# Reef Compiler Guide

Quick reference for compiling to Reef bytecode.

## Bytecode Formats

ReefVM supports two bytecode formats:

1. **String format**: Human-readable text with opcodes and operands
2. **Array format**: TypeScript arrays with typed tuples for programmatic generation

Both formats are compiled using the same `toBytecode()` function.

## Bytecode Syntax

### Instructions
```
OPCODE operand     ; comment
```

### Operand Types

**Immediate numbers** (`#N`): Counts or relative offsets
- `MAKE_ARRAY #3` - count of 3 items
- `JUMP #5` - relative offset of 5 instructions (prefer labels)
- `PUSH_TRY #10` - absolute instruction index (prefer labels)

**Labels** (`.name`): Symbolic addresses resolved at parse time
- `.label:` - define label at current position
- `JUMP .loop` - jump to label
- `MAKE_FUNCTION (x) .body` - function body at label

**Variable names**: Plain identifiers (supports Unicode and emoji!)
- `LOAD counter` - load variable
- `STORE result` - store variable
- `LOAD 💎` - load emoji variable
- `STORE 変数` - store Unicode variable

**Constants**: Literals added to constants pool
- Numbers: `PUSH 42`, `PUSH 3.14`
- Strings: `PUSH "hello"` or `PUSH 'world'`
- Booleans: `PUSH true`, `PUSH false`
- Null: `PUSH null`

**Native function names**: Registered TypeScript functions
- `CALL_NATIVE print`

## Array Format

The programmatic array format uses TypeScript tuples for type safety:

```typescript
import { toBytecode, run } from "#reef"

const bytecode = toBytecode([
  ["PUSH", 42],        // Atom values: number | string | boolean | null
  ["STORE", "x"],      // Variable names as strings
  ["LOAD", "x"],
  ["HALT"]
])

const result = await run(bytecode)
```

### Operand Types in Array Format

**Atoms** (`number | string | boolean | null`): Constants for PUSH
```typescript
["PUSH", 42]
["PUSH", "hello"]
["PUSH", true]
["PUSH", null]
```

**Variable names**: String identifiers
```typescript
["LOAD", "counter"]
["STORE", "result"]
```

**Label definitions**: Single-element arrays starting with `.` and ending with `:`
```typescript
[".loop:"]
[".end:"]
[".function_body:"]
```

**Label references**: Strings in jump/function instructions
```typescript
["JUMP", ".loop"]
["JUMP_IF_FALSE", ".end"]
["MAKE_FUNCTION", ["x", "y"], ".body"]
["PUSH_TRY", ".catch"]
```

**Counts**: Numbers for array/dict construction
```typescript
["MAKE_ARRAY", 3]    // Pop 3 items
["MAKE_DICT", 2]     // Pop 2 key-value pairs
```

**Native function names**: Strings for registered functions
```typescript
["CALL_NATIVE", "print"]
```

### Functions in Array Format

```typescript
// Basic function
["MAKE_FUNCTION", ["x", "y"], ".body"]

// With defaults
["MAKE_FUNCTION", ["x", "y=10"], ".body"]

// Variadic
["MAKE_FUNCTION", ["...args"], ".body"]

// Named args
["MAKE_FUNCTION", ["@opts"], ".body"]

// Mixed
["MAKE_FUNCTION", ["x", "y=5", "...rest", "@opts"], ".body"]
```

### Complete Example

```typescript
const factorial = toBytecode([
  ["MAKE_FUNCTION", ["n", "acc=1"], ".fact"],
  ["STORE", "factorial"],
  ["JUMP", ".main"],

  [".fact:"],
  ["LOAD", "n"],
  ["PUSH", 0],
  ["LTE"],
  ["JUMP_IF_FALSE", ".recurse"],
  ["LOAD", "acc"],
  ["RETURN"],

  [".recurse:"],
  ["LOAD", "factorial"],
  ["LOAD", "n"],
  ["PUSH", 1],
  ["SUB"],
  ["LOAD", "n"],
  ["LOAD", "acc"],
  ["MUL"],
  ["PUSH", 2],
  ["PUSH", 0],
  ["TAIL_CALL"],

  [".main:"],
  ["LOAD", "factorial"],
  ["PUSH", 5],
  ["PUSH", 1],
  ["PUSH", 0],
  ["CALL"],
  ["HALT"]
])

const result = await run(factorial)  // { type: "number", value: 120 }
```

