nescript/docs/language-guide.md

# NEScript Language Guide

NEScript is a statically-typed, compiled language designed for NES game development. It compiles directly to 6502 machine code packaged as iNES-format ROMs -- no external assembler or tooling required.

This guide covers every language feature with practical examples.

---

## Program Structure

Every NEScript program consists of a game declaration, top-level definitions, and a start declaration.

```
game "My Game" {
    mapper: NROM
    mirroring: vertical
}

const SPEED: u8 = 2

var score: u8 = 0

fun helper() -> u8 {
    return 42
}

state Title {
    on frame {
        draw Logo at: (100, 100)
        if button.start {
            transition Playing
        }
    }
}

state Playing {
    on enter {
        score = 0
    }
    on frame {
        // game logic here
    }
}

start Title
```

### Game Declaration

The `game` block is required and must appear first. It names the game and sets hardware configuration.

```
game "Coin Cavern" {
    mapper: NROM
    mirroring: vertical
}
```

Available properties:

| Property     | Values                           | Default      |
|--------------|----------------------------------|--------------|
| `mapper`     | `NROM`, `MMC1`, `UxROM`, `MMC3`  | required     |
| `mirroring`  | `horizontal`, `vertical`         | `horizontal` |

### Start Declaration

Exactly one `start` declaration must exist. It names the initial state entered on power-on.

```
start Title
```

---

## Types

NEScript has four primitive types and fixed-size arrays.

### Primitive Types

| Type   | Size    | Range           | Description                        |
|--------|---------|-----------------|------------------------------------|
| `u8`   | 1 byte  | 0 to 255        | Unsigned 8-bit integer             |
| `i8`   | 1 byte  | -128 to 127     | Signed 8-bit integer               |
| `u16`  | 2 bytes | 0 to 65535      | Unsigned 16-bit integer            |
| `bool` | 1 byte  | `true` / `false`| Boolean                            |

### Arrays

Arrays are fixed-size, homogeneous, and zero-indexed. The size must be a compile-time constant. Maximum 256 elements.

```
var enemies: u8[8]
const TABLE: u8[4] = [10, 20, 30, 40]
```

### Type Casting

NEScript has no implicit coercion. All conversions use `as`:

```
var a: u8 = 200
var b: u16 = a as u16       // zero-extend: 200
var c: i8 = a as i8         // reinterpret bits
var d: u8 = b as u8         // truncate to low byte
```

---

## Variables

### Variable Declarations

Variables are declared with `var` and must have an explicit type:

```
var x: u8                   // uninitialized (zeroed on state entry)
var y: u8 = 100             // initialized
var pos: u16 = 0x0400       // 16-bit value
var alive: bool = true
var scores: u8[4] = [0, 0, 0, 0]
```

### Constants

Constants are evaluated at compile time and stored in ROM:

```
const MAX_ENEMIES: u8 = 5
const SPEED: u8 = 3
const SIN_TABLE: u8[8] = [0, 49, 90, 117, 127, 117, 90, 49]
```

### Enums

Enums declare a named set of `u8` constants. Each variant is assigned an
index starting at 0 in declaration order:

```
enum Direction { Up, Down, Left, Right }
// Up=0, Down=1, Left=2, Right=3

var player_dir: u8 = Up

on frame {
    if button.left  { player_dir = Left }
    if button.right { player_dir = Right }
    if player_dir == Down { /* ... */ }
}
```

Variant names are global — they are flattened into the top-level symbol
table, so a variant cannot share its name with any other constant,
variable, or function (E0501). An enum cannot have more than 256
variants because each is stored as a `u8`.

### Structs

Structs declare composite types with named fields:

```
struct Vec2 {
    x: u8,
    y: u8,
}

struct Player {
    health: u8,
    lives: u8,
}

var pos: Vec2
var hero: Player

on frame {
    pos.x = 100
    pos.y = 50
    hero.health = 3
    hero.lives = 5
    if button.right { pos.x += 1 }
    draw Hero at: (pos.x, pos.y)
}
```

Fields are laid out contiguously in declaration order. A variable of
struct type allocates enough contiguous bytes to hold all its fields;
each field is accessible via the dot operator.

