Standalone WASI commands

ExperimentalWASI Preview 2WASI Preview 31 min read

Use a WASI target when a program should run in a component host without a JavaScript loader. Dream supports command components for WASI Preview 2 and Preview 3.

Save this program as hello.dream:

import system;

fun main(): void {
    System.println("Hello from WASI");
}

Compile it and run the resulting component:

dream --target wasm32-wasip2 hello.dream
wasmtime run hello.wasm

The output is Hello from WASI. Compilation also writes readable hello.wat and hello.abi.json files. The compiler uses a pinned WASI SDK. Install it with dreamer toolchain install wasi-sdk before compiling.

For Preview 3, select --target wasm32-wasip3. With the pinned Wasmtime 49.0.4, run the component with its Preview 3 features enabled:

wasmtime run -S p3=y -W component-model-async=y \
    -W component-model-more-async-builtins=y \
    -W component-model-async-stackful=y hello.wasm

Files and network access

The component host controls access to files and networking. Grant a directory explicitly:

wasmtime run --dir ./data::. application.wasm

An ungranted path produces the usual Result.Err from Dream's filesystem APIs. For example:

import system;
import system.io;

async fun main(): void {
    let contents = File.read("input.txt").await;
    if contents.is_err() {
        System.println(contents.unwrap_err().message());
        return;
    }
    System.println(contents.unwrap());
}

Enable HTTP imports with -S http=y. Socket programs also need the host's network permissions; Wasmtime grants them with -S inherit-network=y. Continue to handle Result.Err from network operations because connection failures and host restrictions can still reject a request.

Command components provide stdio, files, clocks, secure randomness, TCP, UDP, and HTTP. HTTP exposes individual responses; an explicit redirect limit reports an unsupported-operation error. Unsupported host capabilities produce a compiler diagnostic. JavaScript interop, windowing, GPU devices, process spawning, and CLI parsing are unavailable on these targets. Use native output for CLI parsing, and JavaScript interop for browser and Node applications.

Components currently expose the command entry point. Exported component libraries require an explicit WIT interface and are rejected by this target.