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

# miniKanren-on-SX: relational programming on the CEK/VM

miniKanren is not a language to parse — it is an **embedded DSL** implemented as a library
of SX functions. No tokenizer, no transpiler. The entire system is a set of `define` forms
in `lib/minikanren/`. Programs are SX expressions using the miniKanren API.

The unique angle: SX's delimited continuation machinery (`perform`/`cek-resume`, call/cc)
maps almost perfectly to the search monad miniKanren needs. Backtracking is cooperative
suspension, not a separate trail machine. This is the cleanest possible host for miniKanren.

End-state goal: **full core miniKanren** (`run`, `fresh`, `==`, `conde`, `condu`, `onceo`,
`project`, `matche`) + **core.logic-style relations** (`appendo`, `membero`, `listo`,
`numbero`, etc.) + **arithmetic constraints** (`fd` domain, `CLP(FD)` subset).

## Ground rules

- **Scope:** only touch `lib/minikanren/**` and `plans/minikanren-on-sx.md`. Do **not**
  edit `spec/`, `hosts/`, `shared/`, or other `lib/<lang>/`.
- **Shared-file issues** go under "Blockers" below with a minimal repro; do not fix here.
- **SX files:** use `sx-tree` MCP tools only.
- **Architecture:** pure library — no source parser. Programs are written in SX using the API.
- **Reference:** *The Reasoned Schemer* (Friedman/Byrd/Kiselyov) + Byrd's dissertation.
- **Commits:** one feature per commit. Keep `## Progress log` updated and tick boxes.

## Architecture sketch

```
SX program using miniKanren API
    │
    ├── lib/minikanren/unify.sx      — terms, variables, walk, unification, occurs check
    ├── lib/minikanren/substitution.sx — substitution as association list / hash table
    ├── lib/minikanren/stream.sx     — lazy streams of substitutions (via delay/force)
    ├── lib/minikanren/goals.sx      — == / fresh / conde / condu / onceo / project / matche
    ├── lib/minikanren/run.sx        — run* / run n — drive the search, extract answers
    ├── lib/minikanren/relations.sx  — standard relations: appendo, membero, listo, etc.
    └── lib/minikanren/clpfd.sx      — arithmetic constraints (CLP(FD) subset)
```

Key semantic mappings:
- **Logic variable** → SX vector of length 1 (mutable box); `make-var` creates fresh one;
  `walk` follows the substitution chain
- **Substitution** → SX association list (or hash table for performance) mapping var → term
- **Stream of substitutions** → lazy list using `delay`/`force` (Phase 9 of primitives)
- **Goal** → SX function `substitution → stream-of-substitutions`
- **`==`** → unifies two terms, extending substitution or failing (empty stream)
- **`fresh`** → introduces new logic variables; `(fresh (x y) goal)` → goal with x, y bound
- **`conde`** → interleave streams from multiple goal clauses (depth-first with interleaving)
- **`run n`** → drive the stream, collect first n substitutions, reify answers

## Roadmap

### Phase 1 — variables + unification
- [x] `make-var` → fresh logic variable (unique mutable box)
- [x] `var?` `v` → bool — is this a logic variable?
- [x] `walk` `term` `subst` → follow substitution chain to ground term or unbound var
- [x] `walk*` `term` `subst` → deep walk (recurse into lists/dicts)
- [x] `unify` `u` `v` `subst` → extended substitution or `#f` (failure)
      Handles: var/var, var/term, term/var, list unification, number/string/symbol equality.
      No occurs check by default; `unify-check` with occurs check as opt-in.
- [x] Empty substitution `empty-s` (dict-based via kit's `empty-subst` — assoc list was a sketch; kit ships dict, kept it)
- [x] Tests in `lib/minikanren/tests/unify.sx`: ground terms, vars, lists, failure, occurs

