Variables

Declaration

Variables can be declared in two different ways. First, there is the let-style declaration that looks like this:

When declaring a new variable in this way, the type annotation is required. To omit the type annotation, you can use the shorter declaration syntax, which looks like this:

In this case, the type of the variable will be inferred from the type of the initializer expression.

Mutability

By default, variables are declared as immutable, meaning their values cannot be changed after their initial declaration. However, variables can be declared as mutable via the mut keyword, which must precede the variable's name, no matter which declaration option is used. For example, a variable can be declared as mutable like so:

Initializers

Variables are not required to be declared with an initializer, in which case they are initially assigned their type's default value. When omitting a initializer, the variable must be declared with the longer let-style syntax. As an example, this variable declaration:

will result in a variable i whose initial value is 0. Other common default values include 0.0 for floating point numbers and "" for strings. The default value of compound types is composed of the default value for each of the compound type's attributes.