Struct literals initialize or assign all fields at once:

```
struct Vec2 { x: u8, y: u8 }

// as an initializer
var pos: Vec2 = Vec2 { x: 100, y: 50 }

// as an assignment
on frame {
    pos = Vec2 { x: 0, y: 0 }
    if button.right {
        pos = Vec2 { x: pos.x + 1, y: pos.y }
    }
}
```

Inside `if`, `while`, and `for` conditions the struct literal syntax
is reserved for the following block, so wrap the literal in parens if
you ever need one in a condition:

```
if pos == (Vec2 { x: 0, y: 0 }) { /* ... */ }
```

In v0.1 only primitive field types (`u8`, `i8`, `bool`) are supported —
nested structs, `u16`, and array fields are not yet allowed.

### Memory Placement Hints

The NES has 256 bytes of zero-page RAM with faster access. You can hint where variables should be placed:

```
fast var px: u8             // prefer zero-page (faster instructions)
slow var high_score: u16    // prefer upper RAM (saves zero-page space)
var normal: u8              // compiler decides automatically
```

If zero-page is exhausted and `fast` variables cannot be placed, the compiler emits error `E0301`.

### Scope

| Scope    | Declared In    | Lifetime                                   |
|----------|----------------|---------------------------------------------|
| Global   | Top level      | Entire program, permanent RAM allocation    |
| State    | `state` block  | Active while state is active; RAM reusable  |
| Function | `fun` block    | Duration of function call                   |
| Block    | `if`/`while`   | Enclosing block, shares parent allocation   |

---

## Functions

### Declaration

Functions use `fun`, with optional parameters and return type:

```
fun add(a: u8, b: u8) -> u8 {
    return a + b
}

fun reset_score() {
    score = 0
}
```

### Inline Functions

The `inline` keyword hints the compiler to inline the function at call sites:

```
inline fun clamp(val: u8, max: u8) -> u8 {
    if val > max {
        return max
    }
    return val
}
```

`inline` is a hint -- the compiler may decline for large functions.

### Calling Functions

```
var result: u8 = add(10, 20)
reset_score()
```

### Restrictions

- **No recursion.** Both direct and indirect recursion are compile errors (`E0402`).
- **Call depth limit.** The default maximum call depth is 8. Exceeding it produces error `E0401`.

---

## States

States are the top-level organizational unit. Exactly one state is active at any time.

### State Declaration

```
state Playing {
    var timer: u8 = 0           // state-local variable

    on enter {
        // runs once when entering this state
        timer = 60
    }

    on exit {
        // runs once when leaving this state
    }

    on frame {
        // runs every frame (60 Hz) while this state is active
        timer -= 1
        draw Player at: (player_x, player_y)
    }
}
```

`on frame` is syntactic sugar for a loop with an implicit `wait_frame()` at the end. A state can have any combination of `on enter`, `on exit`, and `on frame`.

### State Transitions

```
transition GameOver
```

Transitions are immediate. The current state's `on exit` runs, then the target state's `on enter` runs. The remainder of the current frame handler does not execute.

---

## Expressions

### Literals

```
42              // decimal integer
0xFF            // hexadecimal
0b10110001      // binary
1_000           // underscores allowed for readability (if supported)
true            // boolean
false           // boolean
[1, 2, 3]      // array literal
```

All integer literals must fit in `u16` (0-65535). The compiler narrows to the required type at usage.

### Arithmetic Operators

| Operator | Description    | Example      |
|----------|----------------|--------------|
| `+`      | Addition       | `a + b`      |
| `-`      | Subtraction    | `a - b`      |
| `*`      | Multiplication | `a * b`      |
| `/`      | Division       | `a / b`      |
| `%`      | Modulo         | `a % b`      |

`*`, `/`, and `%` are available but expensive on the 6502 (software routines). The compiler optimizes power-of-two operations to shifts and warns on non-power-of-two multiply/divide.

