# 22 — Typeclasses Milestone — Design Spec

**Date:** 2026-05-09
**Status:** Draft — awaiting user spec review
**Authors:** Brummel (orchestrator) + Claude

## Goal

Ship typeclasses end-to-end as committed in DESIGN.md Decision 11
("typeclasses — Haskell-lite, monomorphised, coherent"). Validate
the user-defined-class path works end-to-end before declaring the
milestone closed — not just the Prelude path.

This is a retrospective milestone spec. 22a (decision iter) and
22b.1 (schema floor + workspace registry) shipped before the
brainstorm-skill pipeline existed. The spec ratifies the existing
design where it holds and corrects iter-slicing where Feature-
Acceptance points to a gap.

## Architecture

Design is fully specified in DESIGN.md Decision 11 §1449–1759. The
five committed semantic axes were re-checked against the Feature-
Acceptance criterion at the time of writing:

1. **Haskell-lite scope** (multi-method, single-param, optional
   defaults, single-superclass) — holds. LLM produces `class Eq a
   where eq, ne` naturally; `Ord extends Eq` is the textbook case.
   Multi-param classes and HKTs remain LLM-distant.
2. **Constraints explicit + mandatory** — holds. The "alles
   sichtbar" line is project-zementiert; constraints visible at
   the function boundary is the LLM-author advantage.
3. **Orphan-free coherence** — holds, with one documentation
   note: primitives (Int/Float/Bool/String) have no defining
   module — `instance MyClass Int` must therefore live in
   MyClass's module. Test fixture `test_22b1_orphan_third`
   confirms. Workspace-closed authoring is the right precondition.
4. **Defaults via explicit `default` keyword** — holds. LLM
   writes `default ne x y = not (eq x y)` more naturally than
   mixed class bodies à la Haskell.
5. **Class-param kind `*` only** — holds. LLM-natural pattern is
   `List.map`/`Tree.map` per type, not Functor abstraction.
   Monomorphisation handles per-type directly.

No axis falls. Decision 11 stays unchanged. The brainstorm
correction is to iter-slicing, not to axis content.

## Components (iterations)

| Iter | Status | Scope | Closes when |
|---|---|---|---|
| 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) |
| 22b.2 | shipped 2026-05-09 | 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 |
| 22b.3 | shipped 2026-05-09 | Monomorphisation pass — synthesised `FnDef`s from `(method, type-hash)` pairs, call rewriting, cache by key. Synthetic class+instance fixture for e2e validation. `__` separator chosen over `#` for LLVM IR identifier legality. | Synthetic fixture compiles, runs, correct stdout; bench gates green |
| 22b.4a | shipped 2026-05-09 | Form-A parser+printer arms for ClassDef / InstanceDef; `Type::Forall.constraints` round-trip gap fix; round-trip skip-list retired (70 → 106 fixtures green). | Round-trip green for every `examples/*.ail.json`; bench gates green |
| 22b.4b | dropped (post-22) | Originally: Prelude module Show#Int + `int_to_str` C-runtime primitive. Removed from milestone-22 scope per Amendments §"22b.4b dropped" — substantively separable runtime work; LLM-utility for primitive Show is weak. | (deferred) |
| 22c | shipped 2026-05-09 | Single end-to-end fixture: `class Foo a` + `data IntBox` + `instance Foo IntBox` + `main` printing 42. Surfaced and fixed a mono-pass user-ADT bug as part of close. | Fixture compiles, runs, correct stdout; bench gates green |

22c was deferred in 22a's queue. This brainstorm flagged it as a
shipping gap: the milestone is "typeclasses" but the Prelude path
alone (compiler-internal setup) does not validate that an LLM-author
can in fact define a class and an instance and have them work.
Promoting 22c with tight scope (one fixture, no `deriving`) closes
the gap without expanding the milestone substantially.

22b.3's synthetic fixture is the matching addition: monomorphisation
must be testable before the Prelude lands, so its iter cannot rely
on 22b.4 for end-to-end validation.

## Data flow

Per DESIGN.md Decision 11 §"Resolution and monomorphisation":

```
.ail.json modules
  ↓  workspace load
  ↓    ↓ collect ClassDef per (class-name → defining-module)
  ↓    ↓ collect InstanceDef per (class-name, type-hash)
  ↓    ↓ coherence + uniqueness + completeness + superclass checks
  ↓  Workspace { defs, registry }
  ↓
  ↓  per-FnDef typecheck
  ↓    ↓ collect residual constraints from method calls
  ↓    ↓ check residuals against declared (+ superclass-expanded)
  ↓    ↓ resolve fully-concrete constraints against registry
  ↓
  ↓  monomorphisation pass
  ↓    ↓ for each (resolved-method, concrete-type) pair:
  ↓    ↓   synthesise FnDef with hash-deterministic name
  ↓    ↓   substitute class param to concrete type in body
  ↓    ↓   rewrite original Call to target synthesised name
  ↓
  ↓  codegen sees only ordinary monomorphic FnDefs and direct calls
LLVM IR
```

No runtime dispatch, no dictionary passing. A class-method call
that cannot be monomorphised (constraint unresolved at entry point)
is a static error, not a runtime one.

