flan/NEXT.md

# Where this is

**Dev loop steps 1, 2 and 3 are done** — see *The reload primitive* below. A list
of top-level forms can be recompiled and installed into a running process; call
sites compiled before they existed follow them, and a `defn` or `defvar` the
process was never built with can be added and then redefined again. That is the
whole of `C-c C-c`, minus an editor: sand.flan takes a redefinition over a
socket and installs it between frames.

What is left is the *session* — something that holds the checker environment
between evaluations, tracks which names the running process was built with, and
speaks a protocol an editor can talk to.

Milestone 4 is done: **sand.flan builds, links raylib and runs**, and its
simulation has a headless acceptance case that runs on the `dune test` path at
`-O0` and `-O2`. Milestones 2 and 3 are behind it (`calc-me.flan` compiles and
runs; the interpreter was dropped — open decision #7, settled, see below).

```
reader ✅ → parse ✅ → load ✅ → check ✅ → emit ✅ → clang ✅
```

| File | What it does |
|---|---|
| `lib/loc.ml` | source locations + `Loc.Error`, the frontend's one exception |
| `lib/form.ml` | reader output: `Sym Kw Int Float Str Byte List Vec Map` |
| `lib/reader.ml` | hand-written S-expression reader, no menhir/ocamllex |
| `lib/ast.ml` | AST: `texpr`, `expr`, `place`, `pattern`, `decl` |
| `lib/parse.ml` | forms → AST; special forms, desugaring, declarations |
| `lib/load.ml` | **imports: a package directory → qualified declarations** |
| `lib/types.ml` | resolved types; structural equality, `Never` fits anywhere |
| `lib/tast.ml` | the typed IR the backend consumes |
| `lib/check.ml` | AST → typed IR; two passes, bidirectional |
| `lib/session.ml` | **a live program: what the process was built from, plus every change since** |
| `lib/wire.ml` | **the editor protocol: one s-expression per message, length framed** |
| `lib/dev.ml` | **`flan dev`: a session, the program running beside it, and a socket** |
| `lib/prelude.ml` | printers + `rand-f32`, written in Flan |
| `lib/emit.ml` | typed IR → LLVM IR text |
| `lib/build.ml` | `.ll` + the shim + the packages' C → clang → executable |
| `runtime/flan_rt.c` | the host ABI: argv, stdout, exit, 4 conversions |
| `runtime/flan_dev.c` | **dev only: the by-name registry a run-time-new name needs** |
| `vendor/raylib/` | **the raylib package: `raylib.flan`, `shim.c`, `link`** |
| `vendor/agent/` | **the dev agent: a socket, a loader thread, install at a frame boundary** |
| `sand-sim/` | **the falling-sand simulation, with no raylib in it** |
| `bin/main.ml` | `flan read \| parse \| check \| emit \| build \| run \| reload \| dev` |
| `test/test_flan.ml` | reader, parser and checker |
| `test/test_acceptance.ml` | expression/result pairs + whole programs + the traps |
| `test/test_reload.ml` | **the reload primitive: recompile one function, load it, call it** |
| `test/test_agent.ml` | **a running program taking a redefinition over a socket** |
| `test/test_session.ml` | **what a running process cannot be told, and recovering from a typo** |
| `test/test_dev.ml` | **the daemon, driven the way an editor drives it** |
| `test/reload_host.c` | the C host that loads and installs two rebuilds, in one process |

```
$ flan run calc-me.flan "1 + 2 * (3 - 0.5) / 2"
3.5
$ flan run test/programs/sand-headless.flan
2256461126764447066
$ flan run sand.flan                      # a window, 120 fps, hold space
```

## What milestone 4 added

**`dotimes`** desugars in `check.ml` to a `Let` plus a `While` — no new IR node.
The bound is evaluated once into a hidden slot before the loop, so a body that
changes it cannot change the trip count, and the loop variable is not
assignable, which makes the generated step its only writer.

**`defer`** is recognised in `check_fn` and nowhere else, because that is the
only place that knows a form is at the top level of a function body. Each one
is checked in place, then registered on the context; it emits nothing where it
stands. Function exit runs them innermost-first, and an explicit `return` runs
the ones registered *above* it — a defer written below a return has not
executed yet and must not fire. A trap runs none of them, which follows from
the bounds-check shape (`noreturn` then `unreachable`) rather than being a
separate decision.

`defer` inside a `let`, a loop or a branch is **rejected**, not accepted with
function scope. It would run once at function exit rather than once per
iteration, and that is the silent-wrongness class the rule below is about.
Block-scoped defer is real work and is not done.