## String Format

### Functions
```
MAKE_FUNCTION (x y) .body       ; Basic
MAKE_FUNCTION (x=10 y=20) .body ; Defaults
MAKE_FUNCTION (x ...rest) .body ; Variadic
MAKE_FUNCTION (x @named) .body  ; Named args
MAKE_FUNCTION (x ...rest @named) .body ; Both
```

### Function Calls
Stack order (bottom to top):
```
LOAD fn
PUSH arg1           ; Positional args
PUSH arg2
PUSH "name"         ; Named arg key
PUSH "value"        ; Named arg value
PUSH 2              ; Positional count
PUSH 1              ; Named count
CALL
```

## Opcodes

### Stack
- `PUSH <const>` - Push constant
- `POP` - Remove top
- `DUP` - Duplicate top

### Variables
- `LOAD <name>` - Push variable value
- `STORE <name>` - Pop and store in variable

### Arithmetic
- `ADD`, `SUB`, `MUL`, `DIV`, `MOD` - Binary ops (pop 2, push result)

### Comparison
- `EQ`, `NEQ`, `LT`, `GT`, `LTE`, `GTE` - Pop 2, push boolean

### Logic
- `NOT` - Pop 1, push !value

### Control Flow
- `JUMP .label` - Unconditional jump
- `JUMP_IF_FALSE .label` - Jump if top is false or null (pops value)
- `JUMP_IF_TRUE .label` - Jump if top is truthy (pops value)
- `HALT` - Stop execution of the program

### Functions
- `MAKE_FUNCTION (params) .body` - Create function, push to stack
- `CALL` - Call function (see calling convention above)
- `TAIL_CALL` - Tail-recursive call (no stack growth)
- `RETURN` - Return from function (pops return value)
- `BREAK` - Exit iterator/loop (unwinds to break target)

### Arrays
- `MAKE_ARRAY #N` - Pop N items, push array
- `ARRAY_GET` - Pop index and array, push element
- `ARRAY_SET` - Pop value, index, array; mutate array
- `ARRAY_PUSH` - Pop value and array, append to array
- `ARRAY_LEN` - Pop array, push length

### Dicts
- `MAKE_DICT #N` - Pop N key-value pairs, push dict
- `DICT_GET` - Pop key and dict, push value (or null)
- `DICT_SET` - Pop value, key, dict; mutate dict
- `DICT_HAS` - Pop key and dict, push boolean

### Exceptions
- `PUSH_TRY .catch` - Register exception handler
- `PUSH_FINALLY .finally` - Add finally to current handler
- `POP_TRY` - Remove handler (try succeeded)
- `THROW` - Throw exception (pops error value)

### Native
- `CALL_NATIVE <name>` - Call registered TypeScript function (consumes entire stack as args)

## Compiler Patterns

### If-Else
```
<condition>
JUMP_IF_FALSE .else
  <then-block>
  JUMP .end
.else:
  <else-block>
.end:
```

### While Loop
```
.loop:
  <condition>
  JUMP_IF_FALSE .end
  <body>
  JUMP .loop
.end:
```

### For Loop
```
<init>
.loop:
  <condition>
  JUMP_IF_FALSE .end
  <body>
  <increment>
  JUMP .loop
.end:
```

### Continue
No CONTINUE opcode. Use backward jump to loop start:
```
.loop:
  <condition>
  JUMP_IF_FALSE .end
  <early-check>
  JUMP_IF_TRUE .loop    ; continue
  <body>
  JUMP .loop
.end:
```

### Break in Loop
Mark iterator function as break target, use BREAK opcode:
```
MAKE_FUNCTION () .each_body
STORE each
LOAD collection
LOAD each
<call-iterator-with-break-semantics>
HALT

.each_body:
  <condition>
  JUMP_IF_TRUE .done
  <body>
  BREAK                  ; exits to caller
.done:
  RETURN
```

### Short-Circuit AND
```
<left>
DUP
JUMP_IF_FALSE .end      ; Short-circuit if false
POP
<right>
.end:                    ; Result on stack
```

