Embedding Wren in Hare August 20, 2025 on Drew DeVault's blog

I’ve been on the lookout for a scripting language which can be neatly embedded into Hare programs. Perhaps the obvious candidate is Lua – but I’m not particularly enthusiastic about it. When I was evaluating the landscape of tools which are “like Lua, but not Lua”, I found an interesting contender: Wren.

I found that Wren punches far above its weight for such a simple language. It’s object oriented, which, you know, take it or leave it depending on your use-case, but it’s very straightforwardly interesting for what it is. I found a few things to complain about, of course – its scope rules are silly, the C API has some odd limitations here and there, and in my opinion the “standard library” provided by wren CLI is poorly designed. But, surprisingly, my list of complaints more or less ends there, and I was excited to build a nice interface to it from Hare.

The result is hare-wren. Check it out!

The basic Wren C API is relatively straightforwardly exposed to Hare via the wren module, though I elected to mold it into a more idiomatic Hare interface rather than expose the C API directly to Hare. You can use it something like this:

use wren;

export fn main() void = {
	const vm = wren::new(wren::stdio_config);
	defer wren::destroy(vm);
	wren::interpret(vm, "main", `
		System.print("Hello world!")
	`)!;
};

$ hare run -lc main.ha
Hello world!

Calling Hare from Wren and vice-versa is also possible with hare-wren, of course. Here’s another example:

use fmt;
use wren;

export fn main() void = {
	let config = *wren::stdio_config;
	config.bind_foreign_method = &bind_foreign_method;

	const vm = wren::new(&config);
	defer wren::destroy(vm);

	wren::interpret(vm, "main", `
	class Example {
		foreign static greet(user)
	}

	System.print(Example.greet("Harriet"))
	`)!;
};

fn bind_foreign_method(
	vm: *wren::vm,
	module: str,
	class_name: str,
	is_static: bool,
	signature: str,
) nullable *wren::foreign_method_fn = {
	const is_valid = class_name == "Example" &&
		signature == "greet(_)" && is_static;
	if (!is_valid) {
		return null;
	};
	return &greet_user;
};

fn greet_user(vm: *wren::vm) void = {
	const user = wren::get_string(vm, 1)!;
	const greeting = fmt::asprintf("Hello, {}!", user)!;
	defer free(greeting);
	wren::set_string(vm, 0, greeting);
};

$ hare run -lc main.ha
Hello, Harriet!

In addition to exposing the basic Wren virtual machine to Hare, hare-wren has an optional submodule, wren::api, which implements a simple async runtime based on hare-ev and a modest “standard” library, much like Wren CLI. I felt that the Wren CLI libraries had a lot of room for improvement, so I made the call to implement a standard library which is only somewhat compatible with Wren CLI.

On top of the async runtime, Hare’s wren::api runtime provides some basic features for reading and writing files, querying the process arguments and environment, etc. It’s not much but it is, perhaps, an interesting place to begin building out something a bit more interesting. A simple module loader is also included, which introduces some conventions for installing third-party Wren modules that may be of use for future projects to add new libraries and such.

Much like wren-cli, hare-wren also provides the hwren command, which makes this runtime, standard library, and module loader conveniently available from the command line. It does not, however, support a REPL at the moment.

I hope you find it interesting! I have a few projects down the line which might take advantage of hare-wren, and it would be nice to expand the wren::api library a bit more as well. If you have a Hare project which would benefit from embedding Wren, please let me know – and consider sending some patches to improve it!