# Authoring surface

## Authoring surface

Form (A) is implemented as
the `ailang-surface` crate (parser + printer). Form-A is
gated against drift by `ailang-surface/tests/round_trip.rs`, which
parses every `.ail` fixture, prints it back, re-parses, and demands
canonical-byte equality. `ail render` and both branches
of `ail describe` project through `ailang_surface::print`, so
form (A) is the **sole** text projection of a module. `pretty.rs`
carries no round-tripping printer — only the diagnostic helpers
(`type_to_string`, `pattern_to_string`, `manifest`) are public there.

## Architectural pin: data structure is the source of truth

The textual surface is **not** a replacement for the JSON-AST. It is
one projection among potentially many. Concretely:

- The [JSON-AST](0002-data-model.md) keeps its role as the canonical, hashable, content-
  addressed representation of a module. All hashing, content-
  addressing, cross-module references, and typecheck/codegen input
  flow through the JSON-AST. **No new hashable form is introduced.**
- The textual surface (form A, this contract) is the **AI authoring
  projection**: optimised for me producing programs token-efficiently
  and for foreign LLMs producing programs from a spec. It is not
  optimised for human authors and does not need to be human-pleasant.
- 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.
- **No human is expected to author AILang seriously.** Authoring is
  AI work. Display and verification, by contrast, are concerns where
  human-facing alternatives may be useful — and which can therefore
  layer their own projections on top of the same AST without
  touching the surface or the core.

In code terms: `ailang-core` owns the AST. `ailang-surface` is one
producer/consumer pair: text-form-A → AST → text-form-A. A hypothetical
`ailang-visual` would be a different producer
of the same AST. `ailang-check` and `ailang-codegen` consume only
the AST and remain projection-agnostic.

## Constraints (hard, in priority order)

1. **Formalizable for a foreign LLM.** The grammar must fit in an
   EBNF/PEG spec of ≤ 30 productions. A model that has never seen
   AILang must be able to read the spec and produce conforming source
   zero-shot. Rules out: precedence between binary operators,
   semantic indentation, maximal-munch lexing, context-sensitive
   reductions.
2. **AST-isomorphic.** Every surface form maps to exactly one AST
   shape. The full bijection between `.ail.json` and `.ail` (both
   directions, BLAKE3-stable hashing, Float-bits-hex encoding,
   workspace-CI enforcement points) is anchored as the top-level
   [Roundtrip Invariant](0009-roundtrip-invariant.md) — this constraint
   records that the surface-design choice must satisfy that
   invariant; the invariant itself lives at top level because the
   property is load-bearing on the language identity, not on this
   file's surface-design rationale.
3. **No external symbols.** ASCII only. No Greek (`∀`), no arrows
   (`→`), no subscripts. Reasoning: I substitute mojibake for
   non-ASCII characters under context pressure; foreign LLMs vary
   in how they tokenize Unicode.
4. **No precedence.** Either everything is parenthesized, or there
   are no infix operators. Prefer the latter — `add(x, 1)` over
   `x + 1`. Removes a fail mode for both me and foreign LLMs.
5. **No semantic indentation.** Block structure expressed by paired
   delimiters or terminator tokens. Indentation is informational
   only; the parser ignores it.
6. **One construct per token-list.** Every AST node corresponds to
   exactly one parenthesized form (or atom). No "sometimes you can
   omit the parens" rules.
7. **AST surface stays frozen.** The surface adapts to the AST, not
   the other way around. We do not change the JSON schema or
   invalidate hashes to make the surface prettier.

Ratified by: `crates/ailang-surface/tests/round_trip.rs`.
