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# Engine Errors

## Overview

Engine configuration validates slot names, primitive names, slot values, and synchronous scopes.
The concrete errors are exported on each `Engine` namespace.

| Error | Description |
| --- | --- |
| [`Engine.AsyncScopeError`](#engineasyncscopeerror) | `Engine.with` receives a function that returns a Promise. |
| [`Engine.InvalidSlotValueError`](#engineinvalidslotvalueerror) | An Engine slot is not an object or `undefined`. |
| [`Engine.UnknownPrimitiveError`](#engineunknownprimitiveerror) | A slot contains an unrecognized primitive name. |
| [`Engine.UnknownSlotError`](#engineunknownsloterror) | An Engine contains an unrecognized slot name. |

## Recipes

### Handle an Asynchronous Scope

Catch `Engine.AsyncScopeError` when a callback passed to
[`Engine.with`](/docs/engine/with) returns a Promise.

```ts twoslash
import { Engine } from 'viem'

try {
  Engine.with({}, async () => {})
} catch (error) {
  if (error instanceof Engine.AsyncScopeError) { // [!code focus]
    console.error('The Engine scope must be synchronous.') // [!code focus]
  } // [!code focus]
}
```

## `Engine.AsyncScopeError`

Thrown when [`Engine.with`](/docs/engine/with) receives an asynchronous function. Initialize an
Engine before entering the scope and keep the callback synchronous.

```ts twoslash
import { Engine } from 'viem'

try {
  Engine.with({}, async () => {})
} catch (error) {
  if (error instanceof Engine.AsyncScopeError) {
    error.name
    //    ^?
  }
}
```

## `Engine.InvalidSlotValueError`

Thrown when an Engine slot is not an object or `undefined`.

## `Engine.UnknownPrimitiveError`

Thrown when a slot contains a primitive that is not part of its contract.

## `Engine.UnknownSlotError`

Thrown when [`Engine.install`](/docs/engine/install), [`Engine.set`](/docs/engine/set), or
[`Engine.reset`](/docs/engine/reset) receives an unrecognized slot.