Skip to main content

Traits

Target version: v0.14.

What​

Static, dispatched-at-monomorphisation typeclass-like traits.

trait Numeric {
    const ZERO: Self
    const ONE:  Self
    fn add(a: Self, b: Self) -> Self
    fn mul(a: Self, b: Self) -> Self
}

impl Numeric for f32 {
    const ZERO: f32 = 0.0
    const ONE:  f32 = 1.0
    fn add(a: f32, b: f32) -> f32 = a + b
    fn mul(a: f32, b: f32) -> f32 = a * b
}

A function may take a generic with a trait bound:

fn sum[T: Numeric, N](v: T[N]) -> T = +/(v)   # works for any Numeric T

Why not today​

esque has shape generics, not type generics. To add traits the type checker needs:

  • Element-type generic parameters (the [T] part).
  • A trait registry, with subtype-of-trait reasoning.
  • Monomorphisation that emits one specialised copy per (shape, trait-impl) pair.

This is a clean feature but not a small one.

Why monomorphisation, not vtables​

Two reasons. First, traits will be used in the hot path (numeric generics for autodiff and for the kernel DSL); a vtable indirection defeats vectorisation. Second, the rest of the language is already monomorphised on shape; type monomorphisation is the same pass.

A dyn Trait form for host-only data is a possible later addition, but not part of the v0.14 work.