Generics, Trait Objects, and Values

The command boundary uses dynamic dispatch because declarations are discovered by name at runtime. EngineState stores cloneable Box<dyn Command> values; CommandClone provides object-safe cloning. Each command advertises a concrete Signature, documentation, examples, and a common run method.

Inside commands, generics recover static composition. Iterator adapters are generic until ListStream::new erases them. Evaluation is generic over a DebugContext marker so normal and debug execution share code without a runtime branch at every operation. Conversion traits such as IntoPipelineData, IntoValue, and typed CallExt flag access connect Rust types to shell values.

Closed runtime diversity uses enums: Value, PipelineData, ShellError, OutDest, expressions, and byte-stream sources. Exhaustive matches are valuable because adding a new data mode must force every core boundary to consider it.

The pattern is layered: trait objects for a runtime registry, enums for the language’s closed data model, generics for implementation helpers, and boxed iterators exactly where arbitrary lazy pipelines must share one representation.