Variables in Sway are immutable by default. This means that, by default, once a variable is declared, its value cannot change. This is one of the ways how Sway encourages safe programming, and many modern languages have this same default. Let's take a look at variables in detail.

Declaring a Variable

Let's look at a variable declaration:

let foo = 5;

Great! We have just declared a variable, foo. What do we know about foo?

  1. It is immutable.
  2. Its value is 5.
  3. Its type is u64, a 64-bit unsigned integer.

u64 is the default numeric type, and represents a 64-bit unsigned integer. See the section Built-in Types for more details.

We can also make a mutable variable. Let's take a look:

let mut foo = 5;
foo = 6;

Now, foo is mutable, and the reassignment to the number 6 is valid. That is, we are allowed to mutate the variable foo to change its value.

Type annotations

A variable declaration can contain a type annotation. A type annotation serves the purpose of declaring the type, in addition to the value, of a variable. Let's take a look:

let foo: u32 = 5;

We have just declared the type of the variable foo as a u32, which is an unsigned 32-bit integer. Let's take a look at a few other type annotations:

let bar: str[4] = "sway";
let baz: bool = true;

If the value declared cannot be assigned to the declared type, there will be an error generated by the compiler.