### Bitwise Operators

| Operator | Description    | Example      |
|----------|----------------|--------------|
| `&`      | Bitwise AND    | `a & 0x0F`   |
| `\|`     | Bitwise OR     | `a \| 0x80`  |
| `^`      | Bitwise XOR    | `a ^ mask`   |
| `~`      | Bitwise NOT    | `~a`         |
| `<<`     | Shift left     | `a << 2`     |
| `>>`     | Shift right    | `a >> 1`     |

### Comparison Operators

| Operator | Description       | Example      |
|----------|-------------------|--------------|
| `==`     | Equal             | `a == 0`     |
| `!=`     | Not equal         | `a != b`     |
| `<`      | Less than         | `a < 10`     |
| `>`      | Greater than      | `a > max`    |
| `<=`     | Less or equal     | `a <= 255`   |
| `>=`     | Greater or equal  | `a >= min`   |

### Logical Operators

NEScript uses keyword-based logical operators:

```
if alive and (health > 0) {
    // ...
}
if not paused or force_update {
    // ...
}
```

| Operator | Description   |
|----------|---------------|
| `and`    | Logical AND   |
| `or`     | Logical OR    |
| `not`    | Logical NOT   |

### Operator Precedence

From highest to lowest:

| Level | Operators                          | Associativity |
|-------|------------------------------------|---------------|
| 1     | `()` grouping                      | --            |
| 2     | `-` (unary), `~`, `not`            | right         |
| 3     | `*`, `/`, `%`                      | left          |
| 4     | `+`, `-`                           | left          |
| 5     | `<<`, `>>`                         | left          |
| 6     | `&`                                | left          |
| 7     | `^`                                | left          |
| 8     | `\|`                               | left          |
| 9     | `==`, `!=`, `<`, `>`, `<=`, `>=`   | left          |
| 10    | `and`                              | left          |
| 11    | `or`                               | left          |

### Button Reads

Read controller input as boolean expressions:

```
if button.right {
    player_x += SPEED
}
if button.a {
    jump()
}
```

Available buttons: `up`, `down`, `left`, `right`, `a`, `b`, `start`, `select`.

For two-player games, prefix with the player:

```
if p1.button.a { /* player 1 */ }
if p2.button.right { /* player 2 */ }
```

Without a prefix, `button` refers to player 1.

### Function Calls in Expressions

```
var clamped: u8 = clamp_x(player_x + SPEED)
```

### Array Indexing

```
var val: u8 = table[i]
table[i] = 0
```

### Type Casting

```
var wide: u16 = narrow as u16
```

---

## Statements

### Assignment

```
x = 10
x += 5
x -= 1
x &= 0x0F
x |= 0x80
x ^= mask
```

All assignment operators:

| Operator | Description         |
|----------|---------------------|
| `=`      | Assign              |
| `+=`     | Add and assign      |
| `-=`     | Subtract and assign |
| `&=`     | AND and assign      |
| `\|=`    | OR and assign       |
| `^=`     | XOR and assign      |

Array element assignment:

```
enemies[i] = 0
scores[player] += 10
```

### If / Else If / Else

Braces are always required. No ternary operator.

```
if health == 0 {
    transition GameOver
} else if health < 3 {
    flash_warning()
} else {
    // normal gameplay
}
```

### While Loop

```
var i: u8 = 0
while i < 10 {
    enemies[i] = 0
    i += 1
}
```

### Match Statement

`match` matches a scrutinee against a sequence of patterns and
executes the body of the first matching arm. Each arm's pattern is
compared against the scrutinee with `==`. An underscore arm `_` acts
as the catch-all:

```
enum State { Title, Playing, GameOver }
var state: u8 = Title

on frame {
    match state {
        Title => {
            if button.start { state = Playing }
        }
        Playing => {
            // ... game logic ...
        }
        GameOver => {
            if button.a { state = Title }
        }
        _ => {}
    }
}
```

`match` desugars to an `if` / `else if` chain at parse time, so
patterns can be any expression that produces a value comparable to
the scrutinee.

### For Loop

The `for` loop iterates over a half-open integer range `[start, end)`:

```
for i in 0..8 {
    total += arr[i]
}
```

The loop variable is a `u8` scoped to the loop body. Both bounds can
be any expression that evaluates to `u8` at runtime, including
constants or variables. The range is half-open, so `0..8` iterates
`0, 1, 2, ..., 7` (8 iterations). For a closed range, use `0..9`.

The loop is desugared into a `while` loop with an index variable, so
`break` and `continue` work the same as in any loop body.

### Loop (Infinite)

```
loop {
    wait_frame()
    if button.start {
        break
    }
}
```

The compiler warns if a `loop` contains neither `break`, `wait_frame`, nor `transition`.

