spec: 22 milestone — typeclasses retrospective
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# 22 — Typeclasses Milestone — Design Spec
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**Date:** 2026-05-09
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**Status:** Draft — awaiting user spec review
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**Authors:** Brummel (orchestrator) + Claude
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## Goal
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Ship typeclasses end-to-end as committed in DESIGN.md Decision 11
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("typeclasses — Haskell-lite, monomorphised, coherent"). Validate
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the user-defined-class path works end-to-end before declaring the
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milestone closed — not just the Prelude path.
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This is a retrospective milestone spec. 22a (decision iter) and
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22b.1 (schema floor + workspace registry) shipped before the
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brainstorm-skill pipeline existed. The spec ratifies the existing
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design where it holds and corrects iter-slicing where Feature-
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Acceptance points to a gap.
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## Architecture
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Design is fully specified in DESIGN.md Decision 11 §1449–1759. The
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five committed semantic axes were re-checked against the Feature-
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Acceptance criterion at the time of writing:
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1. **Haskell-lite scope** (multi-method, single-param, optional
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defaults, single-superclass) — holds. LLM produces `class Eq a
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where eq, ne` naturally; `Ord extends Eq` is the textbook case.
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Multi-param classes and HKTs remain LLM-distant.
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2. **Constraints explicit + mandatory** — holds. The "alles
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sichtbar" line is project-zementiert; constraints visible at
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the function boundary is the LLM-author advantage.
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3. **Orphan-free coherence** — holds, with one documentation
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note: primitives (Int/Float/Bool/String) have no defining
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module — `instance MyClass Int` must therefore live in
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MyClass's module. Test fixture `test_22b1_orphan_third`
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confirms. Workspace-closed authoring is the right precondition.
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4. **Defaults via explicit `default` keyword** — holds. LLM
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writes `default ne x y = not (eq x y)` more naturally than
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mixed class bodies à la Haskell.
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5. **Class-param kind `*` only** — holds. LLM-natural pattern is
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`List.map`/`Tree.map` per type, not Functor abstraction.
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Monomorphisation handles per-type directly.
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No axis falls. Decision 11 stays unchanged. The brainstorm
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correction is to iter-slicing, not to axis content.
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## Components (iterations)
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| Iter | Status | Scope | Closes when |
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|---|---|---|---|
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| 22b.1 | shipped 2026-05-09 | Schema floor (`ClassDef`, `InstanceDef`, `FnDef.type` extension), workspace registry with three coherence checks (orphan/duplicate/missing-method), hash-stability proof, fixture filter for round-trip gate. | JOURNAL entry committed (done) |
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| 22b.2 | queued | Typecheck arms: `FnDef.type.constraints` field activation; class-schema validation (`kind-mismatch`, `invalid-superclass-param`, `constraint-references-unbound-type-var`); per-call-site `missing-constraint` and `no-instance`; `overriding-non-existent-method`, `method-name-collision`, `missing-superclass-instance`. | All new diagnostics fire on dedicated fixtures; bench gates green |
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| 22b.3 | queued | Monomorphisation pass — synthesised `FnDef`s from `(method, type-hash)` pairs, call rewriting, cache by key. **Synthetic class+instance fixture** in the test suite for end-to-end validation of the mono pass before the Prelude lands. | Synthetic fixture compiles, runs, correct stdout; bench gates green |
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| 22b.4 | queued | Prelude module (Show/Eq/Ord on Int/Float/Bool/String), `print` rewiring through `Show.show`, Form-B parser/printer arms for ClassDef/InstanceDef, `test_22b1_*` round-trip filter retired. | Prelude fixture compiles, runs; round-trip green for all `examples/`; bench gates green |
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| 22c | **promoted** (was deferred) | Single end-to-end fixture: user defines `class Foo a`, user defines `data Bar`, user writes `instance Foo Bar`, calls `foo b`. Bench gate. JOURNAL entry = milestone close. | Fixture compiles, runs, correct stdout; full audit suite green |
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22c was deferred in 22a's queue. This brainstorm flagged it as a
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shipping gap: the milestone is "typeclasses" but the Prelude path
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alone (compiler-internal setup) does not validate that an LLM-author
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can in fact define a class and an instance and have them work.
