Variables

The primary way to declare new variables is with the let statement.

let x = 4; // declare the variable `x` with an initial value of `4`
x // 4
x = 6; // assign a new value, `6`, to `x`
x // 6

You can also declare a new variable with no initial value.

let x;
// `x` has no value!
x = 5; // assign `5` to `x`
x // 5

(Note that, in more advanced uses of the let statement, declaring a variable with an initializer is not always equivalent to declaring it no initializer and then assigning to it.)

Using a variable with no value is an error.

let x;
x // error!

All variables have a type, which is inferred by default.

let x = "abc"; // x: String
x = 5; // error!

The type of a variable can also be explicitly specified with a type annotation:

let x: String = "abc";
let y: String = 5; // error!

Shadowing

Declaring a variable with the same name as a variable already in scope shadows the previous variable, making it inaccessible for the duration of the scope.

let x = 1;
x // 1
let x = "abc";
x // "abc"

This is different from reassigning the variable in a number of ways; for one, the new variable can have a different type (like in the above example).

Variables shadowed within a block will no longer be shadowed after the end of the block.

let x = 1;
x // 1
if x > 0 {
  let x = "abc";
  x // "abc"
}
x // 1