Scripting fud2 with Rhai

You can add functionality to fud2 with functionality written in Rhai. Rhai is a scripting language designed to work well with Rust.

All functionality included with fud2 is written in Rhai. They can be found here. These provide a good example of how to add states and operations with Rhai.

Loading Scripts

You can tell fud2 to load a script by including a plugins key in your fud2.toml file.

plugins = ["/my/fancy/plugin.rhai"]

base = "..."

Example Script

We'll walk through how to write a script that adds support for using the calyx compiler.

First, we need to define some states:

export const calyx_state = state("calyx", ["futil"]);
export const verilog_state = state("verilog", ["sv", "v"]);

These two lines define a calyx state and a verilog state. The export prefix means that these variables will be accessible to other scripts that import "calyx".

Next we'll define a setup procedure to define some rules that will be useful.

// allows calyx_setup to be used in other scripts
export const calyx_setup = calyx_setup;

// a setup function is just a normal Rhai function that takes in an emitter
// we can use the emitter in the same way that we use it from rust
fn calyx_setup(e) {
   // define a Ninja var from the fud2.toml config
   e.config_var("calyx-base", "calyx.base");
   // define a Ninja var from either the config, or a default derived from calyx-base
   e.config_var_or("calyx-exe", "calyx.exe", "$calyx-base/target/debug/calyx");
   // define a Ninja var from cli options, or with a default
   e.config_var_or("args", "calyx.args", "");
   // define a rule to run the Calyx compiler
   e.rule("calyx", "$calyx-exe -l $calyx-base -b $backend $args $in > $out");

And now we can define the actual operation that will transform calyx files into verilog files.

  "calyx-to-verilog",      // operation name
  [calyx_setup],           // required setup functions
  calyx_state,             // input state
  verilog_state,           // output state
  |e, input, output| {     // function to construct Ninja build command
    e.build_cmd([output], "calyx", [input], []) ;
    e.arg("backend", "verilog");

Rhai Specifics

String Templates

Rhai has a string templating feature, similar to the format! macro in rust. Templated strings are marked with backticks (`path/${some_var}.ext`) and variables are included with $. You can include expressions that will be evaluated by using brackets: ${1 + 2}.

String Functions

Rhai includes standard string operations. They are described in the documentation. These are useful for constructing more complicated paths.

Export Rules

In Rhai, all top-level variable declarations are private by default. If you want them to be available from other files, you need to export them explicitly.

All functions are exported by default. However, they are only exported in a callable format. If you want to use the function as a variable (when passing them as a setup function or build function), you need to export them explicitly as well.

This is how that looks:

export const my_fancy_setup = my_fancy_setup;
fn my_fancy_setup(e) {


You can import another Rhai script file like so:

import "calyx" as c;

All exported symbols defined in calyx.rhai will be available under c.


The name for an import is always just the basename of the script file, without any extension.


Currently, the Rhai API is almost identical to the Rust API. However Emitter::add_file is not currently supported. And Emitter::var is renamed to _var because var is a reserved keyword in Rhai.

Adding to the API

If there is something that is hard to do in Rhai, it is straightforward to register a Rust function so that it is available from Rhai.

Rust functions are registered in ScriptRunner::new. Refer to ScriptRunner::reg_get_state to see a simple example of how to register a function.