# params

## Category
Compiler Construct

## Description
`params` is a **compiler-level construct** (not a runtime-registered command) that destructures an array into named local variables. It is the primary way to unpack function arguments in SQ#.

Supports:
- Simple variable binding
- Default values for missing elements
- Type-checked binding

## How It Works
The compiler inlines `params` into `_this select N` expressions with optional type checks at compile time. There is no runtime `params` command — it is entirely resolved during compilation.

## Usage

### Basic destructuring
```sqf
_arr = [42, "hello", true];
_arr params ["_num", "_str", "_flag"];
// _num = 42, _str = "hello", _flag = true
```

### With defaults
```sqf
_arr = [42];
_arr params ["_num", ["_opt", 99]];
// _num = 42, _opt = 99 (default used since _arr has < 2 elements)

_arr = [42, 77];
_arr params ["_num", ["_opt", 99]];
// _num = 42, _opt = 77 (default NOT used — value exists)
```

### Type-checked
```sqf
_arr = [42, "player", true];
_arr params [["_num", "number"], ["_name", "string"], ["_alive", "boolean"]];
// Compiler generates type assertions — throws if types don't match
```

### In function context
```sqf
_myFunc = {
    params ["_a", "_b", ["_c", 0]];
    _a + _b + _c
};

_result = [10, 20] call _myFunc;     // 30 (_c defaults to 0)
_result = [10, 20, 5] call _myFunc;  // 35
```

### With _this in spawn/call
```sqf
[100, "player"] spawn {
    params ["_hp", "_name"];
    systemChat f"{_name} HP: {_hp}";
};
```

## Type Check Options
| Syntax | Behavior |
|---|---|
| `"_var"` | Bind variable, no type check |
| `["_var", defaultValue]` | Bind with fallback |
| `["_var", "type"]` | Bind with type assertion |
| `["_var", defaultValue, "type"]` | Bind with fallback AND type check |

## Thread Safety
Compile-time only — no runtime overhead. Safe everywhere.

## See Also
- [call](call.md) — execute code with arguments
- [spawn](spawn.md) — async with arguments
- [typeName](typeName.md) — runtime type checking
- [select](select.md) — manual array element access