### Break and Continue

```
var i: u8 = 0
while i < 20 {
    i += 1
    if enemies[i] == 0 {
        continue            // skip inactive enemies
    }
    if i > 10 {
        break               // stop processing
    }
    update_enemy(i)
}
```

### Return

```
fun abs_diff(a: u8, b: u8) -> u8 {
    if a > b {
        return a - b
    }
    return b - a
}
```

Functions without a return type use `return` with no value (or simply reach the end of the function body).

### Draw

Render a sprite to the screen:

```
draw Player at: (player_x, player_y)
draw Coin at: (COIN_X, COIN_Y) frame: anim_frame
```

The `draw` statement writes to the OAM shadow buffer. The NES supports up to 64 sprites per frame.

Syntax: `draw SpriteName at: (x_expr, y_expr) [frame: expr]`

### Transition

Switch to another state immediately:

```
transition GameOver
```

The current state's `on exit` runs, then the target state's `on enter` runs.

### Wait Frame

Yield execution until the next vertical blank (NMI). Synchronizes to the 60 Hz display refresh.

```
wait_frame()
```

This triggers OAM DMA transfer and PPU updates before yielding. Inside `on frame`, a `wait_frame()` is implicit at the end of each frame.

### Scroll

Set the PPU scroll position:

```
scroll(scroll_x, scroll_y)
```

### Set Palette

```
set_palette NightPalette
```

Queues the named palette for a vblank-safe copy into PPU palette
RAM (`$3F00-$3F1F`). The write is applied by the NMI handler on the
next vblank. See `palette` declarations below.

### Load Background

```
load_background Level1
```

Queues the named background (a full-screen 32×30 nametable + 64-byte
attribute table) for a vblank-safe copy into nametable 0
(`$2000-$23FF`). Applied by the NMI handler at the next vblank. See
`background` declarations below.

### Function Calls as Statements

```
reset_score()
update_physics(player_x, player_y)
```

---

## Assets

### Sprite Declarations

```
sprite Player {
    chr: @chr("assets/player.png")
}

sprite Coin {
    chr: @binary("assets/coin.bin")
}
```

### Palette Declarations

```
palette MainPalette {
    colors: [0x0F, 0x01, 0x11, 0x21,
             0x0F, 0x06, 0x16, 0x26,
             0x0F, 0x09, 0x19, 0x29,
             0x0F, 0x0B, 0x1B, 0x2B,
             0x0F, 0x01, 0x11, 0x21,
             0x0F, 0x16, 0x27, 0x30,
             0x0F, 0x14, 0x24, 0x34,
             0x0F, 0x0B, 0x1B, 0x2B]
}
```

Each byte is an NES master-palette index (`$00-$3F`). The 32 bytes
map directly to PPU palette RAM `$3F00-$3F1F` in the canonical NES
layout:

| Offset          | Contents                            |
|-----------------|-------------------------------------|
| `$00`-`$03`    | Background sub-palette 0 (4 colours) |
| `$04`-`$07`    | Background sub-palette 1             |
| `$08`-`$0B`    | Background sub-palette 2             |
| `$0C`-`$0F`    | Background sub-palette 3             |
| `$10`-`$13`    | Sprite sub-palette 0                 |
| `$14`-`$17`    | Sprite sub-palette 1                 |
| `$18`-`$1B`    | Sprite sub-palette 2                 |
| `$1C`-`$1F`    | Sprite sub-palette 3                 |

The first byte of each sub-palette is typically `$0F` (black); it
serves as the shared background colour. Lists shorter than 32 bytes
are zero-padded; lists longer than 32 are a compile error.

The *first* `palette` declared in a program is loaded into VRAM at
reset time, before rendering is enabled, so the title screen boots
with the right colours on frame 0. Additional declarations sit in
PRG ROM as named data blobs and become active via `set_palette Name`,
which queues the write for the next vblank.

### Background Declarations

```
background TitleScreen {
    tiles: [0x00, 0x01, 0x01, 0x00,  /* ... up to 960 bytes ... */]
    attributes: [0xFF, 0x55,         /* ... up to 64 bytes ... */]
}
```

A background is a 32×30 nametable. `tiles` is the nametable itself
— 960 bytes of CHR tile indices, one per 8×8 cell, in row-major
order (left-to-right, top-to-bottom). `attributes` is the 8×8
attribute table (64 bytes) that controls which sub-palette each
16×16 metatile uses.

