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
- It is immutable.
- Its value is
- 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;
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.
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 = "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.