### Phase 2 — streams + goals
- [x] Stream type: `mzero` (empty), `unit s` (singleton), `mk-mplus` (interleave),
      `mk-bind` (apply goal to stream). Names mk-prefixed because SX has a host
      `bind` primitive that silently shadows user defines.
- [x] Lazy streams via thunks: a paused stream is a zero-arg fn; mk-mplus suspends
      and swaps when its left operand is paused, giving fair interleaving.
- [x] `==` goal: `(fn (s) (let ((s2 (mk-unify u v s))) (if s2 (unit s2) mzero)))`
- [x] `==-check` — opt-in occurs-checked equality goal
- [x] `succeed` / `fail` — trivial goals
- [x] `conj2` / `mk-conj` (variadic) — sequential conjunction
- [x] `disj2` / `mk-disj` (variadic) — interleaved disjunction (raw — `conde`
      adds the implicit-conj-per-clause sugar later)
- [x] `fresh` — introduces logic variables inside a goal body. Implemented as a
      defmacro: `(fresh (x y) g1 g2 ...)` ⇒ `(let ((x (make-var)) (y (make-var)))
      (mk-conj g1 g2 ...))`. Also `call-fresh` for programmatic goal building.
- [x] `conde` — sugar over disj+conj, one row per clause; defmacro that
      wraps each clause body in `mk-conj` and folds via `mk-disj`. Notes:
      with eager streams ordering is left-clause-first DFS; true interleaving
      requires paused thunks (Phase 4 recursive relations).
- [x] `condu` — committed choice. defmacro folding clauses into a runtime
      `condu-try` walker; first clause whose head goal yields a non-empty
      stream commits its first answer, rest-goals run on that single subst.
- [x] `onceo` — `(stream-take 1 (g s))`; trims a goal's stream to ≤1 answer.
- [x] Tests: basic goal composition, backtracking, interleaving (110 cumulative)

### Phase 3 — run + reification
- [x] `run*` `goal` → list of all answers (reified). defmacro: bind q-name as
      fresh var, conj goals, take all from stream, reify each.
- [x] `run n` `goal` → list of first n answers (defmacro; n = -1 means all)
- [x] `reify` `term` `subst` → walk* + build reification subst + walk* again
- [x] `reify-s` → maps each unbound var (in left-to-right walk order) to a
      `_.N` symbol via `(make-symbol (str "_." n))`
- [x] `fresh` with multiple variables — already shipped Phase 2B.
- [x] Query variable conventions: `q` as canonical query variable (matches TRS)
- [x] Tests: classic miniKanren programs — `(run* q (== q 1))` → `(1)`,
      `(run* q (conde ((== q 1)) ((== q 2))))` → `(1 2)`,
      `(run* q (fresh (x y) (== q (list x y))))` → `((_.0 _.1))`. Peano +
      `appendo` deferred to Phase 4.

### Phase 4 — standard relations
- [ ] `appendo` `l` `s` `ls` — list append, runs forwards and backwards
- [ ] `membero` `x` `l` — x is a member of l
- [ ] `listo` `l` — l is a proper list
- [ ] `nullo` `l` — l is empty
- [ ] `pairo` `p` — p is a pair (cons cell)
- [ ] `caro` `p` `a` — car of pair
- [ ] `cdro` `p` `d` — cdr of pair
- [ ] `conso` `a` `d` `p` — cons
- [ ] `firsto` / `resto` — aliases for caro/cdro
- [ ] `reverseo` `l` `r` — reverse of list
- [ ] `flatteno` `l` `f` — flatten nested lists
- [ ] `permuteo` `l` `p` — permutation of list
- [ ] `lengtho` `l` `n` — length as a relation (Peano or integer)
- [ ] Tests: run each relation forwards and backwards; generate from partial inputs

### Phase 5 — `project` + `matche` + negation
- [ ] `project` `(x ...) body` — access reified values of logic vars inside a goal;
      escapes to ground values for arithmetic or string ops
- [ ] `matche` — pattern matching over logic terms (extension from core.logic)
      `(matche l ((head . tail) goal) (() goal))`
- [ ] `conda` — soft-cut disjunction (like Prolog `->`)
- [ ] `condu` — committed choice (already in phase 2; refine semantics here)
- [ ] `nafc` — negation as finite failure with constraint
- [ ] Tests: Zebra puzzle, N-queens, Sudoku via `project`, family relations via `matche`