## Error handling

Diagnostic catalogue, by phase. Codes follow the project's existing
kebab-case CLI convention.

**Workspace-load (registry-build) — landed in 22b.1:**

- `orphan-instance`
- `duplicate-instance`
- `missing-method`

**Workspace-load (registry-build) — 22b.2:**

- `missing-superclass-instance`
- `overriding-non-existent-method`
- `method-name-collision` (across two in-scope classes, or class
  method vs top-level function)

**Class-schema validation — 22b.2:**

- `kind-mismatch` (class param appears in applied position in any
  method signature)
- `invalid-superclass-param` (superclass `type` ≠ class's own `param`)
- `constraint-references-unbound-type-var`

**Per-FnDef typecheck — 22b.2:**

- `missing-constraint` (residual not covered by declared +
  superclass-expanded constraints)
- `no-instance` (fully concrete constraint with no registry entry)

No `ambiguous-instance` diagnostic — coherence makes registry keys
globally unique by construction.

## Testing strategy

**Per iter:**

- **22b.1** — already shipped. Schema-extension hash-stability
  tests + 5 registry-diagnostic fixtures.
- **22b.2** — one fixture per new diagnostic (one-fixture-one-
  diagnostic rule). Plus a positive fixture exercising
  `missing-constraint` recovery (constraint declared correctly
  → green).
- **22b.3** — synthetic class+instance fixture exercising the
  mono pass end-to-end. Hand-written `class Foo a where foo : a
  -> Int` + `instance Foo Int where foo i = i` + `fn main = print
  (foo 5)`. Verifies the synthesised def is emitted, called, and
  produces "5". Plus unit tests on the mono cache (no double
  emission).
- **22b.4** — Prelude fixture (`fn main = print 42` exercising
  `print` → `Show.show` → `show@Int`). Round-trip filter retires;
  all `examples/test_22b1_*` plus the new prose round-trip passes.
- **22c** — single user-class e2e: `class Greet a where greet :
  a borrow -> String` + `data Person { name : String }` +
  `instance Greet Person where greet p = "Hello, " ++ p.name` +
  `fn main = print (greet (Person { name = "world" }))`.

**Bench gates:** all three (`bench/check.py`,
`bench/compile_check.py`, `bench/cross_lang.py`) green at every
iter close. The mono pass is a no-op for fixtures without classes —
pre-22b modules stay bit-identical through compile.

## Acceptance criteria

The 22 milestone closes when **all** of the following hold:

1. 22b.1 + 22b.2 + 22b.3 + 22b.4 + 22c JOURNAL entries committed.
2. Audit suite (architect drift review against DESIGN.md +
   `bench/check.py` + `bench/compile_check.py` +
   `bench/cross_lang.py`) green.
3. The 22c user-class fixture compiles, runs, and produces correct
   stdout — with no Prelude class involvement (ensures the path is
   independently exercised).
4. Round-trip filter `test_22b1_*` is removed; all
   `examples/*.ail.json` pass round-trip.

## Out of scope (deferred, with substantive rationale)

**Operator routing** (`==`, `<`, `<=`, `>`, `>=` through `Eq`/`Ord`).
Deferred to a post-22 milestone bundled with bench re-baselining.
Rationale: the entire bench corpus typechecks differently when
operators route through classes; the new monomorphised trampolines
may codegen non-equivalently to the primitive form, which is a new
baseline rather than a regression. Re-baselining is its own work,
not an iter detail. The 22a "would risk firing the bench gate"
phrasing was an effort-rationale; the substantive form is the new-
baseline argument above.

**`deriving`.** Deferred to a post-22 milestone, contingent on 22
shipping clean. Rationale: `deriving` requires a generic-synthesis
pass at workspace-load (parsing `deriving (Eq, Show)`, synthesising
`InstanceDef` AST nodes, feeding them into the same coherence
pipeline as hand-written instances). That is new infrastructure,
not a 22-arc detail. Synthesising instances for a typeclass
mechanism whose user-class path has not been e2e-validated would
also stack risk.

