Running Rust on Wippy

Build a Rust WebAssembly component and run it as functions, CLI commands, and HTTP endpoints.

What We're Building

A Rust component with four exported functions:

  • greet - Takes a name, returns a greeting
  • add - Adds two integers
  • fibonacci - Computes the nth Fibonacci number
  • list-files - Lists files in a mounted directory

We'll expose these as callable functions, a CLI command, and an HTTP endpoint.

Prerequisites

rustup target add wasm32-wasip1
cargo install cargo-component

Project Structure

rust-wasm-demo/
├── demo/                    # Rust component
│   ├── Cargo.toml
│   ├── wit/
│   │   └── world.wit       # WIT interface
│   └── src/
│       └── lib.rs           # Implementation
└── app/                     # Wippy application
    ├── wippy.lock
    └── src/
        ├── _index.yaml      # Infrastructure
        └── demo/
            ├── _index.yaml  # CLI processes
            └── wasm/
                ├── _index.yaml          # WASM entries
                └── demo_component.wasm  # Compiled binary

Step 1: Create the WIT Interface

WIT (WebAssembly Interface Types) defines the contract between host and guest:

Create demo/wit/world.wit:

package component:demo;

world demo {
    export greet: func(name: string) -> string;
    export add: func(a: s32, b: s32) -> s32;
    export fibonacci: func(n: u32) -> u64;
    export list-files: func(path: string) -> string;
}

Each export becomes a function that Wippy can call.

Step 2: Implement in Rust

Create demo/Cargo.toml:

[package]
name = "demo"
version = "0.1.0"
edition = "2024"

[dependencies]
wit-bindgen-rt = { version = "0.44.0", features = ["bitflags"] }

[lib]
crate-type = ["cdylib"]

[profile.release]
opt-level = "s"
lto = true

[package.metadata.component]
package = "component:demo"

Create demo/src/lib.rs:

#[allow(warnings)]
mod bindings;

use bindings::Guest;

struct Component;

impl Guest for Component {
    fn greet(name: String) -> String {
        format!("Hello, {}!", name)
    }

    fn add(a: i32, b: i32) -> i32 {
        a + b
    }

    fn fibonacci(n: u32) -> u64 {
        if n <= 1 {
            return n as u64;
        }
        let (mut a, mut b) = (0u64, 1u64);
        for _ in 2..=n {
            let next = a + b;
            a = b;
            b = next;
        }
        b
    }

    fn list_files(path: String) -> String {
        let mut result = String::new();
        match std::fs::read_dir(&path) {
            Ok(entries) => {
                for entry in entries {
                    match entry {
                        Ok(e) => {
                            let name = e.file_name().to_string_lossy().to_string();
                            let meta = e.metadata();
                            let (kind, size) = match meta {
                                Ok(m) => {
                                    let kind = if m.is_dir() { "dir" } else { "file" };
                                    (kind, m.len())
                                }
                                Err(_) => ("?", 0),
                            };
                            let line = format!("{:<6} {:>8}  {}", kind, size, name);
                            println!("{}", line);
                            result.push_str(&line);
                            result.push('\n');
                        }
                        Err(e) => {
                            let line = format!("error: {}", e);
                            eprintln!("{}", line);
                            result.push_str(&line);
                            result.push('\n');
                        }
                    }
                }
            }
            Err(e) => {
                let line = format!("cannot read {}: {}", path, e);
                eprintln!("{}", line);
                result.push_str(&line);
                result.push('\n');
            }
        }
        result
    }
}

bindings::export!(Component with_types_in bindings);

The bindings module is generated by cargo-component from the WIT definition.

Step 3: Build the Component

cd demo
cargo component build --release

This produces target/wasm32-wasip1/release/demo.wasm. Copy it to your Wippy app:

mkdir -p ../app/src/demo/wasm
cp target/wasm32-wasip1/release/demo.wasm ../app/src/demo/wasm/demo_component.wasm

Get the SHA-256 hash for integrity verification:

sha256sum ../app/src/demo/wasm/demo_component.wasm

Step 4: Wippy Application

Infrastructure

Create app/src/_index.yaml:

version: "1.0"
namespace: demo

entries:
  - name: gateway
    kind: http.service
    meta:
      comment: HTTP server
    addr: ":8090"
    lifecycle:
      auto_start: true

  - name: api
    kind: http.router
    meta:
      comment: Public API router
      server: demo:gateway
    prefix: /

