# Tuple and tuple indexing expressions

## Tuple expressions

> **Syntax**\
> _TupleExpression_ :\
>    `(` [_InnerAttribute_]\* _TupleElements_? `)`
>
> _TupleElements_ :\
>    ( [_Expression_] `,` )+ [_Expression_]?

Tuple expressions evaluate into [tuple values][tuple type] with the operands
initializing the elements of the tuple.

Tuple expressions are written by listing the [operands] in a parenthesized,
comma-separated list. 1-ary tuple expressions require a comma after their
operand to be disambiguated with a [parenthetical expression].

The number of operands is the arity of the constructed tuple. Tuple expressions
without operands produce the unit tuple. For other tuple expressions, the first
written operand initializes the 0th element and subsequent operands initializes
the next highest element. For example, in the tuple expression
`('a', 'b', 'c')`, `'a'` initializes the value of the 0th element, `'b'` the
1st, and `'c'` the 2nd.

Examples of tuple expressions:

| Expression           | Type         |
| -------------------- | ------------ |
| `()`                 | `()` (unit)  |
| `(0.0, 4.5)`         | `(f64, f64)` |
| `("x".to_string(), )` | `(String, )`  |
| `("a", 4usize, true)`| `(&'static str, usize, bool)` |

### Tuple expression attributes

[Inner attributes] are allowed directly after the opening parenthesis of a
tuple expression in the same expression contexts as [attributes on block
expressions].

## Tuple indexing expressions

> **Syntax**\
> _TupleIndexingExpression_ :\
>    [_Expression_] `.` [TUPLE_INDEX]

Tuple indexing expressions evaluate like [field access expressions], but access
elements of [tuples][tuple type] or [tuple structs].

Tuple index expressions are written as an operand, `.`, and a tuple index. The
index must be written as a [decimal literal] with no leading zeros, underscores,
or suffix. The operand must have the type of a tuple or tuple struct. If the
tuple index is not an element of the tuple or tuple struct, it is a compiler
error.

Examples of tuple indexing expressions:

```rust
let pair = ("a string", 2);
assert_eq!(pair.1, 2);

# struct Point(f32, f32);
let point = Point(1.0, 0.0);
assert_eq!(point.0, 1.0);
assert_eq!(point.1, 0.0);
```

> **Note**: Unlike field access expressions, tuple index expressions can be the
> function operand of a [call expression] as it cannot be confused with a
> method call since method names cannot be numbers.

[_Expression_]: ../expressions.md
[_InnerAttribute_]: ../attributes.md
[attributes on block expressions]: block-expr.md#attributes-on-block-expressions
[call expression]: ./call-expr.md
[decimal literal]: ../tokens.md#integer-literals
[field access expressions]: ./field-expr.html#field-access-expressions
[Inner attributes]: ../attributes.md
[operands]: ../expressions.md
[parenthetical expression]: grouped-expr.md
[tuple type]: ../types/tuple.md
[tuple structs]: ../types/struct.md
[TUPLE_INDEX]: ../tokens.md#tuple-index