**`Num` class.** No concrete promotion path. Decision 11 rationale
holds (class-based numeric overloading invokes literal-defaulting,
which axis-7 excluded). Numeric operations stay primitive and
per-type. Trigger for re-evaluation: a concrete user case that
cannot be expressed otherwise.

## Open commitments (22a → 22b implementer)

Three items 22a explicitly punted to the 22b implementer. The
brainstorm position on each:

**Mono-symbol naming format.** Implementer call in 22b.3.
Constraints: must contain the method name and a type indicator
legible in diagnostics; must be hash-stable; must coexist with the
existing mangling scheme `@ail_<module>_<def>`. Recommendation:
`<method>#<typesurfacename>` for primitive types (`show#Int`),
hash-suffix for compound types — but the implementer chooses.

**Form-B prose projection for ClassDef/InstanceDef.** Implementer
call in 22b.4. Constraints: must round-trip exactly with the JSON
form; must follow the existing C-like prose convention.
Recommendation: `class <Name> <param> { <method> : <FnSig> }` and
`instance <Class> <Type> { <method> = <body> }` parallel to existing
FnDef/DataDef forms — but the implementer chooses.

**Mode annotations on class methods.** Not a "convention" in the
strict sense — class method signatures are full FnSigs and carry
mode annotations per Decision 10. Position for the Prelude (22b.4):
`borrow` for all read-only methods (`show`, `eq`, `ne`, `lt`, `le`,
`gt`, `ge`). User-defined classes choose modes per method.

## Known costs

**Reserved-name footprint of the Prelude.** Once 22b.4 ships, the
following names are class-method-occupied at the workspace top
level: `show`, `eq`, `ne`, `lt`, `le`, `gt`, `ge`. A user-defined
function or class method with any of these names fires
`method-name-collision`. The cost is a small lexicon of forbidden
names; the benefit is no namespace fragmentation across the Prelude
classes.

## Amendments

### 2026-05-09 — Iteration split: 22b.4 → 22b.4a + 22b.4b

Original 22b.4 scope ("Prelude module Show/Eq/Ord on Int/Float/Bool/String,
print rewiring, Form-B parser/printer arms, round-trip filter retired")
splits into two iterations:

- **22b.4a (this iter):** Form-A parser+printer arms for ClassDef
  and InstanceDef; round-trip skip-list retired for `test_22b1_*`,
  `test_22b2_*`, `test_22b3_*`. Pure surface plumbing, no codegen
  or runtime change.
- **22b.4b (queued):** Prelude module containing `class Show a where
  show : (a borrow) -> Str`, `instance Show Int`, the new
  `int_to_str` C-runtime primitive, codegen wiring for it, and an
  end-to-end `show 42` fixture that prints `42` through the existing
  mono pass.

Substantive rationale: 22b.4a touches one crate (`ailang-surface`)
with no runtime/codegen risk; 22b.4b touches `runtime/`,
`ailang-codegen`, `ailang-check::builtins`, plus a new fixture, with
a different review surface and a different bench-gate exposure.
Bundling them was a brainstorm-time underestimate of the
runtime-side work; running them as separate iterations preserves
the bite-sized cycle the skill system enforces.

### 2026-05-09 — Form-A vs Form-B terminology fix

The original 22 spec ("Form-B parser/printer arms for ClassDef
/InstanceDef") and the 22b.1 round-trip-skip-list comment both
loosely call the s-expression projection "Form-B". That is a
terminology error: `crates/ailang-surface` is **Form-A** (the
parseable s-expression). `crates/ailang-prose` is **Form-B**
(human-readable, one-way projection — no parser by design). The
22b.4a arms are Form-A. Form-B printer arms for ClassDef /
InstanceDef are NOT required for the round-trip gate (it is
unparser-side); they are queued as an audit-cycle nicety, not as
a milestone-22 acceptance criterion.

The skip-list comment that read "the surface (Form-B) parser/printer
arms" was retired alongside the filter (Task 5).

### 2026-05-09 — Mono-symbol separator: `__` chosen over `#`

22a's brainstorm recommended `<method>#<typesurfacename>` for the
mono-symbol naming format with the implementer free to choose. 22b.3
implemented `<method>__<typesurfacename>` instead (e.g. `show__Int`,
`eq__Bool`). Reason: `#` is an invalid character in LLVM IR global
identifiers (`@ail_..._foo#Int_clos` is rejected by the LLVM
verifier). `__` is verifier-legal, identifier-legal in C-glue
contexts, and parses unambiguously into method + type surface name
(neither method names nor type surface names contain `__` by
convention). The recommendation in §"Open commitments (22a → 22b
implementer)" is updated to read `<method>__<typesurfacename>`.

Hash-suffix form for compound types (`<method>__<8-hex>`) follows
the same separator.

