flan/DISCUSS.md

# Let's discuss

Open questions, raised and deliberately not answered yet. Nothing here is a decision or a task. Each entry is the
question as asked, plus what is already known in this repo that bears on it — so the investigation starts from what
exists rather than from scratch.

Settled decisions live in `NEXT.md`. Reasons for what already exists live in `BUILT.md`.

---

## 1. `i`, the inspector, and the frame it cannot see — answered and built

Answered, and built as option 2. `BUILT.md`'s "Two ways to root a walk, and why neither subsumes the other" is where
it lives now, including the correction to what this entry said about rooting at an address. The number is kept because
other files cite these by number; nothing here is open.

## 2. Annotating the IR and the disassembly with the source

**The IR half is nearly free and should just be done.** `emit.ml` writes `.ll` as text, so a comment costs nothing and
cannot break anything, and every typed IR node already carries a `Loc.t`.

**The disassembly half, with the optimisation question settled.** `objdump` already interleaves source into a listing
when DWARF is present (`-S`), and the daemon already shells out to objdump — so the `-O0` case is close to a flag.

Settled in conversation: **`-O0` is expected to follow the source and gets the full annotation; `-O2` is not expected to
and gets either nothing or whatever best-effort mapping falls out.** That removes what looked like the blocking
question. `--debug` forcing `-O0` is therefore fine and does not need decoupling for this.

**How SBCL does it, since it came up.** SBCL does *not* shell out. `sb-disassem` is its own disassembler, written in
Lisp, that knows the instruction encodings directly. What that buys is annotation from the inside: it labels constants
the function references, names the functions being called, marks entry points, and shows its own calling conventions —
because it compiled the code object and still holds the metadata.

The relevant observation is that **this project is closer to SBCL's position than the objdump route suggests.** The
daemon owns the build and holds the metadata too; it simply is not feeding much of it into the listing yet. Naming the
function behind an indirection cell, or a constant by its source name, needs no instruction decoder — only the
information the daemon already has. Writing a disassembler is not the interesting part and should stay off the table;
richer annotation of objdump's output is cheap and is where SBCL's advantage actually comes from.

## 3. `def`, `defvar`, `defconst` — three roles, currently two words

There is no `def`. `defvar` is a mutable global that is always re-initialised on redefinition; `defconst` is a
compile-time constant, folded into its use sites.

The problem in practice: `C-c C-k` re-evaluates every top-level form against a *running* program, so today it wipes
state you may have spent a session accumulating.

**Settled in conversation — three roles, three words:**

- **`def`** — always re-initialised. The tweakable: some number you are adjusting and do not care about preserving.
- **`defvar`** — re-initialised *only if it would come out different*. For heavier initialisers: an N×M grid that should
  recompute only when the rows or columns actually changed.
- **`defconst`** — folded, and reserved for what genuinely cannot change for the life of the program. `rows`/`cols` in
  `sand.flan` are honest `defconst`s: the program does not support resizing on the fly and changing them means resetting
  everything anyway.

Note this **`defvar` is not Common Lisp's**, which is "assign only if unbound". This one is value-dependent, which suits
a live-edited game better and is the harder of the two to implement.

**The open question is what "different" means**, and the grid example is what makes it sharp. If the grid is sized by
`rows` and `cols`, the initialising expression's *text* is unchanged when `rows` changes — only its value is. So:

1. **Compare the expression** (textual or AST). Cheap, and does not catch the case above.
2. **Compare the expression plus everything it depends on.** Catches it. Needs dependency tracking.
3. **Evaluate and compare the result.** Correct, and defeats the entire purpose whenever the initialiser is expensive —
   which is the only case this feature exists for.

Option 2 is what is wanted.

**The reason to do it: it is the same machinery `defconst` needs.** A folded constant cannot be tuned live, because its
value is baked into every function that used it — so the values you most want to tweak while the game runs are exactly
the ones you cannot. Fixing that means knowing which functions depend on which constants, and rebuilding those. That is
the same dependency tracking `defvar`'s "if different" requires. Built once, it buys live-tunable constants *and* the
conditional re-initialisation. Neither is worth building alone; together they are clearly worth it.

## 4. Structural typing, row polymorphism, anonymous structs

The goal as stated: make this feel Clojure-like while still being typed.

**The representation is already structural.** `types.ml` does structural equality on resolved types, and a Flan struct is
exactly its C layout with no header and no tag word. What is nominal is the *checking*, not the data — so this is a
front-end question, not a data-model change.

**What it buys, in rough order of value here:**

1. **Your destructuring already looks like this, and that is the strongest argument.** Clojure's `{:keys [x y]}` works in
   binding position today. Structural typing makes the same notation work in *parameter* position — the pattern you write
   to pull fields out becomes the signature saying what you accept. One notation, two places, no separate declaration.
   That is precisely the "Clojure-like but typed" feel being asked for, and it needs no new syntax, only a new meaning
   for syntax that exists.
2. **Functions over "anything with these fields".** Magnitude over anything with an `x` and a `y` — a position, a
   velocity, an enemy, a bullet. In a game this recurs constantly because everything has a position.
3. **Ad-hoc returns.** Returning a hit flag and a point without declaring a type for the pair. The Clojure habit of
   returning a map, typed and free.
4. **The FFI stops needing conversions.** raylib's `Vector2` and a locally-defined vector are byte-identical and are
   today two nominal types. Structurally they are one.

