Context threading and the async model

One rule for scope

Evaluation threads a context: input, output, error accumulator, registries, plus the current read scope (source), write scope (target), and their absolute paths. Descending into a nested mapping or an each element makes a child context by one rule: share everything, rebind source, target, and paths.

The shared parts explain the long-range behaviors. A validation failure three levels deep reaches the envelope because every level appends to the same errors. An output: read inside a nest sees earlier top-level writes because every level shares the same output. The rebound parts explain the local behaviors: inside each, a bare pointer reads the element, and writes land in a fresh target.

The full record is in the context reference.

One place for asynchrony

Pairings run sequentially. Within one pairing, fan-out is parallel: each elements and variant alternatives evaluate concurrently. Plugins are the only asynchronous stage; initializers and transformers are synchronous by rule.

Those three facts compose into a predictable concurrency model. A mapping with no plugins completes without I/O. A plugin inside each fans its calls out in parallel, one per element. Plugins in separate pairings run in sequence, because pairings do. Where you place a plugin in the document decides the concurrency you get, and you can read it off the mapping.