**New builtins:** `zeroed` (takes its type from the place it is stored into),
`min`/`max` (each operand through a slot, so neither is evaluated twice),
`bit-and`/`bit-or`/`bit-xor`/`<<`/`>>` (integers only; `>>` is arithmetic on a
signed type and logical on an unsigned one), and `rand-f32`.

**`rand-f32` is in the prelude, in Flan** — PCG-XSH-RR 32 over a `u64` state.
It is not libc's, because a grid hash is only a regression test if the sequence
is byte-identical on native and wasm32 (plan.org, RNG is ours). `rand-seed`
sets the state. This is what the bitwise operators were added for.

**Enums and keywords.** `(defenum Name [member value ...])` gives a type that
is an `i32` at run time and its own type in the checker, so `:space` at a call
site resolves against the parameter's enum and a typo is an error there rather
than a wrong number later. A keyword means nothing where no enum is expected —
there is no keyword type to fall back on.

## Why the FFI goes through a C shim

The decision that shapes the whole raylib package. What clang generates for
raylib's own prototypes on x86-64:

```
Vector2 {float,float}   →  declare <2 x float> @GetMousePosition()
Color   {u8,u8,u8,u8}   →  declare void @ClearBackground(i32)
Rectangle {4 × int}     →  declare { i64, i64 } @mkrect()
```

None of those is the struct's own LLVM type. A small aggregate's calling
convention is not part of its layout — it is a per-target classification the
*caller* has to reproduce, and x86-64, arm64 and wasm32 classify differently.
Putting that in `emit.ml` is three classifiers to write and then keep correct
forever, and a mistake shows up as `(.y m)` returning garbage rather than as a
link error.

So `vendor/raylib/shim.c` has one wrapper per binding, each one flattening the
aggregates: a struct returns through an out-pointer, a struct argument is
passed by pointer, a Flan string crosses as ptr+len and the shim NUL-terminates
a copy. clang classifies all of it, per target, for free. `check.ml` enforces
the rule — an aggregate in a `declare` signature is rejected with the reason —
so the boundary cannot quietly acquire one. This is plan.org's "one narrow host
ABI, implemented twice", and `flan_rt.c` is the same pattern.

The price is a hand-written wrapper per raylib call. They are one-liners and
mechanical enough to generate if that ever becomes the bottleneck.

`raylib.flan` declares each `-raw` entry point and wraps it in an ordinary Flan
function just below, so the surface sand.flan sees is `(rl/get-mouse-position)`
returning a `Vector2`. Verified end to end, headless: `GetColor(0x11223344)`
comes back as `17 34 51 68`, four separate bytes — a `Color` is *not* the
little-endian reading of the packed integer, so an identity would have passed a
weaker test. That case is in the acceptance table, skipped if `libraylib` is
not installed.

The bindings are 18 calls: window (`init-window`, `close-window`,
`window-should-close?`, `set-target-fps`, `set-trace-log-level`), keyboard
(`key-pressed?`/`down?`/`released?`), mouse (`mouse-button-pressed?`/`down?`/
`released?`, `get-mouse-position`), `get-color`, and drawing (`begin-drawing`,
`end-drawing`, `draw-fps`, `clear-background`, `draw-rectangle`), plus the
`Key`, `MouseButton` and `TraceLogLevel` enums. Adding one is three lines: a
`declare`, an `extern` prototype, and a one-line wrapper.

No raylib headers are needed: `shim.c` declares the prototypes it uses, so the
build depends on the shared library being linkable and not on `raylib-devel`.
`vendor/raylib/link` carries `-l:libraylib.so.550` because Fedora ships the
runtime library without the `.so` symlink.

## Packages

`lib/load.ml` resolves `(import rl "vendor:raylib")` before the checker runs.
The directory is the package; `vendor:` is a collection, resolved by walking up
from the importing file until a directory of that name is found; a path with no
collection is relative to the importing file. Importing is a **rename**: every
top-level name the package declares becomes `alias/name`, and every use of one
— in a type, in a body, in a struct literal, in an *array length* — is
rewritten to match. Local bindings shadow. Nothing downstream knows a package
existed; the checker sees one flat list of declarations whose names contain a
slash.

A package may also carry the C it binds to: every `.c` file in the directory is
compiled into the build, and a file named `link` lists extra linker arguments.

This is not a module system yet. No visibility (hence `rl/get-color-raw` being
callable), no cycle detection, and a package cannot import another one.

## sand.flan is two programs

plan.org wants sand tested twice — interactive at 120 fps, and headless over N
frames with the grid hashed, the version CI runs on native *and* wasm32. Those
cannot be one binary: `Load` collects a package's C sources and linker
arguments unconditionally, so anything importing the raylib package links
libraylib on every target regardless of what its `main` does, and on wasm32
that link cannot succeed.

