Tuples and Unit

Move does not fully support tuples as one might expect coming from another language with them as a first-class value. However, in order to support multiple return values, Move has tuple-like expressions. These expressions do not result in a concrete value at runtime (there are no tuples in the bytecode), and as a result they are very limited: they can only appear in expressions (usually in the return position for a function); they cannot be bound to local variables; they cannot be stored in structs; and tuple types cannot be used to instantiate generics.

Similarly, unit () is a type created by the Move source language in order to be expression based. The unit value () does not result in any runtime value. We can consider unit() to be an empty tuple, and any restrictions that apply to tuples also apply to unit.

It might feel weird to have tuples in the language at all given these restrictions. But one of the most common use cases for tuples in other languages is for functions to allow functions to return multiple values. Some languages work around this by forcing the users to write structs that contain the multiple return values. However, in Move, you cannot put references inside of structs. This required Move to support multiple return values. These multiple return values are all pushed on the stack at the bytecode level. At the source level, these multiple return values are represented using tuples.

Literals

Tuples are created by a comma separated list of expressions inside of parentheses.

Note that (e) does not have type (e): (t), in other words there is no tuple with one element. If there is only a single element inside the parentheses, the parentheses are only used for disambiguation and do not carry any other special meaning.

Sometimes, tuples with two elements are called “pairs” and tuples with three elements are called “triples.”

Examples

module 0x42::example {
  // all 3 of these functions are equivalent
 
  // when no return type is provided, it is assumed to be `()`
  fun returns_unit_1() { }
 
  // there is an implicit () value in empty expression blocks
  fun returns_unit_2(): () { }
 
  // explicit version of `returns_unit_1` and `returns_unit_2`
  fun returns_unit_3(): () { () }
 
 
  fun returns_3_values(): (u64, bool, address) {
    (0, false, @0x42)
  }
  fun returns_4_values(x: &u64): (&u64, u8, u128, vector<u8>) {
    (x, 0, 1, b"foobar")
  }
}

Operations

The only operation that can be done on tuples currently is destructuring.

Destructuring

For tuples of any size, they can be destructured in either a let binding or in an assignment.

For example:

module 0x42::example {
  // all 3 of these functions are equivalent
  fun returns_unit() {}
  fun returns_2_values(): (bool, bool) { (true, false) }
  fun returns_4_values(x: &u64): (&u64, u8, u128, vector<u8>) { (x, 0, 1, b"foobar") }
 
  fun examples(cond: bool) {
    let () = ();
    let (x, y): (u8, u64) = (0, 1);
    let (a, b, c, d) = (@0x0, 0, false, b"");
 
    () = ();
    (x, y) = if (cond) (1, 2) else (3, 4);
    (a, b, c, d) = (@0x1, 1, true, b"1");
  }
 
  fun examples_with_function_calls() {
    let () = returns_unit();
    let (x, y): (bool, bool) = returns_2_values();
    let (a, b, c, d) = returns_4_values(&0);
 
    () = returns_unit();
    (x, y) = returns_2_values();
    (a, b, c, d) = returns_4_values(&1);
  }
}

For more details, see Move Variables.

Subtyping

Along with references, tuples are the only other type that have subtyping in Move. Tuples have subtyping only in the sense that they subtype with references (in a covariant way).

For example:

script {
  fun example() {
    let x: &u64 = &0;
    let y: &mut u64 = &mut 1;
 
    // (&u64, &mut u64) is a subtype of (&u64, &u64)
    // since &mut u64 is a subtype of &u64
    let (a, b): (&u64, &u64) = (x, y);
 
    // (&mut u64, &mut u64) is a subtype of (&u64, &u64)
    // since &mut u64 is a subtype of &u64
    let (c, d): (&u64, &u64) = (y, y);
 
    // error! (&u64, &mut u64) is NOT a subtype of (&mut u64, &mut u64)
    // since &u64 is NOT a subtype of &mut u64
    let (e, f): (&mut u64, &mut u64) = (x, y);
  }
}

Ownership

As mentioned above, tuple values don’t really exist at runtime. And currently they cannot be stored into local variables because of this (but it is likely that this feature will come soon). As such, tuples can only be moved currently, as copying them would require putting them into a local variable first.