Eunomia

Eunomia is the datatype law of the Atlas stack: the single source of truth for numeric and scalar datatype vocabulary. It owns the representations every other Atlas crate computes over — and nothing else.

It is the sibling of themis (placement law — NUMA/tier/worker locality). Where themis supplies the law of where data lives, Eunomia supplies the law of what data is. In myth, Eunomia ("good order") is Themis's daughter; here the datatype law sits one rung below the placement law in the foundation.

What it owns

• Scalar wrapper types — F16, Bf16, F32, F64, I8/I16/I32, and sub-byte F4/F8/Bf4/Bf8 — with bytemuck::Pod/Zeroable and exact layout guarantees. One native conversion kernel provides exact widening and round-to-nearest, ties-to-even narrowing across every reduced format.
• Complex<T> — the native re + im·i vocabulary type replacing the third-party num_complex::Complex across the stack.
• Packed sub-byte formats — Packed4/PackedBf4/PackedF4 storage, COW, rkyv archival, and SIMD-accelerated unpack.
• Conversion lattices (CastFrom/CastTo) and element traits (NumericElement/FloatElement).
• Scalar field traits — RealField/ComplexField (the nalgebra scalar field analogues), so generic numeric code runs over f32/f64 and Complex.

It owns no computation kernels, allocation, scheduling, or backend code, and no vector/matrix/geometry types — those are linear algebra and live in leto (CPU) / hephaestus (GPU). The other layers belong to hermes (SIMD), mnemosyne (allocation), moirai (execution), and the domain crates.

Crate

Crate	Role
eunomia	The datatype vocabulary: scalars, complex, packed formats, casts, element + scalar-field traits. #![no_std]-capable.

Position in the Atlas stack

eunomia        datatype law (scalars, complex, packed, field traits)   ← this repo
themis         placement law (NUMA, tier, worker locality)
   ↑ consumed by
hermes         SIMD execution over eunomia types
mnemosyne      allocation
moirai         execution
   ↑ consumed by
leto           array substrate (cache-tiled over hermes/eunomia)
   ↑ consumed by
coeus / hephaestus / apollo / …   domain

Dependency direction is strictly inward: eunomia depends on nothing Atlas-local. Its core uses bytemuck and libm; rkyv, serde, and the NumPy/PyO3 0.29 element boundary are optional. The half crate is a dev-only differential oracle for Eunomia's native reduced-precision conversion tests.

Provenance

The core numeric vocabulary migrated from hermes-numeric (which declared itself the numeric SSOT but was coupled to hermes's SIMD release cadence). Eunomia promotes it to an independently versioned foundation repo so the stable datatype vocabulary evolves separately from perf-volatile SIMD kernels. See docs/adr/0001-eunomia-datatype-ssot.md.

License

Dual-licensed under MIT or Apache-2.0.