**Two costs, and the second is the real work:**

- **Not an "explicit over convenient" question — that framing was wrong and is recorded so it is not repeated.** The
  standing rules about implicit numeric conversion and about a bare integer not becoming an enum are about *lossy*
  things happening silently. A structural match is not lossy; it is a different notion of type *identity*. The two were
  lumped together in conversation and the author rejected the pairing, correctly. Strong typing and nominal typing are
  separate choices and this only touches the second.
- **Field order is layout, and that is the hard part.** `{x f32, y f32}` and `{y f32, x f32}` have the same fields and
  different memory. So either field order becomes significant — surprising, since nothing else about a structural type
  should be — or matching a differently-ordered type requires a copy, which breaks the zero-cost property the whole data
  model rests on. Accepted in conversation as a limitation to respect rather than fight.

**Writability is what decides whether the copy escape exists.** Fields would be writable on the same terms as any struct:
by value you get a copy and writes are local; through a `(Ptr T)` they reach the original. But that settles the layout
question rather than sitting beside it. *Read-only* structural access has an out — the compiler could gather the
requested fields into a temporary, and then field order would not matter at all. *Writable* structural access cannot:
it has to alias the real storage, so the layout must genuinely line up. Decide writability first; the layout answer
follows from it.

**`defclass` may solve this, for the cases that can afford it.** A class has an implementation-defined representation, so
the compiler owns the layout and access goes through metadata rather than a fixed offset — field order stops being a
user-visible fact. The limit is that a class carries identity and metadata, and a `Vector2` should not pay for either.
So plain structs still need their own answer and classes only cover the cases where the overhead was already accepted.
See item 7.

Also unresolved: the shim generates a C typedef per *named* struct, and an anonymous struct has no name to generate one
from. Either anonymous structs cannot cross the FFI, or the generator learns to name them.

## 5. A JS backend — for web apps, not for games

**The goal, stated in conversation: calling JS libraries, and eventually a hiccup-style DSL for writing web apps in
Flan.** That is a different product from the wasm target and the two do not compete — games go to wasm, web apps go to
JS.

This matters because the obvious objection does not apply. "A JS backend cannot call raylib, so the whole graphics layer
would need reimplementing on canvas" is fatal for a *game* backend and irrelevant here: web apps do not use raylib.

**The design fork, and it should be decided first:**

- **Linear memory** — an `ArrayBuffer` standing in for pointers and structs, with typed-array reads and writes. Fast
  arithmetic, and every value is opaque bytes, so every call into a JS library marshals both ways. This is asm.js, which
  wasm exists to replace, and it is precisely wrong for an interop-motivated backend.
- **Object mapping** — a Flan struct becomes a plain JS object, a `Vec` becomes a JS array. Slower for tight numeric
  loops, natural for interop, readable output. **For this goal this is clearly the right one**, and performance is not
  the constraint people assume: DOM work is dominated by the browser, not by arithmetic.

**The consequence to face early: the memory model does not come along.** Object mapping means garbage collection, which
means no pointers, no manual `free`, no arena, no allocator. So this is a **dialect**, not merely a second backend —
some Flan programs will not compile to JS, and that should be named up front rather than discovered. Worth deciding
which subset is the JS-targetable one, and whether the checker enforces it per target.

**The unexpected upside: the dev loop could work on the web.** Conditions, restarts and a break loop all need the
ability to run new code in a stopped program. wasm cannot have this — `--dev` is refused there because reload is
`dlopen` and the browser has no sockets. JS has no such problem: it evaluates new code trivially. A JS target might be
the *only* place the live dev loop and the browser coexist.

**Reader conditionals, decided in conversation.** A cljc-style subset is accepted, and the author chose Clojure's
inline reader conditionals over file-level target naming. Note what that changes: **this language has no conditional
compilation today**, and that absence drove a decision earlier the same day — `barf` signals on web rather than being
compiled out, precisely because "this bit is desktop-only" is not expressible. Note also that the sand-on-web work
introduced file-level target replacement for *C* sources (`foo.web.c` replaces `foo.c`), so the repo now has a
file-level precedent that this decision deliberately does not follow. Both were put; inline was chosen.

**Also wanted, and independent of all of the above:** `#_` to discard the next form, and repeated (`#_#_`) to discard
that many following forms, as in Clojure. Reader-only, small, and useful immediately rather than whenever a JS backend
happens.

**One dependency:** hiccup is a macro, and the macro expander is blocked on `Form` being a Flan union, which is union
values. The backend can start before that; the DSL cannot.

## 6. C interop as seamless as Zig's — built, with two decisions left

**The mechanism is in** (`lib/cimport.ml`, `lib/cjson.ml`, `vendor/raylib/headers`; BUILT.md, "The header
is read now"). Settled and not worth reopening: clang's JSON AST dump over a shelled-out `clang`, never
libclang — and Zig has since abandoned linking clang too, for Aro, which strengthens the argument rather
than weakening it. The import is bounded by the package's own `defstruct`s rather than by a curated list.
Refusals are demotions in Zig's sense: the name exists, cannot be had, and says why at the use site, which
`Load.refuse_hidden` already did for `main`. Names kebab by a rule that is injective over raylib's 581, and
reversibility is by storage — the C symbol is kept verbatim — so the rule never needs an inverse. Where two
names do collide, neither takes it.