### 2026-05-09 — Forall-constraints surface arm gap (22b.4a.4.5)

Surfaced during 22b.4a Task 5: the existing Form-A parser/printer
arms dropped `Type::Forall.constraints` (the field added in 22b.2 to
carry class constraints on polymorphic types). Four `test_22b2_*`
fixtures regressed when the round-trip skip-list was lifted. Fixed
in 22b.4a.4.5 by extending `parse_forall_type` and the `Type::Forall`
arm of `write_type` to handle an optional `(constraints (constraint
<Class> <type>)+ )` clause. Round-trip is now bit-stable for every
forall-with-constraints fixture in `examples/`.

This was not in the original 22b.4 plan because the brainstorm did
not anticipate the gap — the skip-list was thought to mask only the
ClassDef/InstanceDef arms, not the forall-constraints arm too. The
forall-constraints fix is properly part of 22b.4a (cannot retire the
filter without it) and is recorded here for completeness.

### 2026-05-09 — Acceptance criteria 4 still binding

Acceptance criterion 4 ("Round-trip filter `test_22b1_*` is removed;
all `examples/*.ail.json` pass round-trip") is satisfied by 22b.4a.
The `test_22b2_*` and `test_22b3_*` filters added subsequently are
also retired here. No further round-trip gate work is required for
22 milestone close.

### 2026-05-09 — 22b.4b dropped: Prelude-for-primitives deferred to post-22

The 22b.4 → 22b.4a + 22b.4b split (recorded above) further reduces:
22b.4b is removed from milestone-22 scope entirely. Acceptance
criterion 1 amends to read "22b.1 + 22b.2 + 22b.3 + 22b.4a + 22c
JOURNAL entries committed".

Substantive rationale, per the project's feature-acceptance
criterion (LLM utility):

1. **No runtime primitive for `int_to_str` yet.** Show#Int's body
   needs to convert an `i64` to a heap-allocated `Str`. AILang
   currently only has constant interned strings (`@.str_*` LLVM
   globals). A heap-string ABI — runtime alloc, RC-count, drop —
   is independently new infrastructure. Wiring it through a class
   method first stacks risk: typeclass cascade + heap-string ABI
   + new C-runtime function + new codegen arm in one iter.

2. **Low LLM-utility for primitive Show.** An LLM that needs to
   render an `Int` writes `int_to_str x` (a direct primitive call)
   more naturally than `show x` (which routes through a Prelude
   class with one instance). Class dispatch shines when the LLM
   defines a user type and wants polymorphism; for primitives, the
   Prelude class is a thin wrapper with negative entropy (extra
   indirection, no new property).

3. **22c covers the user-class e2e independently.** The milestone's
   typeclass-machinery property is exercised end-to-end by 22c's
   user-class fixture (class Foo a + data Bar + instance Foo Bar
   + main calling foo). The Prelude path adds no semantic capability
   beyond what 22c demonstrates.

When does Prelude-for-primitives return? When concrete LLM-author
code surfaces a case that genuinely benefits from `Show a` /
`Eq a` / `Ord a` over per-type primitive functions — that case
becomes the brainstorm trigger for a post-22 Prelude milestone.
Until then, `int_to_str` ships as a plain primitive (or not at
all) the day a fixture needs it.

### 2026-05-09 — 22c scope tightened to vocabulary AILang already has

The original 22c spec (§"Components" row) sketched the e2e fixture
as `class Greet a where greet : a borrow -> String` + `data Person
{ name : String }` + `instance Greet Person where greet p = "Hello,
" ++ p.name`. That sketch references three constructs AILang does
not currently support:

- **`++` for `Str`** — no concat operator in `crates/ailang-check/src/builtins.rs::list()`.
- **Named-field record syntax `Person { name = "world" }`** — AILang's `data`/`ctor` form is positional (`(ctor MkPerson Str)`), not record-with-named-fields.
- **`print` polymorphic on `Show`** — see preceding amendment.

22c's acceptance property is "user-defined class + user-defined data
type + instance + call site, monomorphised, runs, prints right value".
That property holds with any vocabulary AILang has. The fixture is
re-scoped to:

```
class Foo a where
  foo : (a borrow) -> Int

data IntBox = MkIntBox Int

instance Foo IntBox where
  foo = λb. (match b { MkIntBox v => v })

fn main : () -> Unit !IO
  body: do io/print_int (foo (MkIntBox 42))
```

Stdout: `42\n`. No new primitives, no new surface syntax, no new
codegen paths beyond what 22b.3 shipped. The property the original
sketch was reaching for (typeclass dispatch over a user-defined ADT,
end-to-end, with a hand-readable expected output) holds identically.