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Promoting 22c with tight scope (one fixture, no `deriving`) closes
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the gap without expanding the milestone substantially.
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22b.3's synthetic fixture is the matching addition: monomorphisation
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must be testable before the Prelude lands, so its iter cannot rely
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on 22b.4 for end-to-end validation.
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## Data flow
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Per DESIGN.md Decision 11 §"Resolution and monomorphisation":
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```
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.ail.json modules
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↓ workspace load
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↓ ↓ collect ClassDef per (class-name → defining-module)
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↓ ↓ collect InstanceDef per (class-name, type-hash)
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↓ ↓ coherence + uniqueness + completeness + superclass checks
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↓ Workspace { defs, registry }
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↓
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↓ per-FnDef typecheck
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↓ ↓ collect residual constraints from method calls
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↓ ↓ check residuals against declared (+ superclass-expanded)
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↓ ↓ resolve fully-concrete constraints against registry
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↓
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↓ monomorphisation pass
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↓ ↓ for each (resolved-method, concrete-type) pair:
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↓ ↓ synthesise FnDef with hash-deterministic name
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↓ ↓ substitute class param to concrete type in body
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↓ ↓ rewrite original Call to target synthesised name
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↓
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↓ codegen sees only ordinary monomorphic FnDefs and direct calls
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LLVM IR
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```
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No runtime dispatch, no dictionary passing. A class-method call
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that cannot be monomorphised (constraint unresolved at entry point)
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is a static error, not a runtime one.
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## Error handling
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Diagnostic catalogue, by phase. Codes follow the project's existing
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kebab-case CLI convention.
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**Workspace-load (registry-build) — landed in 22b.1:**
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- `orphan-instance`
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- `duplicate-instance`
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- `missing-method`
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**Workspace-load (registry-build) — 22b.2:**
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- `missing-superclass-instance`
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- `overriding-non-existent-method`
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- `method-name-collision` (across two in-scope classes, or class
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method vs top-level function)
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**Class-schema validation — 22b.2:**
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- `kind-mismatch` (class param appears in applied position in any
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method signature)
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- `invalid-superclass-param` (superclass `type` ≠ class's own `param`)
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- `constraint-references-unbound-type-var`
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**Per-FnDef typecheck — 22b.2:**
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- `missing-constraint` (residual not covered by declared +
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superclass-expanded constraints)
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- `no-instance` (fully concrete constraint with no registry entry)
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No `ambiguous-instance` diagnostic — coherence makes registry keys
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globally unique by construction.
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## Testing strategy
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**Per iter:**
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- **22b.1** — already shipped. Schema-extension hash-stability
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tests + 5 registry-diagnostic fixtures.
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- **22b.2** — one fixture per new diagnostic (one-fixture-one-
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diagnostic rule). Plus a positive fixture exercising
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`missing-constraint` recovery (constraint declared correctly
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→ green).
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- **22b.3** — synthetic class+instance fixture exercising the
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mono pass end-to-end. Hand-written `class Foo a where foo : a
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-> Int` + `instance Foo Int where foo i = i` + `fn main = print
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(foo 5)`. Verifies the synthesised def is emitted, called, and
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produces "5". Plus unit tests on the mono cache (no double
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emission).
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- **22b.4** — Prelude fixture (`fn main = print 42` exercising
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`print` → `Show.show` → `show@Int`). Round-trip filter retires;
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all `examples/test_22b1_*` plus the new prose round-trip passes.
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- **22c** — single user-class e2e: `class Greet a where greet :
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a borrow -> String` + `data Person { name : String }` +
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`instance Greet Person where greet p = "Hello, " ++ p.name` +
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`fn main = print (greet (Person { name = "world" }))`.
