# CHECKPOINT-REIFY — comptime `type_info` / `reify` (async-first foundation, step 3)

Companion to [PLAN-REIFY.md](PLAN-REIFY.md). Update after every step (one step at a
time, per the cadence rule).

## Last completed step
**Phase 2.1 (green) — `field_type` done.** `field_type($T, i) -> Type` is
implemented over the type table (`fieldTypeOf` in `lower/generic.zig`, re-exported
on `Lowering`): struct field / tagged-union + `union` variant payload (`.void` for
a tagless variant) / tuple element / array + vector element; OOB and memberless
types poison to `.unresolved` with a loud diagnostic (never a silent default).
It folds at lower time, so it composes inside `type_eq` / `type_name` / any type-arg
slot. `examples/0616` green (struct fields name+type, `type_eq` fold, tagged-union
payloads incl. `quit → void`). Full suite green (672 examples, 447 unit). Cadence:
2.0 xfail (empty marker, RED) → 2.1 green (this commit).

**`type_info` is NOT done** — it still bails loudly in
`call.zig:tryLowerReflectionCall`. It builds a full `TypeInfo` *value* (inverse of
reify) and is the larger Phase 2.2 step (see Next step).

### (prior) Phase 1 (type-fn → reify identity) — COMPLETE. A type-fn body that returns
`reify(...)` now mints the enum under the instantiation's name:
`instantiateTypeFunction` (`lower/generic.zig`) detects a reify-returning body
(`findReturnReifyCall`) and routes it to `reifyType(<mangled-or-alias name>,
reify_call)` with the type-arg bindings active (so `payload = T` resolves to the
bound arg) — placed BEFORE the general case, which would otherwise route the
reify call to the inline-position loud bail. Registering under the mangled name
lets the fn's top-of-instantiation cache return the SAME `TypeId` on the second
instantiation → `Box(i64)` at two independent sites is ONE type (**Contract 1**).
`examples/0615` green (`build()`→`consume()` cross-site, plus `b : Box(i64) =
.none`); full suite green (671 examples, 447 unit). Cadence: 1.0 xfail (empty
marker, RED) → 1.1 green (this commit).

### (prior) Phase 0.2 (green) — Phase 0 COMPLETE. Implemented `reify(.enum(...))`:
`Lowering.reifyType` (in `lower/nominal.zig`) reads the flat-enum `TypeInfo`
literal off the AST, synthesizes an `ast.EnumDecl`, and feeds it through the
SAME `type_bridge.buildEnumInfo` path source enums use → the minted type is
byte-identical to a hand-written `enum { value: i64; closed; }` and flows
through enum codegen unmodified (**Contract 2 confirmed** — a source enum
exhibits the exact same construct/match behavior, verified by probe). Wired at
the `E :: reify(...)` const-decl hook in `lower/decl.zig`. `examples/0614`
green (`value 3` / `closed`, exit 0); snapshots captured; full suite green
(670 examples, 447 unit). Unsupported reify shapes bail loudly via
`reifyBail` (never a silent default).

Also landed (user-directed, separate commit `feat(parser):`): reserved keyword
as a member name after `.` (`.enum`, `case .enum:`, `x.enum`) — so the reify
example reads `reify(.enum(...))` without a backtick. readme updated.

### Two earlier Phase-0 commits
- **0.1 (xfail).** Added `examples/0614` + empty `.exit` marker → RED.
- **0.0 (lock).** Added the comptime type-metaprogramming surface as the
on-demand module `library/modules/std/meta.sx` (NOT the prelude — see decision
below): `EnumVariant`/`EnumInfo`/`TypeInfo` data types + bodyless `#builtin`
decls `reify` / `type_info` / `field_type`. Each builtin bails LOUDLY when
reached unimplemented (no silent default). Unit test `src/parser.test.zig`
(registered in `src/root.zig`) locks that the decls parse. `zig build test`
green (447/447 unit, 669/669 examples).

## Current state
- `modules/std/meta.sx` declares the surface; the variant uses the backtick raw
  escape `` `enum `` (reads as the keyword, not a mangled `enum_`).
- **Loud bails wired (unimplemented → diagnostic, never a silent type):**
  - `reify(...)` in a `::` type-alias position → `decl.zig` (the `.call`
    const-decl branch) emits "reify is not yet implemented (REIFY Phase 0.2)"
    and poisons the alias to `.unresolved`. This is also where Phase 0.2 will
    hook the real construction.
  - `reify` / `field_type` in any other type position →
    `generic.zig:resolveTypeCallWithBindings` (defense-in-depth).
  - `type_info(...)` in expression position →
    `call.zig:tryLowerReflectionCall`.
- No interpreter-side construction yet — `reify` mints nothing.