So the simulation moved to `sand-sim/`, which imports nothing. `sand.flan`
imports it as `sim/` and adds the window, the mouse and the drawing;
`test/programs/sand-headless.flan` imports it and adds a seed, four
deterministic clouds, 40 frames and an FNV-1a hash. One copy of the physics.

The headless case is what actually *verifies* milestone 4 — running the
interactive build only proves it enters its loop, because with no mouse input
the grid stays empty and `paint-at`, `settle` and `move-grain` never execute on
real data. Measured through the probe: 168 grains painted around row 4–8, still
168 after 40 frames, lowest occupied row 68. Grains fall, and none are lost.

**Three edits were made to sand.flan's own text**, and they are language
decisions rather than fixes:

- `(defconst gravity 0.05)` → `(defconst gravity f32 0.05)`. An untyped float
  constant is `f64`, `velocity` is `[f32]`, and there is no implicit widening.
- `(defvar current-color u32)` → `i32`. It is an index into `colors`, and
  `(len colors)` is an `i32`.
- The file was split as above, so its body now says `sim/rows` and so on.

`(defn main [])` is unchanged — the short form, as plan.org says.

Painting is on **hold left mouse button** rather than on space, since the mouse
bindings exist now. Space is still what cycles the colour, on release, which is
a leftover and probably wants to move to the right button or to a key press.

## Bounds checks — done at milestone 3

`at` and `slice` emit `icmp` → `br` → cold block → `call` → `unreachable`; a
failure names the source location. Three check sites: `at` on `[n T]` (static
bound, folded by LLVM for a literal index — and a literal that is out of bounds
never reaches emit, `check.ml` rejects it), `at` on a slice or string (runtime
len), and `slice` (two comparisons — `lo <= hi` is not redundant, without it a
reversed range yields a huge unsigned length). All comparisons unsigned.
`Build.opts.checks` is on by default and **not** tied to `opts.opt`, which is
what lets the acceptance table run the same programs at `-O0` and `-O2` with
identical checks. The flag is `--no-bounds-checks`.

The write path is its own case: `(set (at arr n) …)` lowers through
`place`/`Pindex`, not through `At`, so a refactor that split them would break
the write check silently. The test covers both.

Cost, measured: a 50M-iteration dependency chain over a 1024-element array runs
at 0.11–0.12s checked against 0.12–0.13s unchecked. Indistinguishable.

## Why there is no interpreter

Open decision #7 is settled: **the compiled path is the only backend.** Both
arguments for a permanent interpreter had expired — the instrumentation step
debugger that wanted it is cut, and compiled redefinition measured at ~16ms,
perceptually instant for expression eval too. Milestone 3 did not need an
oracle either: the acceptance table is hand-written, so the table *is* the
oracle. Consequences already applied: milestone 2's "interpreted calls per
second" criterion is dropped, and the host ABI moved onto the critical path.

## The layout, which is the whole backend design

```
i8..i64 / u8..u64   i8..i64          signedness lives in the ops
f32 f64             float double
bool                i1
an enum             i32
[T] and string      { ptr, i64 }     ptr+len, non-owning
[n T]               [n x T]          inline, a value
(Ptr T)             ptr              opaque pointers
(Option T)          { i8, T }        tag 0 None, 1 Some
a struct            a literal struct, declaration order
Unit and Never      {}
```

No object headers anywhere, so a Flan struct is exactly its C struct and
nothing marshals. Two consequences carry the semantics:

- **Every slot is an `alloca`.** Reading a local is a `load`, assigning is a
  `store`, and a `store` of an aggregate *is* the copy `spec-memory.md`
  requires. `addr` of a local is then just the alloca, and `mem2reg` removes
  the ones nobody addressed. `test/programs/values.flan` pins this down.
- **A place is a pointer, a value is a load from it.** `(set (.pos c) …)`
  through a `(Ptr Cursor)` becomes a `getelementptr` on the pointer, not on a
  copy. This is the split that would have made a tree-walker silently wrong.

Non-local exit is lowered explicitly: `return`, `some` and a failed bounds
check are branches, never platform unwinding, so wasm32 needs no exception
proposal.

## Sharp edges

Most of these are edges the language keeps and you should know about. Two —
the top-level namespace and the shift count, both found by review after
milestone 4 — were bugs that reached LLVM or ran wrong, and are **fixed**; each
says so. They stay written down because each one is now a rule the checker
enforces, and a later change could quietly drop it.