Both lists are zero-padded up to their fixed sizes; `attributes`
may be omitted entirely, in which case every cell uses background
sub-palette 0.

The *first* `background` declared is loaded into nametable 0 at
reset time and background rendering is enabled automatically.
Additional backgrounds can be swapped in via `load_background Name`,
which queues the update for the next vblank. Full-nametable updates
do not fit inside a single vblank, so large background swaps may
require the program to disable rendering temporarily.

### Asset Sources

Three ways to provide asset data:

| Source                     | Description                           |
|----------------------------|---------------------------------------|
| `@chr("file.png")`        | Convert PNG to CHR tile data          |
| `@binary("file.bin")`     | Include raw binary data verbatim      |
| Inline `[0x00, 0x7E, ...]`| Hex byte array directly in source     |

---

## Audio

NEScript ships with a full data-driven audio subsystem. Sound effects run on pulse channel 1 and music runs on pulse channel 2, both driven by an NMI-time tick that walks per-track data tables compiled into PRG ROM. Programs that never touch audio pay zero ROM or cycle cost — the driver and its period table are only linked in when user code contains at least one `play`, `start_music`, or `stop_music` statement.

### Statements

```
play SfxName          // trigger a one-shot sound effect
start_music TrackName // begin looping background music
stop_music            // silence the music channel
```

Each statement looks up the name in the program's user declarations first, then falls back to the builtin table. Unknown names are a hard error (E0505).

### SFX Declarations

An `sfx` block is a frame-accurate envelope for pulse 1. `pitch` latches the pulse period on trigger; `volume` runs one entry per frame, so the envelope length controls the effect duration.

```
sfx Pickup {
    duty: 2                                   // 0-3, 2 = 50% square (default)
    pitch: [0x50, 0x50, 0x50, 0x50, 0x50]     // period for each frame
    volume: [15, 12, 9, 6, 3]                  // 0-15, one per frame
}
```

Rules:
- `pitch` and `volume` must have the same length (the frame count).
- `volume` values are 0-15 (4-bit pulse volume).
- `duty` is 0-3 and defaults to 2.
- Maximum 120 frames (2 seconds at 60 fps).

### Music Declarations

A `music` block is a flat list of `(pitch, duration)` note pairs played on pulse 2. Pitch 0 is a rest; pitches 1-60 are indices into the builtin 60-note period table (C1 through B5, with middle C at index 37). Duration is in frames (so at 60 fps, `30` is half a second).

```
music Theme {
    duty: 2                                   // 0-3 (default 2)
    volume: 10                                // 0-15 (default 10)
    repeat: true                              // loop when track ends (default true)
    notes: [
        37, 20,    // C4 for 20 frames
        41, 20,    // E4
        44, 20,    // G4
        49, 20,    // C5
        0, 10,     // rest for 10 frames
    ]
}
```

Rules:
- `notes` must contain an even number of bytes (pitch + duration pairs).
- Pitches are 0 (rest) or 1-60 (period table index).
- Duration must be ≥ 1 frame.
- Maximum 256 notes per track.

### Builtin Names

For programs that want classic game audio without writing data tables, NEScript provides a handful of builtin effects and tracks that can be used directly:

**Builtin SFX**

| Name | Description |
|------|-------------|
| `coin`, `pickup`, `collect` | Ascending high blip |
| `jump`, `hop` | Descending arc |
| `hit`, `damage`, `explode` | Low blast |
| `click`, `select`, `confirm` | Sharp beep |
| `cancel`, `back`, `error` | Low longer tone |
| `shoot`, `laser`, `fire` | Very high pulse |
| `step`, `footstep` | Short low thud |

**Builtin Music**

| Name | Description |
|------|-------------|
| `title`, `theme`, `main` | Major arpeggio (looping) |
| `battle`, `boss` | Driving pulse (looping) |
| `win`, `victory`, `fanfare` | Ascending burst (one-shot) |
| `gameover`, `lose`, `fail` | Descending dirge (looping) |

A user-declared `sfx` or `music` block takes priority over a builtin with the same name, so `sfx coin { ... }` will shadow the default coin effect.

