# Milestone-1 source surface the emitter lowers — normative reference

> What a `.wo` program may say and have `woc` produce bytecode for.
> The `.wob` format doc ([`00-wob-format.md`](00-wob-format.md)) names the
> BUILTIN *ids*; this names their **source spellings** and the handful of
> rules that have no other home. Landed with the emitter
> (`compiler/src/emit.ml`, plan 3 task 1). Diagnostics referenced here are
> catalogued in [`01-error-catalog.md`](01-error-catalog.md).
>
> Anything on this page is a contract for corpus fixtures and for every
> later sub-project's `.wo` code — not an emitter implementation detail.

## Builtins

Containers and runtime services are free functions, never methods. Each
maps to one `BUILTIN` id of the format doc.

| source | `.wob` builtin | arity | meaning |
| --- | --- | --- | --- |
| `now()` | `now` | 0 | wall-clock milliseconds (`Int`) |
| `print(t)` | `print` | 1 | a `Text`, newline-terminated |
| `print_int(n)` | `print_int` | 1 | an `Int`, newline-terminated |
| `words(t)` | `words` | 1 | whitespace token count of a `Text` |
| `multi_new()` | `multi_new` | 0 | a fresh `multi T` — see the destination rule below |
| `map_new()` | `map_new` | 0 | a fresh `map<K, V>` — see the destination rule below |
| `push(m, v)` | `multi_push` | 2 | append to a `multi` |
| `count(c)` | `count` | 1 | length of a `multi` or a `map` |
| `latest(m)` | `latest` | 1 | last element of a `multi` (traps `BOUNDS` when empty) |
| `get(c, k)` | `multi_get` / `map_get` | 2 | element by index, or value by key (a missing key traps `KEY`) |
| `set(m, k, v)` | `map_set` | 3 | insert or replace in a `map` |
| `has(m, k)` | `map_has` | 2 | `1`/`0` |

`get`, `set`, `push`, `count` and `has` resolve on the container they are
given, so one source name covers the `multi` and `map` ids the runtime
keeps apart.

**Sugar.** `c[i]` is exactly `get(c, i)` and `m[k] = v` is exactly
`set(m, k, v)`. There is no element *write* into a `multi` — v1 has
`multi_push` and `multi_get` and no element store — so `m[i] = v` on a
`multi` is `WO-E403`.

**Shadowing.** A user-declared free `fn` of the same name always wins. A
declared name is never silently replaced by a builtin.

**`push` and `@gc` elements.** `push(m, v)`'s value argument is never a
resolved callee parameter (`push` has no declared signature), so the
owner pass's ordinary Take-gated transfer never reaches it; a `@gc` value
pushed into a `multi` is special-cased in `owner.ml`'s `analyze_call`
(the value escapes into the container exactly like a ctor field, RC_INC
included) specifically so a `multi`-mediated `@gc` cycle can be built
and later collected (`tests/corpus/gc/`, plan 3 task 5). **`set(m, k, v)`
has no equivalent special case** — a `@gc` key or value handed to `set`
is not retained, so a `map<_, SomeGcClass>` (or a `@gc`-keyed map) built
this way will under-count its element's refcount and the collector will
free it while the map still points at it. Nothing in the corpus
exercises this yet; treat it as an open gap, not a proven-safe pattern,
until `set` gets the same fix `push` did.

## A fresh container needs a destination of declared type

`multi_new()` and `map_new()` carry their element (and key/value) kinds as
an instruction immediate, and those kinds **are** the container's drop
plan at runtime (`runtime/src/gc.c`). They cannot be guessed: assuming
`SCALAR` for a `multi Item` leaks every element, and for a
`map<Text, _>` leaks every key. Milestone-1 `let` has no container type
annotation — its optional annotation is a bare identifier — so a fresh
container must be created where its type is declared:

```wo
class Store {
  items:   multi Item
  by_name: map<Text, Int>
}

fn main() {
  let s = Store { items: multi_new(), by_name: map_new() }   -- kinds from the fields
  push(s.items, Item { n: 7 })
  set(s.by_name, "seven", 7)
}
```

A bare `let m = map_new()` is `WO-E403`, reported at the creation site.
The same rule applies to a `take`/`mut` parameter of declared container
type, which is also a typed destination.

## Calls

- Argument count must match the callee's parameter count (`WO-E403`).
  Nothing upstream checks arity — `types.ml` declares `WO-E203` and never
  raises it — and a mismatched call reserves a register window the callee
  does not read, which the loader rejects outright.
- A method is called as `receiver.method(args)`. When the receiver's
  declared type is an **interface**, the call is dispatched by vtable
  (`ICALL`); satisfaction is structural (same method name, same parameter
  count), Go-style, with no `implements` keyword.
- `self` occupies the callee's `r0`, so a method's argument count is
  `1 + parameters`.

## Program entry

The entry point is the **zero-argument free `fn main`**. A `main` that
takes parameters is not an entry (the format's own rule is a zero-argument
free fn), and `wovm` will report `module has no entry method`.

## Operators with no dedicated opcode

Lowered by the emitter, not added to the format:

| source | lowering |
| --- | --- |
| `a % b` | `a - (a / b) * b` — exact for the VM's truncating `DIV`, which traps on `0` and on `INT64_MIN / -1`, both correct for `%` too |
| `a != b` | `(a == b) == 0` |
| `a > b`, `a >= b` | `LT` / `LE` with the operands swapped |
| `a == b` on `Text` | `EQS` (content equality); `EQ` otherwise |
| `a .. b` | `CONCAT` — `+` is arithmetic only, never string addition |

## Not lowerable in milestone 1

Each is `WO-E403` at the offending site, never invented bytecode:

- an element write into a `multi` (no element-store instruction);
- `for` over a `map` (v1 exposes no key enumeration);
- a name that is neither a local, a parameter, `self`, a declared `fn`,
  nor a builtin;
- a field or method on a type that is not a declared class — including a
  class named only inside `multi T` / `ref T`, which `types.ml`'s
  unknown-type check (`WO-E225`) does not look inside.

## `?T`

A nullable field stores exactly what `T` stores and spells nil as `0`.
The v1 format has no kind byte for it (field kinds run `0..5`; the loader
rejects `6`), and it needs none: every per-kind drop plan already ignores
a zero slot. `?T`'s field kind is therefore `T`'s. Note the consequence
for `@gc`: `?SomeGcClass` is a `GCREF` field like any other, so it
participates in refcounting and cycle detection normally.