The Rhai Expression Engine

Ewwii uses Rhai as its core expression and scripting engine. This means your configuration is more than just static values or simple substitutions—it’s real code, and you can use it anywhere dynamic behavior is needed.

Rhai expressions can:

• Perform logic and branching (if, match, ? :)
• Handle mathematical calculations and string operations
• Access nested data from arrays, maps, or JSON
• Run custom functions
• Be used directly in layout definitions, widget attributes, and style rules

Unlike Yuck, where expressions were embedded in { ... } or ${ ... }, Rhai treats expressions as native. They don’t need to be wrapped in special delimiters. If you can write it in Rhai, it just works.

Example

example.rhai

let value = 12 + foo * 10;

box(#{
  class: "baz"
  orientation: "h",
}, [
  button(#{
    class: button_active ? "active" : "inactive",
    on_click: "toggle_thing",
    label: `Some math: ${value}`,
  }),
]);

Core Features

Rhai supports a wide range of expression capabilities:

• Mathematics: +, -, *, /, %
• Comparisons: ==, !=, <, >, <=, >=
• Boolean logic: &&, ||, !
• Conditionals: if/else, ternary (cond ? a : b)
• Regex matching: =~ operator (Rust-style regex)
• Optional access: ?. and ?.[index]
• Data structures: maps/arrays (obj.field, arr[3], map["key"])
• Function calls: standard and Ewwii-specific built-ins (see below)
• String interpolation: `Value is ${value}` (standard Rhai feature)

Note

---

Rhai only allows string interpolation only for the strings that are quoted in “ similar to JavaScript.

Learn more about it here.

Common Built-in Functions

Examples include:

• round(), floor(), ceil(), powi(), powf()
• min(), max(), sin(), cos(), etc.
• replace(), matches(), captures()
• strlength(), arraylength(), objectlength()
• jq() – run jaq-compatible filters on JSON data
• get_env() – read environment variables
• formattime() – format UNIX timestamps
• formatbytes() – format file sizes (IEC or SI)

Dynamic Usage

Because expressions are just Rhai, you can now write real logic inline or break it into reusable functions:

example.rhai

fn status_text(active) {
  return active ? "enabled" : "disabled";
}

label({
  text: `Status: ${status_text(system_active)}`
});

---

TL;DR

If you’ve used scripting languages like JavaScript or Lua, you’ll feel at home. Rhai gives you real control—not just interpolation tricks.