**The evidence:** against raylib 5.5, all 16 `defstruct`s and all 172 hand-written `declare-c` agree
exactly. Against the 5.1-dev header also on this machine, ten real differences. Both comparisons stop the
build, and a permuted `Texture2D` or an `f64` for a `float` is caught by name.

What is left is two decisions, and both are the author's.

### 6a. Does reading the header stay a build-time step, or become a code generator?

The cost, measured, with the wrappers pruned by `Reach` as they already were:

| | today | + 256 imported |
|---|---|---|
| release build, warm | 0.078s | 0.082s |
| **redefinition** | **31.0ms** | **46.5ms** |
| dev build, cold | 0.649s | 0.982s |

Release is nearly free and that question is answered. **The 15.5ms on redefinition is not nothing on the
branch where the dev loop is the priority** — it is a 50% increase on the number that lane exists to keep
small, and it buys a check of signatures that have not changed since the last build.

So the third option from the original discussion is now the live one: `flan import-c` already prints
`declare-c` lines, so **generate from the header, commit the result, regenerate when raylib moves** costs
nothing more to build. Explicit in the source, checked against reality, no header read at build time, and
the check becomes a thing you run rather than a thing you pay for. Against it: a committed file goes stale
silently, which is the failure the whole lane exists to prevent, and "regenerate when the library moves"
is a discipline rather than a mechanism.

A middle reading worth considering: keep the build-time check but run it only when *not* `--dev`, on the
grounds that a release build is where a wrong signature must not get through and a dev build is where
15.5ms is felt. That is the same shape as `Reach` not pruning dev builds, for a symmetric reason.

### 6b. Do the 172 hand-written lines get migrated?

The diff is clean, so nothing blocks it on correctness. What blocks it is that migration needs the header
present at *every* build, which means vendoring raylib.h into the repo or requiring `raylib-devel` — and
BUILT.md records "a build needs libraylib linkable and not raylib-devel installed" as a property that was
chosen on purpose. That is why `vendor/raylib/headers` is opt-in (`?${FLAN_RAYLIB_H}`) today and the
hand-written lines are untouched.

Worth noting what migration would actually lose, since it is small but real: the hand-written names are
better than the rule's. `IsKeyPressed` is `key-pressed?` by hand and `is-key-pressed` by rule;
`CheckCollisionRecs` is `collision-recs?`. And an enum parameter imports as `i32`, because the header says
`KeyboardKey` and nothing tells the importer the package calls that `Key` — so `(rl/key-down? :space)`
would become an integer at the call site. A migration is therefore not a deletion; it is a deletion plus a
kept list of the lines whose face is deliberately nicer than the header's.

## 7. Watching variables

Raised while designing item 1, and deliberately separated from it.

Item 1 shows globals *per frame*, chosen by what the code in that frame references. That is the right default and it
cannot cover the case where the thing you care about is not named by the frame you are standing in — stopped deep in a
helper, still wanting to see the grid.

So: a way to say "always show me this", surviving a resume and the next break.

Questions it opens:

- **What can be watched.** A global is easy — it has a name and an address that does not move. A *local* is harder: it
  belongs to a frame that is gone as soon as you resume, so "watch `y`" either means a different `y` every time or means
  nothing. A watched expression — `(at velocity 40 12)` — is the most useful and the most expensive, since it has to be
  compiled and run in the program each time, which is what `eval-expr` already does.
- **Where the list lives.** Per project, per session, or in the file. A watch list that vanishes when Emacs restarts is
  one you stop using; one in the repo is one you accidentally commit.
- **When it updates.** Only on entering a break is cheap and probably enough. Live-updating while the program runs is a
  different feature — closer to a HUD than a debugger — and worth not conflating.
- **Whether it belongs in the break buffer at all**, or in its own window that is useful while the program is *running*,
  which is arguably where a game developer wants it.

## 8. Indentation is wrong inside a binding vector

Reported against the `let` in `sand.flan`'s `settle`: the second and later bindings indent one column too far.

**Cause, found by reading `flan-mode.el`.** `flan-indent-function` looks at the head of the *enclosing* form and, when
it is one of Flan's body forms (`defn`, `let`, `if`, `while`, `until`, `dotimes`, `match`, `do`, `loop`, `defer`),
indents one past the open paren. Inside a `let`'s binding *vector* the enclosing open is the `[`, and the symbol after
it is the first binding's **name** — never a body form — so the check fails and it falls through to Emacs's generic
`lisp-indent-function`, which treats the vector as a function call and aligns continuation lines under the first
*argument* rather than under the first *binding*.

**What it is built on:** Emacs's built-in `lisp-indent-function`, with that one override. Not `clojure-mode`, which is
where the missing piece lives — `clojure-mode` special-cases binding vectors and aligns them as pairs.

**The fix, agreed in conversation:** add a binding-vector case to `flan-indent-function`. When the enclosing open is `[`
and the form containing it is a binding form, align continuation lines to the column of the **first binding**, not to
the first argument. Bindings are pairs, so the alignment that reads correctly is name-under-name. Check the other
bracket users in the same pass — `defn` parameter lists and `restart-case` clause parameters have the same shape and
almost certainly the same bug.