### How It Works

Compile time:

1. The resolver compiles each `sfx` into `(period_lo, period_hi, envelope[])` and each `music` into `(header, (pitch, duration)[])`, appending builtins for any referenced name that isn't user-declared.
2. The IR codegen emits `play Name` as: write trigger bytes to `$4002`/`$4003`, load envelope pointer into `$0C/$0D`, set the sfx counter. `start_music Name` stamps a state byte into `$07`, loads the stream pointer into `$0E/$0F` (and the loop base into `$05/$06`), and primes the duration counter.
3. The linker splices the audio tick, the 60-entry period table, and every compiled sfx/music blob into PRG ROM, all guarded on a `__audio_used` marker label so silent programs never pay the cost.

Runtime (every NMI, if audio is in use):

1. **SFX**: if the counter is nonzero, read one envelope byte through `(ZP_SFX_PTR),Y` and write it to `$4000`. A zero sentinel mutes pulse 1 and stops the tick.
2. **Music**: if active and the note counter hits zero, read the next pitch byte. 0 = rest (mute pulse 2). 1-60 = look up the period in the table and write to `$4006`/`$4007`. `0xFF` = loop back to the base pointer (or mute if `repeat: false`). Then read the duration byte and reload the counter.

Total memory cost: 8 bytes of zero page, ~200 bytes for the driver body, 120 bytes for the period table, plus the data for each user-declared sfx/music.

---

## Mappers

The mapper determines cartridge hardware and available ROM size.

| Mapper  | PRG ROM       | CHR ROM        | Features                         |
|---------|---------------|----------------|----------------------------------|
| `NROM`  | 16 or 32 KB   | 8 KB           | No banking, simplest             |
| `MMC1`  | Up to 256 KB  | Up to 128 KB   | Switchable banks                 |
| `UxROM` | Up to 256 KB  | 8 KB CHR RAM   | PRG banking only                 |
| `MMC3`  | Up to 512 KB  | Up to 256 KB   | Scanline counter, banking        |

### Bank Declarations

For mappers with bank switching:

```
bank MainCode {
    // Always-resident code (NMI handler, core engine)
}

bank Level1 {
    state Level1 { ... }
    background Level1BG { ... }
}
```

Banks can hold `prg` (code/data) or `chr` (graphics) content. Transitions between states in different banks automatically emit bank-switch and trampoline code.

---

## Comments

```
// Line comment -- extends to end of line

/* Block comment
   spans multiple lines */
```

---

## Includes

Split your game across multiple files:

```
include "physics.ne"
include "enemies.ne"
```

Includes are resolved relative to the including file. Circular includes are a compile error. Duplicate includes are skipped automatically.

---

## Debug Mode

Compile with `--debug` to enable runtime instrumentation. All debug features are stripped completely in release builds (zero bytes, zero cycles).

### Debug Logging

```
debug.log("Player position: ", px, ", ", py)
```

### Debug Assertions

```
debug.assert(lives > 0, "Lives should never be negative")
```

### Runtime Checks (Debug Only)

In debug mode, the compiler inserts:
- Array bounds checking on indexed access
- Arithmetic overflow warnings
- Stack depth monitoring at function entry
- Frame overrun detection (warns if frame handler exceeds vblank period)

---

## Hardware Intrinsics

For the common case of reading or writing a single PPU/APU/mapper
register, NEScript provides two built-in intrinsics:

```
poke(0x2006, 0x3F)        // write $3F to PPU address register
poke(0x2006, 0x00)        // (second half of the address)
poke(0x2007, 0x0F)        // write a palette byte to PPU data

var status: u8 = peek(0x2002)  // read PPU status register
```

The address argument to both is a compile-time constant. Zero-page
addresses compile to `STA $XX` / `LDA $XX`; anything larger compiles
to absolute addressing.

## Inline Assembly

For more elaborate sequences, use `asm { ... }` blocks:

```
fun fast_shift(input: u8) -> u8 {
    var result: u8 = 0
    asm {
        LDA {input}
        ASL A
        ASL A
        STA {result}
    }
    return result
}
```

Inside an `asm` block, `{name}` is replaced with the resolved
zero-page or absolute address of the variable `name`. Labels
defined with `name:` are local to the block.

### Raw Assembly

```
raw asm {
    LDA #$42
    STA $2007
}
```

`raw asm` skips variable substitution — `{name}` is passed through
verbatim. Useful for completely unmanaged snippets that don't
reference NEScript variables.

