Invariants

- `internal/module` does not import `internal/report`.
- Module IDs are stable strings.
- Each emitted module output has exactly one stanza name and one typed value.
- Snapshot output order is caller-owned and preserved.