rose-ash/plans/haskell-on-sx.md

# Haskell-on-SX: mini-Haskell with real laziness

Mini-Haskell is the research-paper-worthy demo. Laziness is native to the SX runtime (thunks are already a first-class type); algebraic data types map onto tagged lists; typeclasses map onto dictionary passing; IO maps onto `perform`/`resume`. Hindley-Milner inference is the one real piece of new work.

End-state goal: a **Haskell 98 subset** that runs the small classic programs (sieve of Eratosthenes lazy stream, fibonacci as infinite list, naive quicksort, n-queens, expression evaluator) plus a ~150-test corpus.

## Scope decisions (defaults — override)

- **Standard:** Haskell 98 subset. No GHC extensions (no `DataKinds`, no `GADTs`, no `TypeFamilies`, no `TemplateHaskell`).
- **Phase 1-3 are untyped** — we get the evaluator right first with laziness + ADTs, then add HM inference in phase 4. This is deliberate: typing is the hard bit and will take a full phase on its own.
- **Typeclasses:** dictionary passing, no overlap, no orphan instances. Added in phase 5.
- **Layout rule:** yes — phase 1 implements Haskell's indentation-sensitive parsing (painful but required).
- **Test corpus:** custom. No GHC test suite. Bundle classic programs + ~100 hand-written expression-level tests + mini Prelude tests.

## Ground rules

- **Scope:** only `lib/haskell/**` and `plans/haskell-on-sx.md`. No edits to `spec/`, `hosts/`, `shared/`, or other language dirs.
- **SX files:** `sx-tree` MCP tools only.
- **Architecture:** Haskell source → AST → desugared-core → SX AST → CEK. Thunks on the SX side provide laziness natively.
- **Commits:** one feature per commit. Keep `## Progress log` updated.

## Architecture sketch

```
Haskell source
    │
    ▼
lib/haskell/tokenizer.sx  — idents, operators, layout-sensitive indentation
    │
    ▼
lib/haskell/parser.sx     — AST: modules, data decls, type sigs, fn clauses, expressions
    │
    ▼
lib/haskell/desugar.sx    — surface → core: case-of-case, do-notation, list comp, guards
    │
    ▼
lib/haskell/transpile.sx  — core → SX AST, wrapping everything in thunks for laziness
    │
    ▼
lib/haskell/runtime.sx    — force, ADT constructors, Prelude, typeclass dicts (phase 5+)
    │
    ▼
existing CEK / VM
```

Key mappings:
- **Laziness** = every function argument is an SX thunk; `force` is WHNF reduction. SX already has `make-thunk` from the trampolining evaluator — we reuse it.
- **Pattern match** = forces the scrutinee to WHNF, then structural match on the tag
- **ADT** = `data Maybe a = Nothing | Just a` compiles to tagged lists: `(:Nothing)` and `(:Just <thunk>)`
- **Typeclass** = each class becomes a record type; each instance becomes a record value; each method becomes a projection; the elaborator inserts the dict at each call site (phase 5)
- **IO** = `IO a` is a function `World -> (a, World)` internally; in practice uses `perform`/`resume` for actual side effects
- **Layout** = offside rule; inserted virtual braces + semis during a lexer-parser feedback pass

## Roadmap

### Phase 1 — tokenizer + parser + layout rule
- [ ] Tokenizer: reserved words, qualified names, operators, numbers (int, float, Rational later), chars/strings, comments (`--` and `{-` nested)
- [ ] Layout algorithm: turn indentation into virtual `{`, `;`, `}` tokens per Haskell 98 §10.3
- [ ] Parser: modules, imports (stub), top-level decls, type sigs, function clauses with patterns + guards + where-clauses, expressions with operator precedence, lambdas, `let`, `if`, `case`, `do`, list comp, sections
- [ ] AST design modelled on GHC's HsSyn at a surface level
- [ ] Unit tests in `lib/haskell/tests/parse.sx`

### Phase 2 — desugar + eager-ish eval + ADTs (untyped)
- [ ] Desugar: guards → nested `if`s; `where` → `let`; list comp → `concatMap`-based; do-notation stays for now (desugared in phase 3)
- [ ] `data` declarations register constructors in runtime
- [ ] Pattern match (tag-based, value-level): atoms, vars, wildcards, constructor patterns, `as` patterns, nested
- [ ] Evaluator (still strict internally — laziness in phase 3): `let`, `lambda`, application, `case`, literals, constructors
- [ ] 30+ eval tests in `lib/haskell/tests/eval.sx`

### Phase 3 — laziness + classic programs
- [ ] Transpile to thunk-wrapped SX: every application arg becomes `(make-thunk (lambda () <arg>))`
- [ ] `force` = SX eval-thunk-to-WHNF primitive
- [ ] Pattern match forces scrutinee before matching
- [ ] Infinite structures: `repeat x`, `iterate f x`, `[1..]`, Fibonacci stream, sieve of Eratosthenes
- [ ] `seq`, `deepseq` from Prelude
- [ ] Do-notation for a stub `IO` monad (just threading, no real side effects yet)
- [ ] Classic programs in `lib/haskell/tests/programs/`:
  - [ ] `fib.hs` — infinite Fibonacci stream
  - [ ] `sieve.hs` — lazy sieve of Eratosthenes
  - [ ] `quicksort.hs` — naive QS
  - [ ] `nqueens.hs`
  - [ ] `calculator.hs` — parser combinator style expression evaluator
- [ ] `lib/haskell/conformance.sh` + runner; `scoreboard.json` + `scoreboard.md`
- [ ] Target: 5/5 classic programs passing

### Phase 4 — Hindley-Milner inference
- [ ] Algorithm W: unification + type schemes + generalisation + instantiation
- [ ] Report type errors with meaningful positions
- [ ] Reject untypeable programs that phase 3 was accepting
- [ ] Type-sig checking: user writes `f :: Int -> Int`; verify
- [ ] Let-polymorphism
- [ ] Unit tests: inference for 50+ expressions

### Phase 5 — typeclasses (dictionary passing)
- [ ] `class` / `instance` declarations
- [ ] Dictionary-passing elaborator: inserts dict args at call sites
- [ ] Standard classes: `Eq`, `Ord`, `Show`, `Num`, `Functor`, `Monad`, `Applicative`
- [ ] `deriving (Eq, Show)` for ADTs

### Phase 6 — real IO + Prelude completion
- [ ] Real `IO` monad backed by `perform`/`resume`
- [ ] `putStrLn`, `getLine`, `readFile`, `writeFile`, `print`
- [ ] Full-ish Prelude: `Maybe`, `Either`, `List` functions, `Map`-lite
- [ ] Drive scoreboard toward 150+ passing

## Progress log

_Newest first._

- _(not started)_

## Blockers

- _(none yet)_