Explanation

Why Mapper works the way it does:

• The mapping model: target-wise descent, ordered pairings, and why the output shape drives everything.
• Documents as data: mappings are documents, and that is the point.
• Context threading and the async model: one rule for scope, one place for asynchrony.
• Where values come from: one vocabulary of scopes, used the same way everywhere.
• Validation before evaluation: why mapping validity is checkable in one pass, and why the report never throws.
• Determinism and portability: when the same mapping gives the same answer.
• Policy at construction: why deployment policy lives in configuration, never in the mapping.
• Serving mappings: registered vs explicit mappings, and what statelessness buys.
• These docs and the specification: what is normative where.