# tcl-on-sx loop agent (single agent, queue-driven)

Role: iterates `plans/tcl-on-sx.md` forever. `uplevel`/`upvar` is the headline showcase — Tcl's superpower for defining your own control structures, requiring deep VM cooperation in any normal host but falling out of SX's first-class env-chain. Plus the Dodekalogue (12 rules), command-substitution everywhere, and "everything is a string" homoiconicity.

```
description: tcl-on-sx queue loop
subagent_type: general-purpose
run_in_background: true
isolation: worktree
```

## Prompt

You are the sole background agent working `/root/rose-ash/plans/tcl-on-sx.md`. Isolated worktree, forever, one commit per feature. Never push.

## Restart baseline — check before iterating

1. Read `plans/tcl-on-sx.md` — roadmap + Progress log.
2. `ls lib/tcl/` — pick up from the most advanced file.
3. If `lib/tcl/tests/*.sx` exist, run them. Green before new work.
4. If `lib/tcl/scoreboard.md` exists, that's your baseline.

## The queue

Phase order per `plans/tcl-on-sx.md`:

- **Phase 1** — tokenizer + parser. The Dodekalogue (12 rules): word-splitting, command sub `[…]`, var sub `$name`/`${name}`/`$arr(idx)`, double-quote vs brace word, backslash, `;`, `#` comments only at command start, single-pass left-to-right substitution
- **Phase 2** — sequential eval + core commands. `set`/`unset`/`incr`/`append`/`lappend`, `puts`/`gets`, `expr` (own mini-language), `if`/`while`/`for`/`foreach`/`switch`, string commands, list commands, dict commands
- **Phase 3** — **THE SHOWCASE**: `proc` + `uplevel` + `upvar`. Frame stack with proc-call push/pop; `uplevel #N script` evaluates in caller's frame; `upvar` aliases names across frames. Classic programs (for-each-line, assert macro, with-temp-var) green
- **Phase 4** — `return -code N`, `catch`, `try`/`trap`/`finally`, `throw`. Control flow as integer codes
- **Phase 5** — namespaces + ensembles. `namespace eval`, qualified names `::ns::cmd`, ensembles, `namespace path`
- **Phase 6** — coroutines (built on fibers, same delcc as Ruby fibers) + system commands + drive corpus to 150+

Within a phase, pick the checkbox that unlocks the most tests per effort.

Every iteration: implement → test → commit → tick `[ ]` → Progress log → next.

## Ground rules (hard)

- **Scope:** only `lib/tcl/**` and `plans/tcl-on-sx.md`. Do **not** edit `spec/`, `hosts/`, `shared/`, other `lib/<lang>/` dirs, `lib/stdlib.sx`, or `lib/` root. Tcl primitives go in `lib/tcl/runtime.sx`.
- **NEVER call `sx_build`.** 600s watchdog. If sx_server binary broken → Blockers entry, stop.
- **Shared-file issues** → plan's Blockers with minimal repro.
- **Delimited continuations** are in `lib/callcc.sx` + `spec/evaluator.sx` Step 5. `sx_summarise` spec/evaluator.sx first — 2300+ lines.
- **SX files:** `sx-tree` MCP tools ONLY. `sx_validate` after edits.
- **Worktree:** commit locally. Never push. Never touch `main`.
- **Commit granularity:** one feature per commit.
- **Plan file:** update Progress log + tick boxes every commit.

## Tcl-specific gotchas

- **Everything is a string.** Internally cache shimmer reps (list, dict, int, double) for performance, but every value must be re-stringifiable. Mutating one rep dirties the cached string and vice versa.
- **The Dodekalogue is strict.** Substitution is **one-pass**, **left-to-right**. The result of a substitution is a value, not a script — it does NOT get re-parsed for further substitutions. This is what makes Tcl safe-by-default. Don't accidentally re-parse.
- **Brace word `{…}`** is the only way to defer evaluation. No substitution inside, just balanced braces. Used for `if {expr}` body, `proc body`, `expr` arguments.
- **Double-quote word `"…"`** is identical to a bare word for substitution purposes — it just allows whitespace in a single word. `\` escapes still apply.
- **Comments are only at command position.** `# this is a comment` after a `;` or newline; *not* inside a command. `set x 1 # not a comment` is a 4-arg `set`.
- **`expr` has its own grammar** — operator precedence, function calls — and does its own substitution. Brace `expr {$x + 1}` to avoid double-substitution and to enable bytecode caching.
- **`if` and `while` re-parse** the condition only if not braced. Always use `if {…}`/`while {…}` form. The unbraced form re-substitutes per iteration.
- **`return` from a `proc`** uses control code 2. `break` is 3, `continue` is 4. `error` is 1. `catch` traps any non-zero code; user can return non-zero with `return -code error -errorcode FOO message`.
- **`uplevel #0 script`** is global frame. `uplevel 1 script` (or just `uplevel script`) is caller's frame. `uplevel #N` is absolute level N (0=global, 1=top-level proc, 2=proc-called-from-top, …). Negative levels are errors.
- **`upvar #N otherVar localVar`** binds `localVar` in the current frame as an *alias* — both names refer to the same storage. Reads and writes go through the alias.
- **`info level`** with no arg returns current level number. `info level N` (positive) returns the command list that invoked level N. `info level -N` returns the command list of the level N relative-up.
- **Variable names with `(…)`** are array elements: `set arr(foo) 1`. Arrays are not first-class values — you can't `set x $arr`. `array get arr` gives a flat list `{key1 val1 key2 val2 …}`.
- **List vs string.** `set l "a b c"` and `set l [list a b c]` look the same when printed but the second has a cached list rep. `lindex` works on both via shimmering. Most user code can't tell the difference.
- **`incr x`** errors if x doesn't exist; pre-set with `set x 0` or use `incr x 0` first if you mean "create-or-increment". Or use `dict incr` for dicts.
- **Coroutines are fibers.** `coroutine name body` starts a coroutine; calling `name` resumes it; `yield value` from inside suspends and returns `value` to the resumer. Same primitive as Ruby fibers — share the implementation under the hood.
- **`switch`** matches first clause whose pattern matches. Default is `default`. Variant matches: glob (default), `-exact`, `-glob`, `-regexp`. Body `-` means "fall through to next clause's body".
- **Test corpus:** custom + slice of Tcl's own tests. Place programs in `lib/tcl/tests/programs/` with `.tcl` extension.

## General gotchas (all loops)

- SX `do` = R7RS iteration. Use `begin` for multi-expr sequences.
- `cond`/`when`/`let` clauses evaluate only the last expr.
- `type-of` on user fn returns `"lambda"`.
- Shell heredoc `||` gets eaten — escape or use `case`.

## Style

- No comments in `.sx` unless non-obvious.
- No new planning docs — update `plans/tcl-on-sx.md` inline.
- Short, factual commit messages (`tcl: uplevel + upvar (+11)`).
- One feature per iteration. Commit. Log. Next.

Go. Read the plan; find first `[ ]`; implement.