---

## Error Codes

### Lexer Errors (E01xx)

| Code   | Description                |
|--------|----------------------------|
| E0101  | Unterminated string literal |
| E0102  | Invalid character           |
| E0103  | Number literal overflow     |

### Type Errors (E02xx)

| Code   | Description                |
|--------|----------------------------|
| E0201  | Type mismatch              |
| E0203  | Invalid operation for type |

### Memory Errors (E03xx)

| Code   | Description                |
|--------|----------------------------|
| E0301  | Zero-page overflow         |

### Control Flow Errors (E04xx)

| Code   | Description                |
|--------|----------------------------|
| E0401  | Call depth exceeded        |
| E0402  | Recursion detected         |
| E0404  | Transition to undefined state |

### Declaration Errors (E05xx)

| Code   | Description                |
|--------|----------------------------|
| E0501  | Duplicate declaration      |
| E0502  | Undefined variable         |
| E0503  | Undefined function         |
| E0504  | Missing start declaration  |
| E0505  | Multiple start declarations|

### Warnings (W01xx)

| Code   | Description                              |
|--------|------------------------------------------|
| W0101  | Expensive multiply/divide operation      |
| W0102  | Loop without break or wait_frame         |
| W0103  | Unused variable                          |
| W0104  | Unreachable code (after return/break/transition, or state unreachable from start) |

### Example Error Output

```
error[E0201]: type mismatch
  --> game.ne:42:15
   |
42 |   var x: u8 = -5
   |               ^^ expected u8, found negative integer
   |
   = help: use i8 if you need negative values: var x: i8 = -5
```

```
error[E0402]: recursion is not allowed
  --> game.ne:55:5
   |
55 |     flood_fill(x + 1, y)
   |     ^^^^^^^^^^^^^^^^^^^^
   |
   = note: flood_fill calls itself (directly recursive)
   = help: the NES has only 256 bytes of stack; use an iterative algorithm instead
```

---

## Command Line

Compile a `.ne` source file into a `.nes` ROM:

```
nescript build game.ne
nescript build game.ne --output my_game.nes
nescript build game.ne --debug
nescript build game.ne --asm-dump
nescript build game.ne --dump-ir
```

| Flag            | Description                                                    |
|-----------------|----------------------------------------------------------------|
| `--output`      | Set output ROM file path (default: input.nes)                  |
| `--debug`       | Enable debug mode with runtime checks                          |
| `--asm-dump`    | Dump generated 6502 assembly to stdout                         |
| `--dump-ir`     | Dump the lowered IR program (after optimization) to stdout     |
| `--memory-map`  | Dump a memory map of variable allocations to stdout            |
| `--call-graph`  | Dump a call graph (which handler/function calls which) to stdout |

### Check

Type-check a source file without producing a ROM:

```
nescript check game.ne
```

---

## Complete Example

A full game demonstrating states, input, functions, constants, and transitions:

```
game "Coin Cavern" {
    mapper: NROM
}

const SPEED: u8 = 2
const SCREEN_RIGHT: u8 = 240
const COIN_X: u8 = 180
const COIN_Y: u8 = 100

var player_x: u8 = 40
var player_y: u8 = 200
var score: u8 = 0
var coins_left: u8 = 3

fun clamp_x(val: u8) -> u8 {
    if val > SCREEN_RIGHT {
        return 0
    }
    return val
}

state Title {
    on frame {
        draw Logo at: (100, 100)
        if button.start {
            transition Playing
        }
    }
}

state Playing {
    on enter {
        player_x = 40
        player_y = 200
        score = 0
        coins_left = 3
    }

    on frame {
        if button.right {
            player_x += SPEED
            if player_x > SCREEN_RIGHT {
                player_x = SCREEN_RIGHT
            }
        }
        if button.left {
            if player_x >= SPEED {
                player_x -= SPEED
            } else {
                player_x = 0
            }
        }

        if player_x >= COIN_X {
            if player_y >= COIN_Y {
                score += 1
                coins_left -= 1
                if coins_left == 0 {
                    transition GameOver
                }
            }
        }

        draw Player at: (player_x, player_y)
        draw Coin at: (COIN_X, COIN_Y)
    }
}

state GameOver {
    on frame {
        draw Trophy at: (120, 100)
        if button.start {
            transition Title
        }
    }
}

start Title
```

Build and run:

```
nescript build coin_cavern.ne
# produces coin_cavern.nes -- open in any NES emulator
```