# Array Expression

An array expression represents an ordered sequence of values.

```dcdl
[1, 2, 3]
["a", "b", "c"]
```

An array literal is a concrete value and does not require all elements to have the same type.

```dcdl
["api", 8080, true]
```

## Array constraint

Write an array constraint by placing an ellipsis immediately after `[`, followed by an element constraint.

```dcdl
Names = [...String];
Ports = [...(Int & >= 1 & <= 65535)];
```

`[...T]` represents an array of zero or more elements, all of which satisfy `T`.
An empty array satisfies every array constraint.

```dcdl
[...String] & []
[...String] & ["api", "worker"]
```

The element constraint can be an object range.
A field with a default that exists only on the right remains abstract in every element when the array constraint is composed.
Its default is selected only if the array is later materialized.

```dcdl
Services = [...{
    name = String;
    enabled = Bool default true;
}];

Services & [{ name = "api"; }]
```

Applying an array constraint to a concrete array refines each element against the element range using the same rules as `as`.
For an object element range, a field present only on the left produces an error, while a field present only on the right remains abstract.

Decodal does not provide an abstract array type without an element constraint.
The former `Array` primitive type is unavailable, and `T` is required in `[...T]`.
`[String]` is not an array constraint; it is a concrete one-element array containing the unresolved `String` constraint.
An array constraint describes only the element range. It has no length, positional tuple, or uniqueness constraints.

## Array concatenation

`++` concatenates two concrete arrays.

```dcdl
base = ["read", "write"];
extra = ["admin"];
roles = base ++ extra;
```

`roles` has the same value as:

```dcdl
["read", "write", "admin"]
```

`++` does not transform array elements.
Elements from the right side follow those from the left.