Rust Functions
Authoring a Function in the Rust programming language


An AssemblyLift function written in Rust is a library crate. 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.
asml_core = { version = "0.2", package = "assemblylift-core-guest" }
assemblylift_core_io_guest = { version = "0.3", package = "assemblylift-core-io-guest" }
asml_awslambda = { version = "0.3", package = "assemblylift-awslambda-guest" }
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.
Only one handler can be defined per function. Calling the handler! macro more than once will produce a compiler error.
extern crate asml_awslambda;
use asml_core::GuestCore;
use asml_awslambda::{AwsLambdaClient, LambdaContext};
// Input must implement serde Deserialize
type Input = ();
handler!(context: LambdaContext<Input>, async {
// function code
// supports .await
The extern crate statement is required to bring statics & externs into scope which are defined in the crate.
The context value is accessible from within the function closure, and provides access to details of the function invocation, include the input payload and authorization data.

Writing an HTTP Function

If your function has an HTTP API, your function input and output must be compatible with the providers' API Gateway (Amazon API Gateway by default).
The crate assemblylift_awslambda_guest includes structs & macros for working with Amazon API Gateway.
extern crate asml_awslambda;
use asml_core::GuestCore;
use asml_awslambda::{ApiGatewayEvent, AwsLambdaClient, LambdaContext};
handler!(context: LambdaContext<ApiGatewayEvent>, async {
let event: ApiGatewayEvent = context.event;
// Do something with the event
if (function_success) {
let response = "OK"; // This can be any serde::Serialize-able value
} else {
http_error!("There was an error!");
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Handler Definition
Writing an HTTP Function