# Lens

**Last Updated:** 2025-01-31
**Confidence:** High

## Summary

A Lens resolves conflicting assertions into a deterministic answer at read time. Multiple truths coexist; the Lens chooses which to return.

**Key Facts:**
- Stateless compute (no side effects)
- Deterministic (same input = same output)
- Fast (runs on every read, avoid allocations)
- Pluggable (implement `Lens` trait)

**File Pointer:** `crates/stemedb-lens/src/lib.rs` (planned)

## The Trait

```rust
pub trait Lens {
    fn resolve(&self, candidates: &[Assertion], context: &QueryContext) -> LensResult;
}
```

## Standard Lenses

| Lens | Strategy | Use Case |
|------|----------|----------|
| Recency | Latest timestamp wins | News, real-time |
| Consensus | Highest vote count | Democratic truth |
| Authority | Weighted by agent reputation | Expert truth |
| Skeptic | Returns variance/conflict | Finding controversy |

## Query Flow

1. Client: `GET(Subject="Tesla", Predicate="Revenue", Lens="Consensus")`
2. Index lookup: `SP:Tesla:Revenue` -> `[Hash1, Hash2, Hash3]`
3. Hydrate: Load assertions from hashes
4. Resolve: `ConsensusLens.resolve(assertions, context)`
5. Return: Single deterministic answer with confidence

## Related Topics

- [Assertion](./assertion.md)
- [stemedb-lens skill](../../.claude/skills/stemedb-lens/SKILL.md)