# Tcl-on-SX completion plan — SX capabilities first

Tcl phases 1–6 are complete (329/329 tests). This plan covers the remaining
limitations, ordered by the SX work needed to enable them.

## Key audit findings

Several apparent gaps are already solved in SX:

- **Floats** — SX parses `3.14` natively; `(+ 1.5 2.5) → 4.0`; `str` formats
  with `%g` (compact, no trailing zeros). `floor`/`ceil`/`round`/`truncate`
  all exist in `spec/primitives.sx`.
- **Regex** — `regexp-match`, `regexp-match-all`, `regexp-replace`,
  `regexp-replace-all`, `regexp-split` are registered OCaml primitives using
  `Re.Pcre` (`hosts/ocaml/lib/sx_primitives.ml`).
- **`call/cc` multi-shot** — works. `set!` on closed-over vars works. Fibers
  are implementable as a pure SX library.
- **`perform` user-accessible** — `(perform :foo 42)` from user code suspends
  the evaluator and emits an IO request. The algebraic effects model is
  already half-built.
- **No `file-read`/`clock-seconds`** — not yet registered as OCaml primitives.
  Only string ports exist. Would need small OCaml additions.
- **No `env-as-value`** — environments are internal OCaml values, not
  inspectable from SX user code.

---

## Phase 1 — Zero-cost wins (no SX changes, only `lib/tcl/`)

Everything here is pure Tcl implementation work.

| Status | Work | Effort | Unlocks in Tcl |
|---|---|---|---|
| [x] | Float in `expr` — detect `.` in number tokens, route through float ops instead of `parse-int` | half day | `expr {3.14 * 2}`, `expr {sqrt(2.0)}`, float comparisons |
| [x] | `regexp pattern str` and `regsub pattern str repl` wrapping existing SX primitives | few hours | pattern matching, text processing |
| [x] | `apply {args body} ?arg…?` — anonymous proc call | 1 hour | higher-order functions, `lmap` idiom |
| [x] | `array get/set/names/size/exists/unset` commands | half day | array variables (tokenizer already parses `$arr(key)`) |

**Total: ~2 days. Zero SX changes.**

---

## Phase 2 — `lib/fiber.sx` (pure SX library, no OCaml)

| Status | Work |
|---|---|
| [x] | Create `lib/fiber.sx` — `make-fiber` / `fiber-resume` / `fiber-done?` |
| [x] | Rewrite `tcl-cmd-coroutine` to use `make-fiber` (true suspension) |

`call/cc` is multi-shot and `set!` on closed-over vars both work. Fibers are
implementable as a pure SX library using symmetric continuation swapping:

```scheme
; lib/fiber.sx — canonical fiber primitive for all hosted languages
(define make-fiber
  (fn (thunk)
    (define slot-k nil)
    (define slot-caller nil)
    (define slot-done false)
    (fn (resume-val)
      (call/cc (fn (caller-k)
        (set! slot-caller caller-k)
        (if (nil? slot-k)
          (begin (thunk resume-val) (set! slot-done true) (caller-k nil))
          (slot-k resume-val)))))))

(define fiber-yield
  (fn (val)
    (call/cc (fn (k)
      (set! slot-k k)
      (slot-caller val)))))
```

Each coroutine becomes a fiber. `yield` swaps to the caller; calling the
coroutine name swaps back. True suspension, not eager pre-execution.

**Broader value:** Ruby fibers, Python generators, Lua coroutines, async event
loops, cooperative schedulers all sit on top of the same library.

**Alternatively:** `perform` is user-accessible. A Tcl scheduler living outside
the SX evaluator (the OCaml host or an SX event loop) could catch
`(perform :fiber-yield val)` and dispatch it — the algebraic effects model,
already half-built.

**Total: 2–3 days. Produces `lib/fiber.sx` as a lasting SX contribution.**
Tcl coroutines then rewrite using `make-fiber` for true suspension.