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**Bench gates:** all three (`bench/check.py`,
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`bench/compile_check.py`, `bench/cross_lang.py`) green at every
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iter close. The mono pass is a no-op for fixtures without classes —
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pre-22b modules stay bit-identical through compile.
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## Acceptance criteria
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The 22 milestone closes when **all** of the following hold:
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1. 22b.1 + 22b.2 + 22b.3 + 22b.4 + 22c JOURNAL entries committed.
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2. Audit suite (architect drift review against DESIGN.md +
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`bench/check.py` + `bench/compile_check.py` +
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`bench/cross_lang.py`) green.
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3. The 22c user-class fixture compiles, runs, and produces correct
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stdout — with no Prelude class involvement (ensures the path is
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independently exercised).
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4. Round-trip filter `test_22b1_*` is removed; all
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`examples/*.ail.json` pass round-trip.
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## Out of scope (deferred, with substantive rationale)
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**Operator routing** (`==`, `<`, `<=`, `>`, `>=` through `Eq`/`Ord`).
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Deferred to a post-22 milestone bundled with bench re-baselining.
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Rationale: the entire bench corpus typechecks differently when
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operators route through classes; the new monomorphised trampolines
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may codegen non-equivalently to the primitive form, which is a new
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baseline rather than a regression. Re-baselining is its own work,
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not an iter detail. The 22a "would risk firing the bench gate"
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phrasing was an effort-rationale; the substantive form is the new-
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baseline argument above.
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**`deriving`.** Deferred to a post-22 milestone, contingent on 22
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shipping clean. Rationale: `deriving` requires a generic-synthesis
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pass at workspace-load (parsing `deriving (Eq, Show)`, synthesising
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`InstanceDef` AST nodes, feeding them into the same coherence
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pipeline as hand-written instances). That is new infrastructure,
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not a 22-arc detail. Synthesising instances for a typeclass
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mechanism whose user-class path has not been e2e-validated would
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also stack risk.
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**`Num` class.** No concrete promotion path. Decision 11 rationale
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holds (class-based numeric overloading invokes literal-defaulting,
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which axis-7 excluded). Numeric operations stay primitive and
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per-type. Trigger for re-evaluation: a concrete user case that
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cannot be expressed otherwise.
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## Open commitments (22a → 22b implementer)
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Three items 22a explicitly punted to the 22b implementer. The
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brainstorm position on each:
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**Mono-symbol naming format.** Implementer call in 22b.3.
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Constraints: must contain the method name and a type indicator
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legible in diagnostics; must be hash-stable; must coexist with the
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existing mangling scheme `@ail_<module>_<def>`. Recommendation:
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`<method>#<typesurfacename>` for primitive types (`show#Int`),
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hash-suffix for compound types — but the implementer chooses.
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**Form-B prose projection for ClassDef/InstanceDef.** Implementer
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call in 22b.4. Constraints: must round-trip exactly with the JSON
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form; must follow the existing C-like prose convention.
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Recommendation: `class <Name> <param> { <method> : <FnSig> }` and
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`instance <Class> <Type> { <method> = <body> }` parallel to existing
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FnDef/DataDef forms — but the implementer chooses.
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**Mode annotations on class methods.** Not a "convention" in the
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strict sense — class method signatures are full FnSigs and carry
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mode annotations per Decision 10. Position for the Prelude (22b.4):
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`borrow` for all read-only methods (`show`, `eq`, `ne`, `lt`, `le`,
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`gt`, `ge`). User-defined classes choose modes per method.
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## Known costs
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**Reserved-name footprint of the Prelude.** Once 22b.4 ships, the
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following names are class-method-occupied at the workspace top
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level: `show`, `eq`, `ne`, `lt`, `le`, `gt`, `ge`. A user-defined
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function or class method with any of these names fires
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`method-name-collision`. The cost is a small lexicon of forbidden
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names; the benefit is no namespace fragmentation across the Prelude
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classes.
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