### Phase 6 — arithmetic constraints CLP(FD)
- [ ] Finite domain variables: `fd-var` with domain `[lo..hi]`
- [ ] `in` `x` `domain` — constrain x to domain
- [ ] `fd-eq` `x` `y` — x = y (constraint propagation)
- [ ] `fd-neq` `x` `y` — x ≠ y
- [ ] `fd-lt` `fd-lte` `fd-gt` `fd-gte` — ordering constraints
- [ ] `fd-plus` `x` `y` `z` — x + y = z (constraint)
- [ ] `fd-times` `x` `y` `z` — x * y = z
- [ ] Arc consistency propagation — when domain narrows, propagate to constrained vars
- [ ] Labelling: `fd-run` drives search by splitting domains when propagation stalls
- [ ] Tests: send-more-money, N-queens with CLP(FD), map coloring, cryptarithmetic

### Phase 7 — tabling (memoization of relations)
- [ ] `tabled` annotation: memoize calls to a relation using a hash table
- [ ] Prevents infinite loops in recursive relations like `patho` on cyclic graphs
- [ ] Producer/consumer scheduling for tabled relations (variant of SLG resolution)
- [ ] Tests: cyclic graph reachability, mutual recursion, Fibonacci via tabling

## Blockers

_(none yet)_

## Progress log

_Newest first._

- **2026-05-07** — **Phase 3 done** (run + reification): `lib/minikanren/run.sx` (~28 lines).
  `reify`/`reify-s`/`reify-name` for canonical `_.N` rendering of unbound vars in
  left-to-right occurrence order; `run*` / `run` / `run-n` defmacros. 18 new tests
  in `tests/run.sx`, including the **first classic miniKanren tests green**:
  `(run* q (== q 1))` → `(1)`; `(run* q (fresh (x y) (== q (list x y))))` →
  `((_.0 _.1))`. 128/128 cumulative.
- **2026-05-07** — **Phase 2 piece D + done** (`condu` / `onceo`): `lib/minikanren/condu.sx`.
  Both are commitment forms: `onceo` is `(stream-take 1 ...)`; `condu` walks clauses
  and commits the first one whose head produces an answer. 10 tests in `tests/condu.sx`,
  110/110 cumulative. Phase 2 complete — ready for Phase 3 (run + reification).
- **2026-05-07** — **Phase 2 piece C** (`conde`): `lib/minikanren/conde.sx` — single
  defmacro folding clauses through `mk-disj` with internal `mk-conj`. 9 tests in
  `tests/conde.sx`, 100/100 cumulative. Confirmed eager DFS ordering for ==-only
  streams; true interleaving is a Phase 4 concern (paused thunks under recursion).
- **2026-05-07** — **Phase 2 piece B** (`fresh`): `lib/minikanren/fresh.sx` (~10 lines).
  defmacro form for nice user-facing syntax + `call-fresh` for programmatic use.
  9 new tests in `tests/fresh.sx`, 91/91 cumulative.
- **2026-05-07** — **Phase 2 piece A** (streams + ==/conj/disj): `lib/minikanren/stream.sx`
  (mzero/unit/mk-mplus/mk-bind/stream-take, ~25 lines of code) + `lib/minikanren/goals.sx`
  (succeed/fail/==/==-check/conj2/disj2/mk-conj/mk-disj, ~30 lines). Found and noted
  a host-primitive name clash: `bind` is built in and silently shadows user defines —
  must use `mk-bind`/`mk-mplus` etc. throughout. 34 tests in `tests/goals.sx`,
  82/82 cumulative all green. fresh/conde/condu/onceo still pending.
- **2026-05-07** — **Phase 1 done**: `lib/minikanren/unify.sx` (53 lines, ~22 lines of actual code) +
  `lib/minikanren/tests/unify.sx` (48 tests, all green). Kit consumption: `walk-with`,
  `unify-with`, `occurs-with`, `extend`, `empty-subst`, `mk-var`, `is-var?`, `var-name`
  all supplied by `lib/guest/match.sx`. Local additions: a miniKanren-flavoured cfg
  (treats native SX lists as cons-pairs via `:ctor-head = :pair`, occurs-check off),
  `make-var` fresh-counter, deep `mk-walk*` (kit's `walk*` only recurses into `:ctor`
  form, not native lists), and `mk-unify` / `mk-unify-check` thin wrappers. The kit
  earns its keep ~3× over by line count — confirms lib-guest match kit is reusable
  for logic-language hosts as designed.