**`clojure-mode` should be the reference, not the ancestor and not a dependency.** The distinction matters and was
muddled once in conversation: the proposal is *not* `define-derived-mode` on `clojure-mode`, and not adding an external
package. It is to rewrite `flan-mode`'s indentation and font-lock as our own code, **ported from `clojure-mode`'s source
rather than from `lisp-mode`'s**.

The current mode has the wrong ancestor. It is built on Emacs's `lisp-indent-function`, and Emacs Lisp has none of the
shapes Flan actually uses: no vectors as binding forms, no keywords as map keys, no destructuring, no bracket variety.
So every rule has to be added by hand and the binding-vector bug is simply the first one hit. Clojure's rules already
cover brackets-mean-binding, pairs-align, maps, keywords, `#_`, and reader conditionals — the last two now wanted in
their own right.

Where Flan diverges it diverges deliberately rather than by discovery: `defn` carries a return type between the
parameters and the body; field access is `(.x v)`; field labels are moving from `:x` to `.x`, so `{.x 1.0}` is a struct
literal and not a map with symbol keys. Those get written on purpose.

Deriving at runtime stays rejected — it would add an external dependency to a mode that ships inside this repo and
currently needs nothing beyond stock Emacs, and that community is mid-transition to a tree-sitter mode, so the base is
moving.

## 9. Instrumenting with `(pause)` from Emacs, without editing the buffer

Requested, not scheduled. The Clojure model: `C-u` before an eval marks the form so it stops when it runs. Wanted here
for **three targets** — the top-level form, the last expression, and **the form point is inside**, so that with the
cursor at `(+ 1| 1)` the program stops at that `(+ ...)`.

**What already exists, so this is mostly assembly:**

- **`(pause)` itself**, landed today in `lib/prelude.ml`. It is `error` under a `restart-case` with a `continue`
  clause — a breakpoint is not a compiler feature here, it is what a condition system already gives you. Taking
  `continue` resumes at the call.
- **`flan-eval-defun`** takes the top-level form's bounds and sends the text; **`flan-eval-last-sexp`** sends the sexp
  before point. Both hand a *string* to the daemon, which is the hook — the buffer never has to be modified.
- **`flan-dev--enclosing-head`** (`flan-dev.el:1009`) already walks out to the enclosing form to find its head, for
  eldoc. The third target's hard part is already written.
- `current-prefix-arg` is already read in one place (`flan-dev.el:1429`), so the `C-u` convention has precedent.

**So the work is a source-to-source rewrite in Emacs before sending:** find the target form's bounds, send the
top-level form with that span replaced by `(do (pause) <span>)`. Nothing on the compiler side changes.

**The one real difficulty: source locations.** Splicing text shifts every line and column after the insertion, so
error overlays, `layout`, the break loop's frame locations and DWARF would all point slightly wrong for an
instrumented form. Options, none free: pad the inserted text to preserve offsets (ugly, fragile across newlines); send
the instrumentation as a separate field the daemon splices *after* parsing, where locations are already attached; or
accept the drift and say so. **The second is almost certainly right** and makes this a small daemon change rather than
a pure-Emacs one — which also means it should not be built as a pure-Emacs hack first.

**Settled: it sticks, like Clojure's.** The mark stays until the form is evaluated again plainly. That is the
behaviour the workflow expects — you mark it, run the game, hit it repeatedly, and clear it with an ordinary `C-c C-c`
rather than having to re-mark before every run.

## 10. A second backend for dev builds — study SBCL and drop LLVM there?

Raised at the end of the session, not answered. The proposal: write our own code generator for **debug builds and the
interactive workflow**, keep LLVM for release. The motivation is that evaluating arbitrary code — full functions, real
test code, not just constant arithmetic — costs a compile, a file and a `dlopen` every time.

**What SBCL actually does**, since it is the model: its own assembler and code generator, emitting machine code
straight into the heap. No files, no external tools, no dynamic loading. Code objects are ordinary heap objects and are
collected when unreachable. That is also why its `disassemble` can annotate richly and why it can show machine state —
it owns everything.

**The argument against, and it is the same one that killed the interpreter.** Two backends must agree about arithmetic,
struct layout, overflow, evaluation order and calling convention. When they disagree you get *works interactively,
breaks when shipped* — the worst bug class in a live-programming system, and arguably worse than the interpreter
version because the divergence is in code generation rather than in semantics you can read.

Note what plan.org's dev-vs-release table already accepts: indirection cells, a shadow stack, version words, code
never freed. Those are **additive instrumentation on one code generator**, not a second one. This proposal is a
different kind of divergence.

And the cost is enormous: an instruction selector and assembler per architecture — x86-64, arm64, wasm32 — which is
where SBCL's decades went.

**The cheaper route to the same goal, and the thing to weigh it against: ORC, LLVM's in-process JIT.** Measured today,
a 35ms redefinition is **21ms of `llc` plus `ld`** — external tools generating and linking machine code. An in-process
JIT replaces exactly that, with **the same IR and therefore the same semantics**, no second implementation, and code
that can be unloaded. It gets what the proposal wants without the divergence hazard.

plan.org already says ORC "remains addable later behind the same typed IR without touching the language", and the only
column text-IR-plus-clang loses in its comparison table is the JIT one.

**So the honest position:** the thing that would justify revisiting the LLVM decision is not slower eval in the
abstract — it is wanting *arbitrary interactive evaluation of real code* as a first-class workflow. That is now stated
as a want. If it stays a want, ORC is the answer and a hand-written backend is not.