---

## Phase 3 — Small OCaml additions (`sx_primitives.ml`)

Each is ~10–20 lines of OCaml. All are useful across the whole platform, not
just Tcl.

| Status | Primitive | OCaml effort | Unlocks |
|---|---|---|---|
| [x] | `(file-read path)` → string | tiny | Tcl `open`/`read`, SX scripts reading files |
| [x] | `(file-write path str)` → nil | tiny | Tcl `open`/`puts` to files |
| [x] | `(file-exists? path)` → bool | tiny | Tcl `file exists` |
| [x] | `(file-glob pattern)` → list | small | Tcl `glob` |
| [x] | `(clock-seconds)` → int | tiny | Tcl `clock seconds` |
| [x] | `(clock-format n fmt)` → string | small (wraps `strftime`) | Tcl `clock format` |

**Total: 1 day. One focused afternoon of OCaml.**

---

## Phase 4 — env-as-value (architectural) ✓



`uplevel`/`upvar` required an explicit frame stack because SX environments
aren't inspectable from user code. Adding:

```scheme
(current-env)            ; → env value
(eval-in-env env expr)   ; → result
(env-lookup env key)     ; → value or nil
(env-extend env key val) ; → new env (non-mutating)
```

...would let `uplevel N` be literally "look up env N levels up, eval in it."
The Tcl frame stack (hundreds of lines) collapses to ~10 lines.

Also benefits: metacircular evaluators, REPL tooling, live debugging (inspect
any scope), the sx_docs server's eval endpoint.

More invasive — touches `sx_types.ml` and `sx_server.ml` — but a meaningful
architectural improvement worth doing when the moment is right.

**Total: 2–3 days. High architectural value, not urgent.**

---

## Suggested order

1. **Phase 1** — immediate Tcl wins, zero risk, proves the approach
2. **Phase 2** (`lib/fiber.sx`) — the interesting SX work, benefits all hosted languages
3. **Phase 3** (OCaml primitives) — quick practical completions
4. **Phase 4** — architectural cleanup when it's worth the invasiveness

Phases 1+2+3 ≈ one focused week. Tcl is genuinely complete, and `lib/fiber.sx`
becomes a lasting SX contribution used by every future hosted language.

---

## Progress log

_Newest first._

- 2026-05-06: Phase 4 env-as-value — current-env (special form via Sx_ref.register_special_form), eval-in-env (primitive in setup_evaluator_bridge), env-lookup + env-extend (in setup_env_operations); 5 idiom tests; 342/342 green
- 2026-05-06: Phase 3 OCaml primitives — file-read/write/append/exists?/glob + clock-seconds/milliseconds/format in sx_primitives.ml + unix dep; tcl-cmd-clock/file wired up; 337/337 green
- 2026-05-06: Phase 2 coroutine rewrite — `tcl-cmd-coroutine` now creates a `make-fiber`; `tcl-cmd-yield` calls `:coro-yield-fn` (threaded through interp); true suspension; 337/337 green
- 2026-05-06: Phase 2 fiber.sx — `make-fiber`/`fiber-resume`/`fiber-done?` using call/cc + set!; bidirectional value passing; generator and echo tests pass
- 2026-05-06: Phase 1 array — `tcl-cmd-array` get/set/names/size/exists/unset; frame-local key scanning with prefix `arrname(`; 337/337 tests green
- 2026-05-06: Phase 1 apply — `tcl-cmd-apply` wraps `tcl-call-proc`, parses `{args body}` funcList, full frame isolation; 329/329 tests green
- 2026-05-06: Phase 1 regexp/regsub — `tcl-cmd-regexp`/`tcl-cmd-regsub` wrapping `make-regexp`/`regexp-match`/`regexp-match-all`/`regexp-replace`/`regexp-replace-all`; -nocase/-all/-inline/-all flags; matchVar + subgroup capture; 329/329 tests green
- 2026-05-06: Phase 1 float expr — `tcl-num-float?`, `tcl-parse-num`, float-aware `tcl-apply-binop`/`tcl-apply-func`/unary-minus/`**`; `sqrt`/`floor`/`ceil`/`round`/`sin`/`cos`/`tan`/`pow`/`exp`/`log` all float-native; 329/329 tests green

---

## What stays out of scope

- `package require` of binary loadables
- Full `clock format` locale support  
- Tk / GUI
- Threads (mapped to coroutines only, as planned)
- Full POSIX file I/O (seek/tell/async) — stubs are fine
