# One condition language for quests, dialogue and spawns

> Lantern Valley writes quest, dialogue and spawn rules in one plain-data condition language, evaluated by a pure function that tests can drive.

Canonical page: https://andersonmancini.dev/blog/lantern-valley-quest-conditions

Published: 2026-09-30 · 1 min read · Tags: game-dev, javascript, data-driven, lantern-valley, design

In [Lantern Valley](https://andersonmancini.dev/blog/lantern-valley.md), quests, dialogue rules and spawns are all written in **one condition language** made of plain data, evaluated by one pure function. That is what keeps content out of engine code.

## What a condition looks like

Combinators first:

```js
{ all: [ ... ] }   { any: [ ... ] }   { not: c }   // a bare list means "all"
```

Then the questions you can ask:

```js
{ flag: 'name' }                         { quest: 'id', state: 'done' }
{ questStep: ['id', 's2'] }              { has: { item: 'id', n: 1 } }
{ hasKind: { kind: 'fish', n: 2 } }      { entity: 'id', is: 'broken' }
{ time: [20, 5] }                        { weather: 'rain' }
{ island: 'atol' }                       { friend: { npc: 'id', min: 20 } }
{ caught: 'ebb_snook' }
```

Time ranges wrap past midnight. `hasFamily` counts whole fruit of one tree kind, and the bruised do not count. `hasMix` asks for n items out of a list of at least k different ones, which is how a cart asks for "six pies, two kinds".

## The evaluator is pure

`evalCond(cond, ctx)` takes a context that answers the questions: flags, quest state, counts, entities. It has no access to the world, so a test can build a fake context and check every rule.

## Why one language

The same sentences work in three places: a quest can be available, a dialogue line can be chosen, and a spawn can happen. A designer learns one vocabulary, and the cost of a new rule kind is paid once.

## The same idea elsewhere

The tide is [a function of the hour](https://andersonmancini.dev/blog/lantern-valley-tides-as-rules.md), and the state this language reads is stored by [the save system](https://andersonmancini.dev/blog/lantern-valley-save-that-never-loses-progress.md). For the quest guide on top of it, see [the quest guide post](https://andersonmancini.dev/blog/lantern-valley-quest-guide.md). The project is on its [project page](https://andersonmancini.dev/projects/lantern-valley.md).

## Related projects

- [Lantern Valley](https://andersonmancini.dev/projects/lantern-valley.md) — Cozy adventure game for the browser