# Tutorial 8: Expressions

The expression system enables declarative logic that compiles to portable canonical JSON. All expressions are evaluable at runtime without recompilation.

## Path Navigation

| Prefix | Source | Example |
|--------|--------|---------|
| `start.*` | Start request fields | `start.policyId` |
| `state.*` | Mutable workflow state | `state.customerName` |
| `payload.*` | Task completion payload | `payload.answer` |
| `result.*` | Step result (by resultKey) | `result.productInfo.lob` |

## Built-in Functions

| Function | Description | Example |
|----------|-------------|---------|
| `coalesce` | First non-null | `coalesce(state.id, start.id, 0)` |
| `concat` | String join | `concat("Policy #", state.policyNo)` |
| `add` | Sum | `add(state.attempt, 1)` |
| `if` | Conditional | `if(state.isVip, "VIP", "Standard")` |
| `isNullOrWhiteSpace` | Null/empty check | `isNullOrWhiteSpace(state.name)` |
| `length` | String/array length | `length(state.items)` |
| `upper` | Uppercase | `upper(state.annexType)` |
| `first` | First array element | `first(state.objects)` |
| `mergeObjects` | Deep merge | `mergeObjects(state, payload)` |

## Variants

- [C# Expression Builder](csharp/)
- [JSON Expression Format](json/)

## Next

[Tutorial 9: Testing](../09-testing/) — unit test setup with recording transports.