- **An index converts from a narrower integer and never from a wider one.**
  `(nth colors current-color)` with a `u32` index works — anything above 2³¹
  truncates to a negative `i32` and the unsigned bounds check rejects it. An
  `i64` index is refused with the reason: 2³²+5 truncates to 5 and would read
  the wrong element with no trap at all.
- **There is one top-level namespace, and `check.ml` now enforces it.** The
  environment's tables are per-kind — structs, unions, aliases, enums,
  functions, externs and globals each have their own — so only a function was
  ever checked for a duplicate. `(defn item …)` beside `(defvar item …)` type
  checked and then died in LLVM as `redefinition of function '@flan.item'`, a
  message about an emitted symbol with no source location left, and two
  colliding *type* declarations were not caught anywhere. One pass over
  `Ast.declared_name` now runs before every other collection pass and rejects
  the second declaration of a name whatever kind either one is. `declared_name`
  lives in `ast.ml` because `Load` needs exactly the same set — the names an
  import renames — and two copies of that list would drift.
- **A shift count is bounded, two different ways.** A shift by the operand's
  own width or more is *poison* in LLVM, not a wrong number: `(defn main [] i32
  (<< 1 32))` compiled at -O2 to a bare `retq`, returning an undefined value. A
  literal count out of range is now rejected in `check.ml` — that is the typo
  case — and `emit.ml` masks a computed count to `width - 1`, which is what the
  hardware does anyway and which LLVM folds away whenever the count is
  constant. The prelude's rotate masks its own count; that is now redundant but
  harmless.
- **A `u64` literal is its 64-bit pattern**, so `0xcbf29ce484222325` is a real
  `u64` and not an error. The cost is that a negative *decimal* literal is
  accepted as a `u64` too, because the reader records the value and not how it
  was written. Narrower unsigned types keep the strict check, which is where a
  typo like `300` for a `u8` actually shows up.
- **A folded constant skips `check`.** `(defconst rows (/ h c))` is emitted
  from the folding pass's value, because a global's initialiser has to be a
  compile-time constant and only that pass knows this one is. Its range check
  is therefore its own call to `in_range`; there is a regression test.
- A `let` binding takes no type annotation, which is why `sand-sim` names its
  FNV constants instead of writing them inline.
- `(defn f [] f65 0.0)` still says *unknown name* rather than *did you mean
  f64*: with a single body form the parser cannot tell a return type from the
  first expression. Only the parameter position and `(Option …)` are
  unambiguous.

## The reload primitive — dev loop steps 1 and 2, measured

`llc` → `ld -shared` → `dlopen` → call, with no protocol and no daemon.
`dune test` runs it: one function is recompiled into its own object and called
inside a process that is already running, twice, with a changed body the second
time.

| Step | Cost |
|---|---|
| `Emit.redefinition` | below the timer (<0.1ms) |
| `llc -O2 -filetype=obj` | 15–17ms |
| `ld -shared` | 3ms |
| `dlopen` + `dlsym` | **0.04ms** |

**~19ms end to end**, and the load itself is free. plan.org's 16ms was measured
with clang somewhere else; this is the number from this codebase. For contrast,
`clang -shared` on the same IR is 50ms — the driver is again most of the cost,
which is why the dev path skips it. `llc` and `clang` are both 20.1.8 here;
check that before trusting the `.ll`, since the driver absorbs IR the bare
tools reject.

`ld -shared` rather than `clang -shared` for a second reason: a shared object
is allowed undefined symbols, and that *is* the mechanism. What the new module
does **not** define is the whole design:

- **a global is `external`.** This settles the open question below in the only
  direction that supports the demo: a redefinition can change a function's
  body and can never re-initialise the program's data. Define the global and
  the loaded object gets a second copy — sand's `grid` would reset on every
  reload, and "edit the code, keep the sand" is the thesis.
- **every other function is a `declare`**, so a redefined `settle` calls the
  host's `move-grain` rather than freezing a private copy of it.
- **no `main`.** This module is loaded, not started.

Its string constants still come along; omitting them is an undefined `@.str.N`
at link time, and it is easy to miss because a one-function module usually has
none. `Emit.signature` is now the single place a function's LLVM signature is
spelled, because a `define` here and a `declare` there drift the moment one of
them grows a case for `Unit` or for a slice parameter.

**`-rdynamic` is load-bearing.** A normal executable exports nothing: `nm -D
calc-me | grep 'flan\.'` is empty, so a loaded module's `declare`s would have
nothing to bind to. The test passes it through `lflags`, which keeps it a
property of the dev build rather than of every build. `dlsym` on `"flan.bump"`
works — a dot is legal in an ELF symbol.

Two things about the test are deliberate and are what make it prove anything:
both loads happen in **one process**, since two runs would pass while saying
nothing about an in-process swap; and the versions are **two paths**, since
`dlopen` caches by path and re-opening one would hand back the handle it
already had, so the check would lie. And `helper` is `(* x 2)` in one fixture
and `(* x 3)` in the other: the second body is dead text, since the module
declares `helper` rather than defining it, so the expected 1024 coming back
instead of 1036 is what proves the call landed on the host's copy. With the two
bodies identical nothing at run time would notice a module that grew its own.

