AILang/design/models/0006-prose-projection.md

Form (B) — prose projection whitepaper

Form (B) — human prose projection

AILang ships a second textual projection of the AST: ailang-prose, a one-way projection from Module → human-readable text. It is not an authoring surface; it is the "display" projection that the authoring surface's architectural pin explicitly anticipated:

"Future projections are explicitly anticipated: a visual / graphical front-end is a plausible second projection for human review and inspection (display being the one case where non-AI eyes matter). The architecture leaves room: any producer of well-formed ailang-core::ast::Module values is a valid front-end."

Form (B) targets the specific failure mode where a human reviewer needs to read an AILang module quickly. Form (A) was designed to fit a 30-production EBNF spec and to be parsed zero-shot by foreign LLMs; that prioritisation makes it dense and visually noisy for human readers. Form (B) inverts the trade-offs:

• Rust-flavoured surface. Braces and => for match arms, Rust-aligned 4-level operator precedence, infix arithmetic (a + b, not +(a, b)), unary ! for not.
• Lossy by design. Projection elides machinery the LLM can re-derive: (con T) wrappers ((con Int) → Int), the (fn-type (params ...) (ret ...)) wrap, (term-ctor T C ...) collapses to C(...), redundant parens. Only the AST machinery whose information is recoverable from typecheck context.
• Lossless on load-bearing detail. Mode annotations (own T, borrow T), effects (with IO), explicit clone, reuse-as, doc strings, type annotations on signatures and lambdas, the tail flag — all preserved verbatim.

Critically, form (B) has no parser. Form (A) is round-trippable by construction (see roundtrip invariant); form (B) deliberately is not. Re-integrating prose edits requires an external LLM mediator, not a compiler pass — the prompt template ail merge-prose composes the six-step cycle.

Form (B) does not weaken any authoring surface invariant:

• The JSON-AST remains the only hashable artefact. Prose is not hashed, not content-addressed, not load-bearing for any cross-module reference.
• Form (A) remains the canonical authoring surface. Foreign LLMs still author against form (A); humans review and edit through form (B).
• The 30-production grammar of form (A) is unchanged.
• ailang-check and ailang-codegen remain projection-agnostic; ailang-prose is a downstream consumer of ailang-core::ast, parallel to ailang-surface but in the rendering direction only.

The CLI gains ail prose <m.ail.json> (the deterministic projection) and ail merge-prose <m.ail.json> <edited.prose.txt> (the mediator-prompt composer); both are listed under pipeline.

Form-A spec embedding

An earlier merge-prose prompt instructed the LLM to emit JSON-AST and offered a 12-line schema-essentials reminder; that combination did not give a foreign LLM enough to produce valid output. The current prompt revises this:

• The LLM emits Form-A (the canonical authoring surface), not JSON. JSON-AST stays the only hashable artefact, but it is not a writing surface. The user runs ail parse foo.new.ail before ail check to produce the canonical JSON.
• crates/ailang-core/specs/form_a.md is the complete LLM-targeted Form-A specification — grammar, every term / pattern / type / def keyword, schema invariants, pitfall catalogue, four few-shot modules drawn from examples/*.ail. It is exported as ailang_core::FORM_A_SPEC and embedded verbatim in every merge-prose prompt.
• crates/ailang-core/tests/spec_drift.rs walks every variant of Term, Pattern, Type, Def, Literal via exhaustive match and asserts an anchor for each appears in the spec. The exhaustive match is the load-bearing piece: adding a new variant without updating the match fails compilation in this test, before its assertions even run. Hand-written content, mechanical drift detection.

The cycle's lowest-common-denominator path is the static prompt (ail merge-prose | client | ail parse | ail check), which works with any client.