Build a Smaller Nushell

Build a structured shell whose lazy values interoperate with child-process byte streams and whose parallel map makes ordering and bounds explicit.

1. Define values and spans

Support int, string, record, list, nothing, and error. Give every value a source span. Implement field access and display without parsing source strings again.

2. Parse a concrete pipeline

Parse command names, flags, literals, and | into calls. Give commands signatures with accepted input/output types and report multiple spanned parse errors before execution.

3. Add PipelineData

Implement empty, value, list stream, and byte stream. Back list streams with Box<dyn Iterator<Item = Value> + Send>. Write lazy where, get, and first, plus an explicit collect.

4. Build the command registry

Define a cloneable object-safe Command with signature and run. Separate immutable EngineState from mutable Stack. Move input and output so a stream has one clear consumer.

5. Run external processes

Pipe a byte stream directly into stdin. Encode structured rows on a helper thread when necessary. Return a child-backed byte stream that owns stdout, stderr, an exit-status receiver, and cleanup.

6. Prevent pipe deadlocks

Capture large stdout and stderr simultaneously. Drain them concurrently and wait for exit only after both make progress. Turn nonzero status into a spanned shell error.

7. Add cooperative Ctrl-C

Share an atomic signal. Wrap list iterators and check it per item; check inside long loops; terminate foreground children. Verify an infinite source stops.

8. Add par-each

Use a dedicated fixed Rayon pool and bounded result channel. Stream unordered results first. Add --keep-order with indices and documented collection cost. Test nested parallel calls with a private inner pool.

9. Compare with production

Map to Value, PipelineData, ListStream, ByteStream, ChildProcess, Signals, Command, EngineState, Stack, evaluator, plugin streams, and par-each. Nushell adds a rich language, IR, job control, custom values, plugins, formats, table rendering, completion, and cross-platform process behavior. Preserve its center: structured values are repeatable; streams and processes are owned, lazy lifecycles.