String constants are emitted `private unnamed_addr`, so the module's own
`@.str.N` cannot be interposed by the host's — worth knowing, because with
external linkage a redefined function would silently print the *old* text and
nothing would fail at link time. The fixtures each print a literal so that path
is actually exercised.

### Cells — how a call site follows a redefinition

Loading a new body is not installing it. A call bound at link time cannot be
made to notice one, so **a dev build routes every Flan-to-Flan call through a
cell**: a mutable global holding the address of the function that is current.

```
@"flan.cell.bump" = global ptr @"flan.bump"        ; the host defines it
%p = load ptr, ptr @"flan.cell.bump"               ; every call site
%r = call i64 %p()
```

Redefinition is then one store. A redefinition module declares the cells
`external`, exactly like the globals, and exposes `flan_reload_install()` that
stores its own body into its own cell — cost **below a microsecond**, which is
what makes a frame-boundary swap a non-event.

The cell load is emitted *after* the arguments, so a redefinition landing
between two calls cannot land in the middle of one.

Four things about this that are not free choices:

- **`flan_reload_install` is a named function and not an ELF constructor.** A
  constructor runs during `dlopen`, on whatever thread called it, mid-frame.
  The agent has to choose when the store happens. Loading and installing are
  separate on purpose.
- **A redefinition's own body is `hidden`.** Default visibility in a shared
  object is interposable, and that applies to *taking the address* too: plain
  `@"flan.bump"` inside the module resolves to the host's copy, so the
  installer would publish the very function it was replacing and the reload
  would appear to do nothing. There is a test on the linkage, because the
  failure is silent.
- **This also fixes the self-call edge**, which the previous version of this
  section listed as a sharp edge: a redefined function calling itself goes
  through the cell like any other call, so it reaches the new body. v2 of the
  fixture recurses on purpose, and would print the old body's text if it did
  not.
- **`-rdynamic` is what exports the cells**, so it and cells are one flag:
  `Build.opts.dev`, `flan build --dev`. This is the first time `opts` means
  something semantic rather than an optimisation level.

LLVM cannot fold the indirection away — the cell is an external mutable global
— and a `--dev` build of calc-me keeps 46 indirect calls at `-O2`. The
acceptance table now runs `values`, `machine` and `sand-headless` as dev builds
as well; the sand hash is the case that matters, since it is the one result
that would notice a call reaching the wrong function.

### Names that did not exist when the process started

Editing a `defvar` or a `defn` is a symbol the host exports. *Adding* one is
not: there is no symbol to bind to and ELF cannot grow one. Those go through
`runtime/flan_dev.c`, which is two lookups and nothing else:

```
void **flan_dev_cell(const char *name);              /* a new function's cell */
void  *flan_dev_global(const char *name, uint64_t);  /* a new global's storage */
```

Both are idempotent, so the second module to mention a name gets what the first
one got — which is the entire point. The compiler picks per name: a name the
host has is a symbol (one load at a call site), a name it lacks is a registry
lookup cached at install time in a module-local slot (two loads). So the common
case pays nothing for the general one.

**The unit is a list of top-level forms**, not one function — `Emit.redefinition
~fns`. `C-c C-c` passes one name, `C-c C-k` passes a file's worth, one code
path either way. It has to be: v3 of the fixture adds `extra` and uses it from
a redefined `bump`, and splitting that into two loads would leave a module
referring to storage that does not exist yet.

Four rules, each of which is a silent failure if broken:

- **Every lookup resolves before any body is published.** Publish first and a
  caller reaches a function whose slots are still null. Not race-testable, so
  it is asserted on the emitted `flan_reload_install`.
- **`flan_dev_global` refuses a size change.** The running process has already
  laid that memory out; handing back the old allocation for a differently
  shaped type means the new body reads fields at the wrong offsets and nothing
  says so. This is the layout-drift rule's first enforcement point. Retyping a
  var needs a restart.
- **Nothing is ever `dlclose`d.** A cell holds an address inside a module's
  text; unloading it leaves every call site pointing at unmapped memory. That
  is a constraint on the agent too.
- **The registry never moves.** A module holds a cell's address for as long as
  it is loaded, so the table is fixed capacity with a loud failure rather than
  growable.

The test that separates this from a plausible wrong version is **v4**, which
redefines `added` — a name v3 introduced at run time. v3's `bump` is already
installed and is not rebuilt, so it picks v4 up only if its call goes through a
*cell* both modules found by the same name. Had v3 cached the function's
address instead, every other assertion would still pass and the transcript
would read 246 instead of 432.

