ADR 0006 — Build own Quantity, not unxt/quax

0006 — Build own Quantity (pure-equinox) instead of adopting unxt or quax¶

Context¶

jaxstro needed unit-aware array support for the emerging need to track dimensions at I/O edges. Options were to adopt unxt (SoTA, astropy-backed), adopt quax (transparent dispatch), or build a minimal custom Quantity class from scratch.

Decision¶

Build our own jaxstro.quantity: Quantity class as a pure-equinox module (value + dimension) with explicit .to()/.value unwrap at JAX boundaries. Zero new dependencies. Opt-in and experimental in 0.1.0; no sibling package forced to migrate.

Rationale¶

• Lightweight foundation: unxt uses astropy as a backend (violates thin-foundation posture), adds heavy weight to everything depending on jaxstro, and overlaps/subsumes jaxstro’s units module.

• No treadmill: quax adds jaxtyping-compatible transparent dispatch but requires a permanent primitive-coverage treadmill (new @quax.register for every new function/transform).

• Philosophically aligned: pure-equinox Quantity (explicit units, host-side resolution) matches jaxstro’s ethos; zero new deps; sidesteps the primitive long-tail.

• Boundary clarity: Quantity (dimensions-on-values) and UnitSystem (code-unit scaling) are complementary: use Quantity at I/O edges, unwrap to raw array + UnitSystem for dimensionless compute interior.

• Future interop: if transparent interop becomes a hard requirement later, add a thin quax adapter at the consuming layer, not in the foundation.

Note: Lands as experimental-only in 0.1.0 if it does not delay the release; else deferred to 0.2.0.