> For the complete documentation index, see [llms.txt](https://docs.assemblylift.akkoro.io/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.assemblylift.akkoro.io/master/learn-assemblylift/functions/rust-functions.md).

# Rust Functions

## Dependencies

An AssemblyLift function written in Rust is a [library crate](https://learning-rust.github.io/docs/a4.cargo,crates_and_basic_project_structure.html). It requires several crates to be imported, which provide the necessary plumbing to make our Rust crate an AssemblyLift function. 

When making a new function via `asml make` the generated Cargo.toml should have included these crates. However, you should also ensure that you are using the latest patch version of each.

{% code title="Cargo.toml" %}

```
[dependencies]
asml_core = { version = "0.2", package = "assemblylift-core-guest" }
asml_core_io = { version = "0.2", package = "assemblylift-core-io-guest" }
asml_awslambda = { version = "0.2", package = "assemblylift-awslambda-guest" }
```

{% endcode %}

The *core* crate provides the `GuestCore` trait, which defines an interface for communicating with the cloud runtime (logging, low-level success/error response, etc). This trait is implemented by `AwsLambdaClient` which is provided by the *awslambda* crate.

The *core-io* crate provides the IO system; this is AssemblyLift's `Future` execution system supporting async/await.

## Handler Definition

The *awslambda* crate provides a macro called `handler!` which wraps up all the details of initializing the module, and provides a concise entry point for our function.

The `handler!` macro exports a function called `handler` which provides the entry-point from the runtime host. 

{% hint style="warning" %}
Only one handler can be defined per function. Calling the handler! macro more than once will produce a compiler error.
{% endhint %}

{% code title="lib.rs" %}

```rust
extern crate asml_awslambda;

use asml_core::GuestCore;
use asml_awslambda::{AwsLambdaClient, LambdaContext};

handler!(context: LambdaContext<ApiGatewayEvent>, async {
    // function code
    // supports .await
})
```

{% endcode %}

The `extern crate` statement is required to bring `static`s & `extern`s into scope which are defined in the crate.

The `context` value is accessible from within the function code, and provides access to details of the function invocation, include the input payload and authorization data.