  - name: processes
    kind: process.host
    lifecycle:
      auto_start: true

  - name: terminal
    kind: terminal.host
    lifecycle:
      auto_start: true

WASM Functions

Create app/src/demo/wasm/_index.yaml:

version: "1.0"
namespace: demo.wasm

entries:
  - name: assets
    kind: fs.directory
    meta:
      comment: Filesystem with WASM binaries
    directory: ./src/demo/wasm

  - name: greet_function
    kind: function.wasm
    meta:
      comment: Greet function via payload transport
    fs: demo.wasm:assets
    path: /demo_component.wasm
    hash: sha256:YOUR_HASH_HERE
    method: greet
    pool:
      type: inline

  - name: add_function
    kind: function.wasm
    meta:
      comment: Add function via payload transport
    fs: demo.wasm:assets
    path: /demo_component.wasm
    hash: sha256:YOUR_HASH_HERE
    method: add
    pool:
      type: inline

  - name: fibonacci_function
    kind: function.wasm
    meta:
      comment: Fibonacci function via payload transport
    fs: demo.wasm:assets
    path: /demo_component.wasm
    hash: sha256:YOUR_HASH_HERE
    method: fibonacci
    pool:
      type: inline

Key points:

  • A single fs.directory entry provides the WASM binary
  • Multiple functions reference the same binary with different method values
  • The hash field verifies binary integrity at load time
  • inline pool creates a fresh instance per call

Functions with WASI

The list-files function accesses the filesystem, so it needs WASI imports:

  - name: list_files_function
    kind: function.wasm
    meta:
      comment: Filesystem listing with WASI mounts
    fs: demo.wasm:assets
    path: /demo_component.wasm
    hash: sha256:YOUR_HASH_HERE
    method: list-files
    imports:
      - wasi:cli
      - wasi:io
      - wasi:clocks
      - wasi:filesystem
    wasi:
      mounts:
        - fs: demo.wasm:assets
          guest: /data
    pool:
      type: inline

The wasi.mounts section maps a Wippy filesystem entry to a guest path. Inside the WASM module, /data points to the demo.wasm:assets directory.

CLI Commands

Create app/src/demo/_index.yaml:

version: "1.0"
namespace: demo.cli

entries:
  - name: greet
    kind: process.wasm
    meta:
      comment: Greet someone via WASM
      command:
        name: greet
        short: Greet someone via WASM
    fs: demo.wasm:assets
    path: /demo_component.wasm
    hash: sha256:YOUR_HASH_HERE
    method: greet

  - name: ls
    kind: process.wasm
    meta:
      comment: List files from mounted WASI filesystem
      command:
        name: ls
        short: List files from mounted directory
    fs: demo.wasm:assets
    path: /demo_component.wasm
    hash: sha256:YOUR_HASH_HERE
    method: list-files
    imports:
      - wasi:cli
      - wasi:io
      - wasi:clocks
      - wasi:filesystem
    wasi:
      mounts:
        - fs: demo.wasm:assets
          guest: /data

The meta.command block registers the process as a named CLI command. The greet command needs no WASI imports since it only uses string operations. The ls command needs filesystem access.

HTTP Endpoint

Add to app/src/demo/wasm/_index.yaml:

  - name: http_greet
    kind: function.wasm
    meta:
      comment: Greet exposed via wasi-http transport
    fs: demo.wasm:assets
    path: /demo_component.wasm
    hash: sha256:YOUR_HASH_HERE
    method: greet
    transport: wasi-http
    pool:
      type: inline

  - name: http_greet_endpoint
    kind: http.endpoint
    meta:
      comment: HTTP POST endpoint for WASM greet
      router: demo:api
    method: POST
    path: /greet
    func: http_greet

The wasi-http transport maps HTTP request/response context to WASM arguments and results.

Step 5: Initialize and Run

cd app
wippy init

Run CLI Commands

# List available commands
wippy run list

Available commands:
  greet    Greet someone via WASM
  ls       List files from mounted directory

# Run greet
wippy run greet

# Run ls to list mounted directory
wippy run ls

Run as a Service

wippy run

This starts the HTTP server on port 8090. Test the endpoint:

curl -X POST http://localhost:8090/greet

Call from Lua

WASM functions are called the same way as Lua functions:

local funcs = require("funcs")

local greeting, err = funcs.call("demo.wasm:greet_function", "World")
-- greeting: "Hello, World!"

local sum, err = funcs.call("demo.wasm:add_function", 6, 7)
-- sum: 13

local fib, err = funcs.call("demo.wasm:fibonacci_function", 10)
-- fib: 55

Next Steps