**Where the shared objects live**, since it was asked: `Build.workdir ()` — a per-build temporary directory — with the
object cache separately at `Build.cachedir ()`, stable across builds. Nothing sweeps the workdir, and nothing is ever
`dlclose`d, so a long session accumulates both files and mappings. Sweeping at session end is cheap and unrelated to
any of the above.

## NEXT SESSION STARTS HERE

**The agreed plan: investigate putting the compiler inside the running program.**

Today there are two programs. The *daemon* is the compiler. The *game* is a separate program it launches. They talk
over a socket, so everything the editor shows — locals, globals, the stack, evaluation results — has to be packed up
in one program, sent across, and unpacked in the other. A large share of what `BUILT.md` documents exists only to move
data between those two address spaces.

Putting the compiler inside the game removes all of that. There is nothing to send, because the compiler can read the
program's memory directly. It is what SBCL does, and it is why interactive work in SBCL is instant.

**Read, in this order:** item 13 (what is actually being decided, and why the order matters), then item 11 (the shape,
the two objections and how the author answered them), then item 12 (the specific questions a spike must answer).

**First task, and it is research not building:** the spike in item 12 — what it costs to link the OCaml compiler into
a dev build. OCaml 5.2's multicore runtime has already removed the objection that would have been fatal. If it embeds
acceptably, everything else is reachable.

**Do not start with the backend question.** Merge first, measure, then choose. Item 13 says why.

---

## 11. One process, not two — the biggest open question

Raised at the very end of the session and **not resolved**. It subsumes items 9 and 10, and possibly several decisions
recorded elsewhere as settled. Do not treat any of those as closed until this is answered.

**The claim:** the daemon and the running program should not be two processes. SBCL is one image; Clojure is one
image; in both, the interactive workflow is cheap *because nothing is transported*. The author's reading is that the
split here was forced by not having threads — and that is worth checking, because **threads are already in use**: the
agent runs a listener thread and a loader thread inside the program today.

**The concrete shape**, if it were done: load the *program* into the daemon's process as a shared library rather than
launching it as a child. The reload primitive already `dlopen`s modules; the program itself would be one.

**What would evaporate, and it is a lot.** No socket and no wire protocol. Cell updates become plain pointer stores.
Locals and globals become memory the compiler can read directly — no render thunks compiled per inspection, no 4K
result cap, no seqlock, no snapshot copying, no generation stamping. The macro expander stops needing the compiler's
own `dlopen`. `flan reload` stops being a fresh process paying startup every time. A large amount of machinery
recorded in `BUILT.md` exists **only** to move data between two address spaces.

**What is given up, in order of sharpness:**

1. **Crash isolation.** A segfault in the game currently cannot touch the compiler. `plan.org`'s jank note — corrected
   earlier for citing the wrong mechanism — concludes "we are safe from the repro *because we compile out of process*".
   In one image a bad pointer takes the session with it. Lisp users live with this and mostly accept it; it is a real
   loss and should be chosen knowingly, not discovered.
2. **macOS requires a window on the main thread.** "Run raylib on a separate thread" is fine on Linux and not portable.
   The inversion — compiler on the side thread, window on main — probably works, but it needs deciding rather than
   assuming, and it changes who owns the main loop.
3. **The OCaml runtime and the game in one address space**: signals, `SIGSEGV` handling for the break loop, the OCaml
   GC running while game code holds raw pointers into its own arenas. Each is probably fine; none is free.

**What it does NOT fix, and this is where an earlier answer in this session was wrong.** A hand-written debug backend
was discussed as the way to make evaluation instant. The correction: a new backend removes `llc` and `ld` — about
21ms of a 35ms redefinition — but **not the transport**, because the transport exists due to the process split, not
the backend. Conversely, **merging the processes removes the transport but not the code generation**. They are
independent, and merging is the larger prize of the two.

**Decisions to revisit if this is taken:** the watch design (push was chosen partly because polling costs a compile);
the 4K result cap and its seqlock; the snapshot machinery and its generation stamping; the render-thunk-per-inspection
design for locals and globals; the inspector's inability to retain a value; and whether ORC or a hand-written lowering
is needed at all.

**Both objections were answered by the author, and the second reframed the design — record the corrected shape.**

**Crash isolation: accepted knowingly.** Conditions already catch what the *language* signals — a failed bounds check,
an exhausted allocator, a missing file — and those keep stopping in the break loop in one process exactly as they do
now. What they cannot catch is a real memory fault: a bad pointer through the FFI, a use-after-free in an arena. That
is an OS signal, not a condition, and it takes the image with it. Same deal every Lisp makes.

**macOS: invert the relationship, and this is the right shape rather than a workaround.** Not "load the program into
the daemon" as sketched above — the opposite. **The agent is already a server running inside the program.** So the
compiler moves *into the program's process*, beside the listener that is already there. The game starts, takes the
main thread for its window, and the compiler and listener run on a side thread. That is SLIME's model: you start the
image, it serves, the editor connects.

This also fits a distinction `plan.org` already draws — dev and release builds are deliberately different. A dev build
links the compiler in; a release build ships neither compiler nor server. The same split SBCL has between a
development image and a delivered executable.

One thing that gets *better* rather than merely cheaper: a break loop in the same process as the compiler can offer
restarts that recompile and retry, with no transport in between.

