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
- [x] Tokenizer: reserved words, qualified names, operators, numbers (int, float, Rational later), chars/strings, comments (`--` and `{-` nested)
- [x] Layout algorithm: turn indentation into virtual `{`, `;`, `}` tokens per Haskell 98 §10.3
- Parser (split into sub-items — implement one per iteration):
  - [x] Expressions: atoms, parens, tuples, lists, ranges, application, infix with full Haskell-98 precedence table, unary `-`, backtick operators, lambdas, `if`, `let`
  - [x] `case … of` and `do`-notation expressions (plus minimal patterns needed for arms/binds: var, wildcard, literal, 0-arity and applied constructor, tuple, list)
  - [x] Patterns — full: `as` patterns, nested, negative literal, `~` lazy, infix constructor (`:` / consym), extend lambdas/let with non-var patterns
  - [ ] Top-level decls: function clauses, type signatures, `data`, `type`, `newtype`, fixity decls
  - [ ] `where` clauses + guards
  - [ ] Module header + imports (stub)
  - [ ] List comprehensions + operator sections
- [ ] AST design modelled on GHC's HsSyn at a surface level
- [x] Unit tests in `lib/haskell/tests/parse.sx` (43 tokenizer tests, all green)

### 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._

- **2026-04-24** — Phase 1: full patterns. Added `as` patterns
  (`name@apat` → `(:p-as NAME PAT)`), lazy patterns (`~apat` →
  `(:p-lazy PAT)`), negative literal patterns (`-N` / `-F` resolving
  eagerly in the parser so downstream passes see a plain `(:p-int -1)`),
  and infix constructor patterns via a right-associative single-band
  layer on top of `hk-parse-pat-lhs` for any `consym` or reservedop `:`
  (so `x : xs` parses as `(:p-con ":" [x, xs])`, `a :+: b` likewise).
  Extended `hk-apat-start?` with `-` and `~` so the pattern-argument
  loops in lambdas and constructor applications pick these up.
  Lambdas now parse apat parameters instead of bare varids — so the
  `:lambda` AST is `(:lambda APATS BODY)` with apats as pattern nodes.
  `hk-parse-bind` became a plain `pat = expr` form, so `:bind` now has
  a pattern LHS throughout (simple `x = 1` → `(:bind (:p-var "x") …)`);
  this picks up `let (x, y) = pair in …` and `let Just x = m in x`
  automatically, and flows through `do`-notation lets. Eight existing
  tests updated to the pattern-flavoured AST. Also fixed a pragmatic
  layout issue that surfaced in multi-line `let`s: when a layout-indent
  would emit a spurious `;` just before an `in` token (because the
  let block had already been closed by dedent), `hk-peek-next-reserved`
  now lets the layout pass skip that indent and leave closing to the
  existing `in` handler. 18 new tests in
  `lib/haskell/tests/parser-patterns.sx` cover every pattern variant,
  lambda with mixed apats, let pattern-bindings (tuple / constructor /
  cons), and do-bind with a tuple pattern. 138/138 green.

- **2026-04-24** — Phase 1: `case … of` and `do`-notation parsers. Added `hk-parse-case`
  / `hk-parse-alt`, `hk-parse-do` / `hk-parse-do-stmt` / `hk-parse-do-let`, plus the
  minimal pattern language needed to make arms and binds meaningful:
  `hk-parse-apat` (var, wildcard `_`, int/float/string/char literal, 0-arity
  conid/qconid, paren+tuple, list) and `hk-parse-pat` (conid applied to
  apats greedily). AST nodes: `:case SCRUT ALTS`, `:alt PAT BODY`, `:do STMTS`
  with stmts `:do-expr E` / `:do-bind PAT E` / `:do-let BINDS`, and pattern
  tags `:p-wild` / `:p-int` / `:p-float` / `:p-string` / `:p-char` / `:p-var`
  / `:p-con NAME ARGS` / `:p-tuple` / `:p-list`. `do`-stmts disambiguate
  `pat <- e` vs bare expression with a forward paren/bracket/brace-balanced
  scan for `<-` before the next `;`/`}` — no backtracking, no AST rewrite.
  `case` and `do` accept both implicit (`vlbrace`/`vsemi`/`vrbrace`) and
  explicit braces. Added to `hk-parse-lexp` so they participate fully in
  operator-precedence expressions. 19 new tests in
  `lib/haskell/tests/parser-case-do.sx` cover every pattern variant,
  explicit-brace `case`, expression scrutinees, do with bind/let/expr,
  multi-binding `let` in `do`, constructor patterns in binds, and
  `case`/`do` nested inside `let` and lambda. The full pattern item (as
  patterns, negative literals, `~` lazy, lambda/let pattern extension)
  remains a separate sub-item. 119/119 green.