### Short-Circuit OR
```
<left>
DUP
JUMP_IF_TRUE .end       ; Short-circuit if true
POP
<right>
.end:                    ; Result on stack
```

### Try-Catch
```
PUSH_TRY .catch
  <try-block>
  POP_TRY
  JUMP .end
.catch:
  STORE err
  <catch-block>
.end:
```

### Try-Catch-Finally
```
PUSH_TRY .catch
PUSH_FINALLY .finally
  <try-block>
  POP_TRY
  JUMP .finally         ; Compiler must generate this
.catch:
  STORE err
  <catch-block>
  JUMP .finally         ; And this
.finally:
  <finally-block>       ; Executes in both paths
.end:
```

**Important**: VM only auto-jumps to finally on THROW. For successful try/catch, compiler must explicitly JUMP to finally.

### Closures
Functions automatically capture current scope:
```
PUSH 0
STORE counter
MAKE_FUNCTION () .increment
RETURN

.increment:
  LOAD counter          ; Captured variable
  PUSH 1
  ADD
  STORE counter
  LOAD counter
  RETURN
```

### Tail Recursion
Use TAIL_CALL instead of CALL for last call:
```
MAKE_FUNCTION (n acc) .factorial
STORE factorial
<...>

.factorial:
  LOAD n
  PUSH 0
  LTE
  JUMP_IF_FALSE .recurse
  LOAD acc
  RETURN
.recurse:
  LOAD factorial
  LOAD n
  PUSH 1
  SUB
  LOAD n
  LOAD acc
  MUL
  PUSH 2
  PUSH 0
  TAIL_CALL             ; Reuses stack frame
```

## Key Concepts

### Truthiness
Only `null` and `false` are falsy. Everything else (including `0`, `""`, empty arrays/dicts) is truthy.

### Type Coercion

**toNumber**:
- `number` → identity
- `string` → parseFloat (or 0 if invalid)
- `boolean` → 1 (true) or 0 (false)
- `null` → 0
- Others → 0

**toString**:
- `string` → identity
- `number` → string representation
- `boolean` → "true" or "false"
- `null` → "null"
- `function` → "<function>"
- `array` → "[item, item]"
- `dict` → "{key: value, ...}"

**Arithmetic ops** (ADD, SUB, MUL, DIV, MOD) coerce both operands to numbers.

**Comparison ops** (LT, GT, LTE, GTE) coerce both operands to numbers.

**Equality ops** (EQ, NEQ) use type-aware comparison with deep equality for arrays/dicts.

**Note**: There is no string concatenation operator. ADD only works with numbers.

### Scope
- Variables resolved through parent scope chain
- STORE updates existing variable or creates in current scope
- Functions capture scope at definition time

### Identifiers
Variable and function parameter names support Unicode and emoji:
- Valid: `💎`, `🌟`, `変数`, `counter`, `_private`
- Invalid: Cannot start with digits, `.`, `#`, `@`, or `...`
- Invalid: Cannot contain whitespace or special chars: `;`, `()`, `[]`, `{}`, `=`, `'`, `"`

### Break Semantics
- CALL marks current frame as break target
- BREAK unwinds call stack to that target
- Used for Ruby-style iterator pattern

### Parameter Binding Priority
For function calls, parameters bound in order:
1. Positional argument (if provided)
2. Named argument (if provided and matches param name)
3. Default value (if defined)
4. Null

### Exception Handlers
- PUSH_TRY uses absolute addresses for catch blocks
- Nested try blocks form a stack
- THROW unwinds to most recent handler and jumps to finally (if present) or catch
- VM does NOT automatically jump to finally on success - compiler must generate JUMPs
- Finally execution in all cases is compiler's responsibility, not VM's

### Calling Convention
All calls push arguments in order:
1. Function
2. Positional args (in order)
3. Named args (key1, val1, key2, val2, ...)
4. Positional count (as number)
5. Named count (as number)
6. CALL or TAIL_CALL

### CALL_NATIVE Behavior
Unlike CALL, CALL_NATIVE consumes the **entire stack** as arguments and clears the stack. The native function receives all values that were on the stack at the time of the call.

### Empty Stack
- RETURN with empty stack returns null
- HALT with empty stack returns null