# Enums: plain and tagged

## Plain enums

A plain enum is a C-like set of named constants. It lowers to `i32` and is `Copy`:

```cplus
enum Color { Red, Green, Blue }

let c = Color::Red;
```

## Tagged enums (sum types)

A tagged enum carries data with each variant. This is how C+ models "one of several shapes", and it is the foundation of [error handling](/docs/error-handling):

```cplus
enum Shape {
    Circle(f64),
    Rectangle(f64, f64),
    Square(f64),
}

let s = Shape::Circle(3.14);
```

## Generic enums

Enums can be generic over a type parameter, written with `[T]` (not `<T>`):

```cplus
enum Maybe[T] {
    Some(T),
    None,
}

let m: Maybe[i32] = Maybe[i32]::Some(7);
let n: Maybe[i32] = Maybe[i32]::None;
```

**Always write the type arguments at the source level**: `Option[i32]::Some(v)`, `Option[i32]::None`. Internal mangled names like `Option__i32` exist but are never something you type.

You consume an enum by [pattern matching](/docs/pattern-matching) on its variants.