**Suggested next step:** the macOS question is answered, so start instead with what it costs to link the OCaml compiler
into a dev build — the OCaml runtime and the game sharing an address space, signal handling for the break loop, and the
GC running while game code holds raw pointers into its own arenas. It is the only one of the three costs that could
make the whole idea non-portable, and it is answerable by reading raylib and GLFW rather than by building anything.

## 12. What it costs to put the OCaml compiler in the game's process — and whether to port the compiler

Follows from item 11. **Investigate before deciding anything**; this entry is the brief, not the answer.

**One fact established already: this project is on OCaml 5.2.** That is multicore OCaml — real threads, domains, no
global runtime lock. It removes the objection that would have been fatal: the runtime can sit on its own domain beside
the game's threads rather than serialising everything.

**What to find out:**

- **Linking OCaml into a C/native binary.** `ocamlopt -output-obj` and friends are supported; how well, with dune, with
  the C stubs this project already has (`dynload_stubs.c`, the runtime, the shims)?
- **Signals.** The OCaml runtime installs handlers. The break loop wants `SIGSEGV` for the crash case, and the agent
  already handles socket work. Who wins, and can they coexist?
- **The GC and raw pointers.** OCaml's collector moves its own heap; Flan's arenas and `Vec`s are raw memory OCaml
  never sees. That should be fine — they do not point at each other — but "should be" is not an answer.
- **Startup cost and binary size** for a dev build that links the whole compiler.
- **Whether the main thread can be the game's** while the compiler runs on a domain (see item 11's macOS note).

**And the larger question the author raised: port the compiler to another language?**

The honest end state for a Lisp that wants one image is **self-hosting** — the compiler written in Flan. That is what
SBCL is, and it dissolves this entire question: no foreign runtime to embed, no GC to reconcile, no signal conflict,
and the compiler becomes the largest test of the language. It is also an enormous undertaking and needs macros,
unions, `Map` and a string library first — three of which landed today.

Intermediate options if OCaml proves genuinely hostile: C or C++ (no runtime to embed, painful to write a compiler
in), Rust (no GC, embeds cleanly, large rewrite), or Zig (same, and its own C-interop story is already being read for
item 6). **None of these should be entertained on aesthetics.** The bar is a specific, demonstrated obstacle to
embedding OCaml 5.2 — and the multicore runtime means the most likely obstacle has already gone away.

**Order of work:** answer the embedding questions above with a spike, not a rewrite. If OCaml embeds acceptably, item
11 is buildable and no port is needed. Self-hosting stays a long-term ambition rather than a prerequisite.

## 13. The two questions are independent, and the order matters

The end of the session, and the clearest statement of what is actually being decided. Read this before items 10, 11
and 12, which were each written mid-argument.

**Two separate decisions, repeatedly conflated in conversation:**

1. **One process or two** — whether the compiler runs inside the program's process. This is about *transport*: how
   compiled code and inspection data move between the compiler and the running program.
2. **How machine code is produced and installed** — the current `llc` + `ld` + `dlopen`, libLLVM's in-process JIT
   (ORC), or a hand-written lowering. This is about *code generation*.

Neither implies the other. A hand-written backend removes `llc` and `ld` (about 21ms of a 35ms redefinition) and
leaves the transport. Merging the processes removes the transport and leaves code generation exactly as it is.
**Merging is the larger prize.**

**And there is a third option for question 2 that keeps getting lost: change nothing.** In one process you can still
write a `.so` and `dlopen` it. You lose nothing that works today, and you still delete the socket, the wire protocol,
the 4K result cap, the seqlock, the snapshot copying, the render-thunk-per-inspection, and `flan reload` paying process
startup every time.

**So the order is:**

1. **Spike the embedding** (item 12). OCaml 5.2's multicore runtime removes the objection that would have been fatal.
   If it embeds acceptably, everything below is reachable.
2. **Merge the processes**, keeping the existing code path. Biggest win, no new dependency, deletes the most
   machinery.
3. **Measure what is left.** With transport gone, a redefinition is whatever code generation costs. That may simply be
   fine.
4. **Then decide the backend, on evidence.** libLLVM's JIT means linking the library `plan.org` rejected — for
   reasons that were right at the time, when the JIT was worth ~13ms and nothing else. In one process it is worth the
   file, the load, the unbounded accumulation *and* cheap evaluation of arbitrary real code, which the author has since
   named as a first-class want. The cost is also more bounded than "breaks routinely" suggests: the bindings track
   LLVM major versions, so it is a pin you own and upgrade deliberately.

   A hand-written lowering buys the same speed plus no dependency and a disassembler you own, at the cost of an
   instruction selector per architecture, maintained forever. The drift risk is real but testable — the `@sanitize`
   alias already builds 28 programs twice and compares output and exit status, which is exactly the harness two
   backends would need.

**The principle to keep:** the original decision was made on a measurement and it held up for months. Make this one the
same way — merge first, measure, then choose. Do not pre-empt step 4 at step 1.

**Also settled in passing:** a release build can include the compiler and server and simply not be connected to, the
way an SBCL executable can. The dev/release split is a build flag, not an architectural constraint.

**And one correction to keep:** OCaml's own wasm support is irrelevant to any of this. OCaml is the compiler's
implementation language; Flan programs reach wasm through LLVM. The two only meet if the *compiler* should run in a
browser, which is not a goal.