Sizes are spelled LLVM's way — `ptrtoint (ptr getelementptr (T, ptr null, i32
1) to i64)` — rather than by a layout calculator in OCaml that would have to
agree with LLVM's on every target.

### The agent — dev loop step 3

`vendor/agent/` is a package like any other: `agent.flan` declares three calls,
`flan_agent.c` implements them, `link` asks for `-lpthread`.

```
(agent/start path)   listen on a unix socket; once, at startup
(agent/poll)         install whatever has arrived; returns how many
(agent/wait ms)      the same, but waits for something first
```

The split between them is the design. `dlopen` relocates a module and takes the
loader lock — milliseconds, unbounded — so it happens on the listener thread.
`flan_reload_install` is one store per function and must not land while a
redefined function is on the stack, so it happens on the game thread, at the
top of the frame, when the program asks. The two are connected by a
single-producer/single-consumer ring and two atomics; the game thread never
blocks on the loader.

`wait` exists for tests. A test that races the frame rate fails on a loaded
machine, so `test/programs/agent.flan` waits for the reload instead of sleeping
past it. It takes **two** reloads, which is the daemon's actual loop: the first
introduces a global the process was never built with, the second only reads it,
and the second can only answer 1007 if it found the storage the first one
allocated rather than a fresh zeroed copy. One reload would not have shown
that.

Two details found by running it:

- **stdout is line buffered**, set in `flan_rt_init`. The C default when
  stdout is a file or a pipe is a 4K block, so a program running with a REPL
  attached shows nothing until it exits — and a test driving one cannot see
  its progress at all, which is how this was found.
- **The reply goes out before the module is queued.** The other way round, the
  game thread can install and the program can exit between the two, and the
  answer reaches the sender as a connection reset rather than as `ok`.
- **`ok` means queued, not installed.** The sender does not get to know when
  the swap happened; only the program knows when it is between frames.

**sand.flan calls `agent/poll` at the top of its loop**, which is what step 3
was for. Verified: with sand running under Xvfb, `flan reload sand-probe.flan
game-draw` and one line on the socket, and 455 consecutive frames drew from a
body that did not exist when the process started. Building without `--dev` is
fine — there are no cells, so a module is refused on the listener thread and
the loop never notices.

`flan reload <file.flan> <fn>... [-o out.so] [--new name,...]` builds one
module the way the daemon will. `--new` is the names the host was *not* built
with; it is the one thing the command cannot work out for itself, and it is
exactly what the session will track automatically.

### The session

`lib/session.ml` is the program as a live thing: the declarations the running
process was built from, plus every change accepted since.

**Transactionality came for free and needed no machinery.** `Check.program`
builds a fresh environment from a declaration list on every call, so a form
that fails to check mutates nothing — the accumulated list is simply not
replaced. Re-checking the whole program each evaluation costs the entire
frontend, under 10ms, less than the `llc` that follows. There is a test for the
case that actually matters: a typo, then a good form, in the same session.

Two things the session knows that no single evaluation could:

- **Which names the running process was built with.** It comes from the
  *checked* program, not from any accumulated AST, because `Check.program`
  prepends the prelude and no AST contains it. Derive it from declarations and
  `print-line` reads as new, gets a registry cell nobody publishes, and the
  first call jumps to null with no diagnostic.
- **What that process's memory looks like.** Three changes are refused with a
  reason rather than loaded:

  | Change | What it would have broken |
  |---|---|
  | a function's signature | a cell is a bare `ptr`; every call site compiled before the change still passes the old arguments through it |
  | a global's type | the storage exists and has a shape — reuse reads at the wrong offsets, replacement discards the state the reload exists to preserve |
  | a struct's fields | the values the process is holding have the old layout |

  Note what the checker catches on its own: change `helper`'s parameter type
  and the *caller* fails to type check first, loudly. The session's rules only
  get a turn on a change the checker accepts — one to a name nothing else in
  the program uses, which is exactly where the silent version lives. The
  fixtures carry an unused `defvar` and a C-called `defn` for that reason.

The accumulated list is the **post-`Load`** one, so an evaluated `(import …)`
is spliced as its expansion. Otherwise re-evaluating a file that imports
something appends a second import, `Load` expands it again, and the
duplicate-name pass rejects it. `C-c C-k` on sand.flan's own text is the test.

`flan reload <program.flan> <forms.flan>` is that path from the command line: a
session over the program the process was built from, and a file of the forms
that changed. Verified against a running sand under Xvfb — a one-form
`game-draw` and 910 consecutive frames drew it.