## Decision (0.0)
**Meta lives in `modules/std/meta.sx`, not the prelude (`core.sx`).** Declaring
the data types in the always-loaded prelude interns them into every module's
type table and shifts every `.ir` snapshot (broke 37 examples in a trial). An
on-demand import keeps the prelude clean; reify users `#import
"modules/std/meta.sx"`. (User-directed.)

## Next step
**Phase 2.2 (`type_info($T) -> TypeInfo`).** Reflect a type into a `TypeInfo`
*value* — the inverse of reify. Two sub-pieces, both non-trivial: (a) widen the
`meta.sx` `TypeInfo` data model beyond `` `enum `` (struct / tuple variants); (b)
build the value at comptime — a `[]EnumVariant`-style slice of structs holding
strings (`name`) + `Type` tags (`payload`), populated from the type table. This is
comptime value-CONSTRUCTION (allocating slice/struct/string/type-tag values in the
interpreter), materially larger than 2.1's fold-to-a-TypeId. Currently bails loudly
in `call.zig:tryLowerReflectionCall`. Best done as its own session for context room.

NOTE (round-tripping): `reifyType` still reads a LITERAL `TypeInfo` off the AST
(fine for the inline-literal + type-fn-over-literal cases of Phases 0–1). Once 2.2
produces a COMPUTED `TypeInfo`, feeding it back into `reify` needs the reader
generalized (or interp evaluation of the `TypeInfo` value) — handle that when 2.2
enables round-tripping.

Alternatively jump to **Phase 3** (`make_enum` + `RecvResult`/`TryResult` sx lib over
`reify`) — that only needs reify (have it) + type-fns (have them), not `type_info`.

SELF-REFERENCE = Phase 4, API DECIDED (user-directed): explicit **`declare()` →
`define(h, info)`** (the declaration-vs-definition split; NOT a `reify_rec((self)=>…)`
closure). `declare()` returns a forward nominal `Type` handle (named from the `::`
LHS) usable freely in any later `TypeInfo` — `*List`, `[]List`, and across types for
MUTUAL recursion the one-`self` closure couldn't express; `define(handle, info)` fills
the body. `reify(info)` stays as the one-shot sugar. Maps onto existing machinery:
declare = empty nominal slot (`reserveShadowEnumSlot`-style), define = `buildEnumInfo`
+ `updatePreservingKey`, struct-stub→tagged_union re-key already handled
(`internNamedTypeDecl`'s `adoptsForwardStructStub`). Loud invariants: never-defined
declare = hard error; by-value self-inclusion rejected (infinite size). Full write-up
in PLAN-REIFY Phase 4.

## Known issues
None yet.

## Log
- **2.1 (green) — `field_type` done.** `fieldTypeOf` over the type table
  (struct/tagged-union/union/tuple/array/vector; OOB+memberless = loud poison);
  folds at lower time, composes in `type_eq`/`type_name`; `0616` green. `type_info`
  still pending (Phase 2.2 — builds a TypeInfo value).
- **2.0 (xfail).** `0616` + empty `.exit` marker → RED (field_type bailed).
- **1.1 (green) — Phase 1 done.** Type-fn body `return reify(...)` routes through
  `reifyType` under the instantiation name (`findReturnReifyCall` +
  mangled-name registration); `Box(i64)` at two sites = one type (Contract 1);
  `0615` green.
- **1.0 (xfail).** `0615` + empty `.exit` marker → RED (reify bailed in type-fn body).
- **0.2 (green) — Phase 0 done.** `reifyType` mints a flat enum from a literal
  `TypeInfo` via the shared `buildEnumInfo` path; `0614` green; Contract 2 confirmed
  (reify'd enum == source enum, same construct/match). Payload-less variant idiom in
  the example is `c : E = .closed;` (enum-literal target), same as a source enum.
- **parser (user-directed).** Keyword as member name after `.` — see `feat(parser)`.
- **0.1 (xfail).** `0614` + empty `.exit` marker → RED.
- **0.0 (lock).** Meta surface in `modules/std/meta.sx` (data types + 3 bodyless
  `#builtin` decls), loud bails at all three reach points, `src/parser.test.zig`
  parse-lock. Two user-directed refinements folded in: variant uses `` `enum ``
  raw escape; surface moved out of the prelude into its own module to avoid
  type-table / `.ir`-snapshot churn. `zig build test` green.
- **Stream carved.** Selected as the first async-first foundation: `reify` gates both
  channel result types (`RecvResult($T)`) and `race`'s synthesized union, is fully
  validated (3 reviewers), and is a self-contained compiler/type-system feature
  testable in isolation (`06xx` comptime). Generic-enum syntax dropped in its favor.