---
sidebar_position: 3
title: Extractors
description: "A type that implements FromContext becomes a handler parameter, resolved from the delivery before the body runs."
---
import Tabs from '@theme/Tabs';
import TabItem from '@theme/TabItem';
# Extractors
A type that implements [`FromContext`](api:ruststream::runtime::FromContext) becomes a handler parameter, resolved from the delivery before the body runs.
Every parameter of a `#[subscriber]` function after the message and the optional `&mut Context` is an extractor.
The generated handler resolves the extractors in the order they are declared, then runs the body.
`from_context` reads the [context](./delivery-context.md) mutably, so an extractor can read headers, broker fields and the state.
It is async, so it may also do I/O, such as a lookup.
An error is the `Rejection`, which converts into a [`HandlerOutcome`](api:ruststream::runtime::HandlerOutcome): the delivery settles with that [outcome](../messages/handler-outcome.md), and the body does not run.
`HandlerOutcome` itself is a valid rejection; an extractor that cannot fail uses `Infallible`.
A manual handler calls `from_context` at the top of `handle` and returns a rejection as its error.
The impl is generic over the context type `C` and the state `S`, so the extractor works on any subscription.
An extractor that reads a broker field or a typed state implements the trait for those types instead.
[`State<T>`](api:ruststream::runtime::State), the extractor of the application [state](../../application/state/any-struct.md), and [`Ctx<K>`](./ctx.md) are extractors of this kind that the crate provides.
## Example
<Tabs groupId="api">
<TabItem value="macros" label="Macros">
```rust reference="examples/from_context.rs#extractor"
```
</TabItem>
<TabItem value="manual" label="Manual">
```rust reference="examples/manual/from_context.rs#extractor"
```
</TabItem>
</Tabs>