Two limits of that command specifically, neither of them true of sessions:
it builds a fresh session from source on every invocation, so if the program
file has been edited since the process launched, its idea of which names the
host has and what its memory looks like describes a binary that is not running.
And `Session.eval`'s `origin` defaults to `<eval>`, so an error in forms sent
without one reports positions in a file that does not exist — the daemon has to
pass the real buffer path, which is the same key CIDER's `eval` carries.

### The daemon — `flan dev`

`flan dev <program.flan>` holds one `Session`, builds the program, launches it,
and listens on `.flan-dev.sock` beside the source. What it adds over `flan
reload` is that the session *persists* — a `defvar` added by one evaluation is
part of what the next one is checked against — and that it **owns the build**,
which is what makes its layout rules describe the process that is actually
running rather than a guess about it.

**The protocol is s-expressions, not bencode.** nREPL was the plan and the
argument for it evaporated once the client became ours too: there is no CIDER
to be compatible with, `eval` is string-in/string-out with no slot for *which
form, from which file*, and Emacs already has `read` and `prin1`. So it is one
sexp per message — no parsing code on the editor side, and on this side the
parser is the language's own reader, where `:op` is already a keyword and a
payload of Flan source is already a string literal. Framing is a decimal byte
count and a newline, because the payload contains newlines. An nREPL front end
can sit on the same `Session` later; it should not have gated the editor.

```
(:op "describe")                        → (:status "ok" :fns (…) :globals (…) :alive t)
(:op "eval" :code "…" :file "/buf.flan") → (:status "ok" :names (…) :fns (…) :ms 19.0)
                                        → (:status "error" :message "…" :loc "/buf.flan:1:19")
(:op "close")
```

`:file` is not decoration: `Session.eval`'s origin defaults to `<eval>`, so
without it every error an editor shows points into a file that does not exist.

Two things the daemon must not paper over, both of which would look like a
successful evaluation:

- **The agent socket is chosen by the daemon**, not by the program. A program's
  source has to name some path — sand.flan says `/tmp/flan-sand.sock` — and the
  daemon overrides it through `FLAN_AGENT_SOCKET` before spawning. Guessing
  instead fails silently: the module compiles, is built, and nobody receives
  it.
- **Delivery is checked.** `agent/start` returning 0 means a socket was bound,
  not that anyone connected. A failed connect or a reply that is not `ok`
  becomes an error the editor sees.

It waits for the program to bind before accepting an evaluation — one arriving
first would fail for a reason that reads like a compiler bug — and it accepts
with a timeout so that a program which has exited takes the daemon with it
rather than leaving an editor waiting on a socket nobody is serving.

### What is left

`C-c C-c` works end to end today; what is missing is the two hops between an
editor and it.

- **The Emacs client**, not a CIDER fork. Deliberately last: the protocol is
  mechanical once the daemon exists, and the client is where the taste is.
- **Expression eval** (`C-x C-e`) is a *different primitive* and is not built.
  Redefining a name installs a body; evaluating an expression means
  synthesizing a function around a form, calling it, and rendering the value.
  It needs no cells — wrap, compile as a redefinition module, `dlsym`, call —
  so it is not downstream of any of the above. The open question is the value:
  the compiler knows the type, so emit the print call into the thunk and
  capture the output rather than marshalling anything. The prelude prints
  `i64`, `f64`, bytes and strings, and nothing else; a struct, an `(Option T)`
  or a slice of structs has no printer. Either derive one per type in the
  checker or restrict v1 to scalars and say so. That choice is the difference
  between eval feeling like Lisp and feeling like gdb.

**Session identity is the daemon that owns the build.** A session's struct
layouts and global types have to describe the memory of the process it is
talking to, which is only guaranteed if it is the session that compiled the
running binary. Attaching to a process someone else built is not a thing to
support by default.

## Where build time goes

`flan build calc-me.flan` was ~160ms, and ~95% of it was clang. **The object
cache is in**, and it is now ~110ms:

| Step | Cost |
|---|---|
| frontend: read → parse → load → check → emit | <10ms, below the timer |
| `clang` on the `.ll` | 60ms — `llc` does the same codegen in **20ms** |
| `clang` on `flan_rt.c` | 40ms — **now cached, paid once** |
| link | 20ms |

Every C translation unit a build needs — the host shim and each package's shim
— goes through `Build.compile_c`, which compiles to a `.o` under
`$TMPDIR/flan-objcache` and reuses it. The key is a digest of the source text,
the compiler (its path, size and mtime, so an upgrade invalidates without
paying a `clang --version` subprocess per build), `opts.opt` and `opts.target`.
The opt level has to be in there: the acceptance table builds the same programs
at `-O0` and `-O2`, and an `-O2` object must not serve an `-O0` build. The
object is written to a temporary name and `rename`d into place, so two
concurrent builds cannot see a half-written one.

