Expressions

<Rectangle transform={
    rotate(engine.frames_elapsed / 200.0) *
    translate(in.mouse_x, in.mouse_y)
}/>

Consider the above snippet of Pax. If you're familiar with a templating language like JSX, you might expect that the Expression code within the braces above { ... } is inline Rust code. In fact, this code has many syntactic similarities with Rust, but it is not Rust.

It is PAXEL — part of Pax, a special-purpose language for declaring computed properties in the spirit of spreadsheet formulas.

Anytime you write in between {}s in Pax, you are writing PAXEL. You can create a PAXEL Expression anywhere you can declare a settings value, in template definitions or in @settings blocks.

For example the expression self.activeColor.adjustBrightness(50%) might live in a template:

<Rectangle fill={ self.activeColor.adjustBrightness(50%) } />

or in a settings block:

@settings {
  #my_rectangle {
    fill: { self.activeColor.adjustBrightness(50%) }
  }
}

Pax expressions are ultimately compiled to machine code (WASM or LLVM), and the Pax runtime manages each expression as its own compiled function in a vtable.

Because Pax Expressions are pure, side-effect-free functions, the Pax runtime can (one day) make aggressive optimizations: caching values and only recomputing when one of the stated inputs changes. Expressions are also readily parallelizable, a prospective future performance optimization.

You can read more about PAXEL, the Pax Expression Language, in this chapter.