## 14. The embedding spike, answered: OCaml 5.2 goes into a Flan dev build, and nothing objects

Item 12's brief, run. **Feasible.** No obstacle was found that argues for porting the compiler, and the one expected to
be sharpest — signals — turned out not to exist on the platform measured.

The apparatus is `spike/embed/`: four shell scripts and sixteen small sources, deliberately not a dune target, driving
`ocamlfind` and `clang` by hand against the `flan.cmxa` dune already builds. `bash spike/embed/run.sh` reproduces
everything below; `sig.sh`, `symbols.sh` and `merged.sh` each answer a question on their own. Nothing under `spike/` is
wired into the build, and `dune test --root .` is green either side of it.

### The headline: one binary, and it compiles itself

`spike/embed/merged.sh` builds a single executable out of, in one `clang` link:

- the emitted Flan program (`test/programs/edn.flan`, `--dev`, `@main` renamed to `flan_program_main`),
- `runtime/flan_rt.c`, `runtime/flan_dev.c`, `vendor/agent/flan_agent.c`,
- and the entire OCaml compiler as one `-output-complete-obj` object.

It runs. A C `main()` holds the main thread and runs the Flan program there; `caml_startup` happens on a `pthread`
beside it, next to where `flan_agent_start` already puts its listener. The compiler inside the binary then compiles
`edn.flan` — the source the program itself was built from — and emits 336,579 bytes of LLVM IR. That single result
answers questions 1 and 2 together; the ladder of smaller probes underneath it is support, not evidence in its own
right.

Nothing is wired up. The two halves share an address space and do not speak to each other. That is the point: the
question was whether they *can*, not what they would say.

### Question 1 — does OCaml link into a native binary here?

Yes, and with less friction than expected.

- `ocamlopt -output-complete-obj` is the one to use, not `-output-obj`: it bundles the runtime, so there is no hunt for
  `libasmrun`. The final link needs `-lm -lpthread -ldl` and, on 5.2, **`-lzstd`** — 5.x's marshaller is compressed,
  and the missing `ZSTD_*` symbols are the first thing a naive link fails on. That is the whole of the surprise.
- **dune is not in the way, because it does not have to be involved.** dune builds `lib/flan.cmxa` as it does today;
  the `-output-complete-obj` step consumes that artifact afterwards. No dune rule had to change, and `lib/dune` and
  `bin/dune` are untouched. A real merge would want a dune rule to drive that step, but the spike shows the artifact
  boundary is clean, which is the part that could have failed.
- **The existing C stubs come through.** `lib/dynload_stubs.c` was taken verbatim from the unmerged `9e0ae3a` dlopen
  branch and compiled into the same object; from inside the embedded runtime, `flan_mem_alloc`/`poke`/`peek`
  round-trip correctly and `dlopen`+`dlsym` work. `-output-complete-obj` carries a `foreign_stubs`-shaped C file
  through without special handling.
- **No symbol collides** (`symbols.sh`). Flan's own C — `flan_rt.c`, `flan_dev.c`, `flan_agent.c`, `dynload_stubs.c` —
  defines 211 symbols; `libasmrun.a` defines 1,379; the intersection is empty, and the four Flan files do not collide
  with each other either. Worth checking rather than assuming: those four are compiled into *two different processes*
  today, and merging puts them in one link for the first time.

### Question 2 — threads, and who owns the main loop

**OCaml 5.2 is confirmed multicore**, and `Domain.recommended_domain_count ()` reports 16 here. A spawned domain does
real work in parallel with the main thread. The objection that would have been fatal is gone.

The macOS shape works, and it was tested as the thing that matters rather than as "can OCaml use threads":

- `caml_startup` can be called from a **C-created, non-main pthread**, while `main()` goes on to a loop it does not
  leave. `harness4.c` runs a 298-frame mock game loop on the main thread that never once enters OCaml.
- **A second C thread — one the runtime never created, which is exactly the agent's listener — can call into OCaml**
  after `caml_c_thread_register()`, bracketed by `caml_acquire_runtime_system`/`caml_release_runtime_system`. It
  compiled the same program successfully from that thread. This is the specific capability the merged design needs
  from `flan_agent.c`, and it exists.

The spike could not test macOS. Nothing here is macOS-specific — the inversion is a portable pthread arrangement — but
it is a Linux measurement.

### Question 3 — signals

**On Linux/amd64, OCaml 5.2 installs no signal handlers at all.** Not SIGSEGV, not SIGINT, not SIGFPE, not SIGPIPE,
nothing. The expected conflict does not exist.

`sig.sh` sweeps fifteen signals from inside the runtime at four moments — before `caml_startup`, at module init, from
inside a spawned domain, and after `Domain.join` — and every one is `SIG_DFL`. The reading has to be taken from inside
OCaml rather than from C after `caml_startup` returns, because OCaml 5 starts domains later; the first pass got this
wrong and read `SIG_DFL` for the wrong reason. A plain `ocamlopt` executable was built as a control and behaves
identically, so embedding changes nothing about signals.

The reason is structural, not incidental: OCaml 5 detects stack overflow with an explicit stack-limit check and calls
`caml_raise_stack_overflow` directly, rather than with a guard page and a SIGSEGV handler. `nm` on `libasmrun.a` shows
`sigaltstack` referenced but no SIGSEGV handler defined, which matches.

