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Gabriel Kaszewski 6ad76b98a2 add domain glossary, design spec, and ADRs

CONTEXT.md: domain model with entities (WidgetConfig, DataSource, LayoutPreset),
value objects (Layout, LayoutNode, KeyMapping, WidgetState, Sizing),
architecture decisions (CQRS, static dispatch, postcard), client domain model,
and design constraints.

2026-06-18 18:05:19 +02:00

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0002 — Static dispatch over trait objects

Status: accepted Date: 2026-06-18

Context

Hexagonal architecture uses port traits extensively. Rust offers two ways to consume them: trait objects (Box<dyn Port>, dynamic dispatch) or generics (T: Port, static dispatch / monomorphization).

Decision

Use generics for all port traits. No Box<dyn> anywhere. Composition roots (bootstrap on server, firmware crate on client) are the only places that know concrete types. Generic parameters propagate through application and presentation layers.

Alternatives considered

Trait objects — simpler type signatures, less generic noise. But adds vtable indirection, heap allocation per trait object, and prevents compiler optimizations. On ESP32 with limited RAM, every allocation matters.

Consequences

Zero-cost abstractions — port calls are monomorphized, inlined where possible.
More verbose type signatures — structs and functions carry generic parameters.
Compile times may increase due to monomorphization.
Consistent approach across server and client codebases.

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