Measured: calc-me 160ms → 110ms; sand ~720ms → ~700ms, since sand's time is
mostly linking libraylib and its `shim.c` was never the cost. The cache is
keyed by content, so it never needs invalidating by hand — `rm -rf` on the
directory is only ever a disk-space decision.

The other cheap win is still open: skip the clang driver for the `.ll` (`llc` +
link directly), worth another ~40ms. It is a subset of the dev path's
machinery. Check `llc`'s major version against clang's before relying on it —
the emitted IR text is currently absorbed by the driver behind
`-Wno-override-module`, and a version mismatch surfaces as IR parse errors.

**There is still no REPL.** Nothing does redefinition, `dlopen`, or nREPL.
`build` is the only way to run code.

## Next — the REPL is the priority

Decided in conversation: wasm32 can wait (it is believed to be a solved problem
once the builtins archive is in place), and **the dev loop is the thesis of the
project**, so it comes first. Staged so each step is runnable on its own —
the failure mode is building a daemon and a protocol before knowing the reload
primitive works.

1. ~~**The reload primitive, measured.**~~ **Done** — `Emit.redefinition`,
   `Build.shared`, `test/reload_host.c`, ~19ms. See the section above.
2. ~~**Indirection cells.**~~ **Done** — `Build.opts.dev` / `flan build --dev`,
   `flan_reload_install`, `runtime/flan_dev.c` for names introduced at run
   time, and a fixture where an untouched call site follows the swap and a
   run-time-added function is itself redefined. See the section above.
3. ~~**The agent, in C.**~~ **Done** — `vendor/agent/`, a listener thread that
   loads and a game thread that installs, and sand.flan polling at the top of
   its frame. See the section above.
4. **The daemon and nREPL** (bencode over a socket; `eval`, `load-file`,
   `describe`, `interrupt`), then **5. the Emacs client** — a focused ~3–5k
   line client, not a CIDER fork. Deliberately last and deliberately separate:
   the protocol is mechanical once 1–3 exist, and the editor client is where
   the taste is.

**One decision left to settle before step 2**, because both change codegen and are
painful to retrofit:

- ~~**Do cells cover globals, or only functions?**~~ **Settled by step 1:
  functions only.** A redefinition module declares every global `external`, so
  globals live in the host and survive a reload — which is what "edit the code,
  keep the sand" needs. The consequence to watch is the other half: adding a
  `defvar` to a file cannot take effect on reload, and changing one's type is a
  silent mismatch against storage the host already laid out. Nothing detects
  that yet.
- **What is a redefinition unit — one function, or a file?** A file is much
  easier to make correct and is what `load-file` wants anyway; one function is
  what `C-c C-c` wants and is where the 16ms number comes from.

Deferred until after the dev loop:

6. **wasm32.** The user installed `wasi-libc-devel` and `wasi-libc-static`; the
   sysroot is `/usr/wasm32-wasi` and `wasm-ld` is present. `clang
   --target=wasm32-wasi --sysroot=/usr/wasm32-wasi` gets past the headers and
   then **fails to link**: it wants
   `lib/clang/20/lib/wasm32-unknown-wasi/libclang_rt.builtins.a`, which no
   Fedora package provides (`dnf provides '*libclang_rt.builtins*wasm*'` finds
   nothing). It has to come from a wasi-sdk release, dropped into clang's
   resource directory. After that: teach `build.ml` `--sysroot`, and run the
   acceptance table — `sand-headless.flan` included, which is exactly why it
   does not import raylib — on both targets in CI.
   Note plan.org has the *web* build linking raylib via emscripten, which
   brings its own sysroot: wasi-sdk is right for the headless table, not
   necessarily for the eventual game build.
7. **Loose ends from milestone 4**, none of them blocking: block-scoped
   `defer`; package visibility, so `rl/get-color-raw` is not callable; a
   package importing a package; imported unions.

## Watch for

The rule that caught the two misparse bugs applies unchanged: **anything that
binds a name, alters control flow, or is not yet implemented must be recognised
explicitly and rejected if unsupported.** `check.ml` rejects `Vec`, `Map`,
`Result`/`try`, union values, closures, quoted symbols, generics and function
values *by name*, each with the milestone it belongs to; `load.ml` rejects the
package shapes it does not handle; and the FFI boundary rejects an aggregate.
The tests assert on the reason, not just on the failure.

## Untracked on purpose

`calc-me` and `sand`, the executables `flan build` drops beside their sources,
are now in `.gitignore` — anchored (`/calc-me`, `/sand`) so the patterns cannot
also match `sand-sim/` or anything nested.

`old-ocaml/` — the pre-rewrite menhir/ocamllex frontend, kept as reference and
excluded from the build by the root `dune` file. Its contents are also in git
history at `2c232dd`.