So **the break loop can take `SIGSEGV` outright, and it does not have to install first or last** — order does not
matter when there is nothing to displace. Measured directly: with the break loop's handler installed, deep OCaml
recursion still raises `Stack_overflow` normally, and a genuine fault at `0x10` reaches the break loop's handler with
`si_addr` correct. Chaining was implemented and compared; it is unnecessary here, but `harness5b.c` keeps it, because
it is what the merged build should do on any platform where the sweep comes back non-empty.

**The caveat, and it is the one thing in this report to re-check rather than trust:** this is `x86_64-pc-linux-gnu`
only. macOS/arm64 OCaml 5 was not testable here, and item 11 raises signals in the same breath as macOS. Re-run
`sig.sh` there before relying on it. `flan_agent.c` needs nothing from this either way — it sends with `MSG_NOSIGNAL`
throughout rather than depending on a SIGPIPE disposition.

### Question 4 — the GC and raw memory

Confirmed, and the confirmation is narrow on purpose. An 8 MiB arena was filled with a checkable pattern and 64 raw
interior pointers taken into it; OCaml then allocated 26.4 million words, took 7 major collections and a full
`Gc.compact ()`. Afterwards: the arena base is unmoved, **0 of 8,388,608 bytes altered**, 0 of 64 interior pointers
invalidated.

What that proves is that the collector traces its own roots and foreign memory is invisible to it. Arenas, `Vec`s and
`Map`s are safe because OCaml never learns they exist.

**What would break the assumption**, stated so it is not rediscovered the hard way:

1. Storing an OCaml `value` in Flan memory — an arena, a `Vec`, a global — across any allocation. The collector will
   move the block and will not update that word, because it is not a root. `caml_register_global_root` (or the
   generational one) is the only way that is legal, and it is a rule the merged design has to hold: **the boundary
   passes pointers and scalars, never `value`s into Flan storage** — the same rule `dynload_stubs.c` already states
   for its own reason.
2. A `value` held in a C local across a call that allocates, without `CAMLparam`/`CAMLlocal`. Ordinary OCaml-FFI
   discipline; it applies to every stub the merge adds.
3. Long C work on the compiler thread without releasing the runtime system — which corrupts nothing but stalls
   whichever domains want a stop-the-world. `llc` and `ld` are `exec`s and would want `caml_release_runtime_system`
   around them.

### Question 5 — what it costs

| | bytes |
|---|---|
| `edn.flan`, release build | 71,824 |
| `edn.flan`, dev build, as built today | 114,296 |
| the same dev build with the whole compiler linked in | 4,257,624 |
| **what the compiler adds** | **~4.14 MB** |

**Startup: `caml_startup` takes 0.58–0.72 ms** across six runs — the runtime coming up and every module initialiser in
the compiler running. Measured with `clock_gettime` around the call itself, not `time(1)` on the process, because exec
and dynamic linking are paid today anyway.

The size reads as 37x, and that framing is misleading. **A dev session today runs two binaries, and the daemon alone is
4,815,368 bytes.** The merged dev build is *smaller than today's compiler process by itself*, and there is one of it
instead of two. Sub-millisecond startup and ~4 MB is not a cost worth designing around.

One more number, recorded because item 13's step 3 will want it and for no other reason: **a full in-process compile of
`edn.flan` — read, parse, load, typecheck, emit — is 12.1–12.5 ms**, warm and cold alike, with `llc` and `ld` excluded
because they are separate processes. That is where the remaining cost sits once transport is gone. It argues for
nothing; item 13 says the backend is decided at step 4 on a measurement taken at step 3, and this is not that
measurement.

### What this does not answer

- **The agent's handlers are a port, not a recompile.** `harness4.c` proves the *pattern* — a C-created listener
  thread can register with the runtime and call OCaml. It does not port `flan_agent.c`'s handlers, which today answer
  requests out of the program's own memory and would instead be calling into the compiler.
- **Crash isolation is gone by construction.** Not a finding; item 11 already accepts it knowingly. A bad pointer
  through the FFI takes the session, and conditions still catch everything the *language* signals. Noted only so this
  entry stands alone.
- **macOS, for signals and for the main-thread inversion.** See above.
- **The backend.** Untouched deliberately.

### The order the real work goes in

1. **Make the `-output-complete-obj` step a dune rule**, producing the compiler-as-object that a dev build links. This
   is the only build-system work, and `lib/dune`/`bin/dune` did not need changing to prove it.
2. **Land the `dynload_stubs.c` branch** (`9e0ae3a`, currently reverted). The merged build needs the same
   pointer-and-scalar boundary, and it is already written.
3. **Invert the startup**: the Flan program keeps `main()`, and `flan_agent_start` also brings up the OCaml runtime on
   its side thread. `merged_main.c` is the sketch.
4. **Port the agent's handlers** from answering out of the program's memory to calling the compiler directly — this is
   where the socket, the wire protocol, the 4K result cap, the seqlock, the snapshot copying, the generation stamping
   and the render-thunk-per-inspection all get deleted. It is the bulk of the work and the whole of the prize.
5. **Keep `llc` + `ld` + `dlopen` exactly as they are.** Item 13's third option. Nothing here argues against it.
6. **Measure what is left.** Then, and only then, item 13 step 4.