- **2026-04-24** — Phase 1: expression parser (`lib/haskell/parser.sx`, ~380 lines).
  Pratt-style precedence climbing against a Haskell-98-default op table (24
  operators across precedence 0–9, left/right/non assoc, default infixl 9 for
  anything unlisted). Supports literals (int/float/string/char), varid/conid
  (qualified variants folded into `:var` / `:con`), parens / unit / tuples,
  list literals, ranges `[a..b]` and `[a,b..c]`, left-associative application,
  unary `-`, backtick operators (`x \`mod\` 3`), lambdas, `if-then-else`, and
  `let … in` consuming both virtual and explicit braces. AST uses keyword
  tags (`:var`, `:op`, `:lambda`, `:let`, `:bind`, `:tuple`, `:range`,
  `:range-step`, `:app`, `:neg`, `:if`, `:list`, `:int`, `:float`, `:string`,
  `:char`, `:con`). The parser skips a leading `vlbrace` / `lbrace` so it can
  be called on full post-layout output, and uses a `raise`-based error channel
  with location-lite messages. 42 new tests in `lib/haskell/tests/parser-expr.sx`
  cover literals, identifiers, parens/tuple/unit, list + range, app associativity,
  operator precedence (mul over add, cons right-assoc, function-composition
  right-assoc, `$` lowest), backtick ops, unary `-`, lambda multi-param,
  `if` with infix condition, single- and multi-binding `let` (both implicit
  and explicit braces), plus a few mixed nestings. 100/100 green.

- **2026-04-24** — Phase 1: layout algorithm (`lib/haskell/layout.sx`, ~260 lines)
  implementing Haskell 98 §10.3. Two-pass design: a pre-pass augments the raw
  token stream with explicit `layout-open` / `layout-indent` markers (suppressing
  `<n>` when `{n}` already applies, per note 3), then an L pass consumes the
  augmented stream against a stack of implicit/explicit layout contexts and
  emits `vlbrace` / `vsemi` / `vrbrace` tokens; newlines are dropped. Supports
  the initial module-level implicit open (skipped when the first token is
  `module` or `{`), the four layout keywords (`let`/`where`/`do`/`of`), explicit
  braces disabling layout, dedent closing nested implicit blocks while also
  emitting `vsemi` at the enclosing level, and the pragmatic single-line
  `let … in` rule (emit `}` when `in` meets an implicit let). 15 new tests
  in `lib/haskell/tests/layout.sx` cover module-start, do/let/where/case/of,
  explicit braces, multi-level dedent, line continuation, and EOF close-down.
  Shared test helpers moved to `lib/haskell/testlib.sx` so both test files
  can share one `hk-test`. `test.sh` preloads tokenizer + layout + testlib.
  58/58 green.

- **2026-04-24** — Phase 1: Haskell 98 tokenizer (`lib/haskell/tokenizer.sx`, 490 lines)
  covering idents (lower/upper/qvarid/qconid), 23 reserved words, 11 reserved ops,
  varsym/consym operator chains, integer/hex/octal/float literals incl. exponent
  notation, char + string literals with escape sequences, nested `{- ... -}` block
  comments with depth counter, `-- ... EOL` line comments (respecting the
  "followed by symbol = not a comment" Haskell 98 rule), backticks, punctuation,
  and explicit `newline` tokens for the upcoming layout pass. 43 structural tests
  in `lib/haskell/tests/parse.sx`, a lightweight `hk-deep=?` equality helper
  and a custom `lib/haskell/test.sh` runner (pipes through the OCaml epoch
  protocol, falls back to the main-repo build when run from a worktree). 43/43
  green.

  Also peeked at `/root/rose-ash/sx-haskell/` per briefing: that directory is a
  Haskell program implementing an **SX interpreter** (Types.hs, Eval.hs,
  Primitives.hs, etc. — ~2800 lines of .hs) — the *opposite* direction from this
  project. Nothing to fold in.

  Gotchas hit: `emit!` and `peek` are SX evaluator special forms, so every local
  helper uses the `hk-` prefix. `cond`/`when`/`let` clauses evaluate ONLY the
  last expression; multi-expression bodies MUST be wrapped in `(do ...)`. These
  two together account for all the tokenizer's early crashes.

## Blockers

- _(none yet)_