Primitive Types

For simple values, Move has a number of built-in primitive types. They're the base that makes up all other types. The primitive types are:

• Booleans
• Unsigned Integers
• Address - covered in the next section

However, before we get to the types, let's first look at how to declare and assign variables in Move.

Variables and assignment

Variables are declared using the let keyword. They are immutable by default, but can be made mutable using the let mut keyword. The syntax for the let mut statement is:

let <variable_name>[: <type>]  = <expression>;
let mut <variable_name>[: <type>] = <expression>;

Where:

• <variable_name> - the name of the variable
• <type> - the type of the variable, optional
• <expression> - the value to be assigned to the variable

let x: bool = true;
let mut y: u8 = 42;

A mutable variable can be reassigned using the = operator.

y = 43;

Variables can also be shadowed by re-declaring.

let x: u8 = 42;
let x: u16 = 42;

Booleans

The bool type represents a boolean value - yes or no, true or false. It has two possible values: true and false which are keywords in Move. For booleans, there's no need to explicitly specify the type - the compiler can infer it from the value.

let x = true;
let y = false;

Booleans are often used to store flags and to control the flow of the program. Please, refer to the Control Flow section for more information.

Integer Types

Move supports unsigned integers of various sizes: from 8-bit to 256-bit. The integer types are:

• u8 - 8-bit
• u16 - 16-bit
• u32 - 32-bit
• u64 - 64-bit
• u128 - 128-bit
• u256 - 256-bit

let x: u8 = 42;
let y: u16 = 42;
// ...
let z: u256 = 42;

Unlike booleans, integer types need to be inferred. In most of the cases, the compiler will infer the type from the value, usually defaulting to u64. However, sometimes the compiler is unable to infer the type and will require an explicit type annotation. It can either be provided during assignment or by using a type suffix.

// Both are equivalent
let x: u8 = 42;
let x = 42u8;

Operations

Move supports the standard arithmetic operations for integers: addition, subtraction, multiplication, division, and remainder. The syntax for these operations is:

For more operations, including bitwise operations, please refer to the Move Reference.

The type of the operands must match, otherwise, the compiler will raise an error. The result of the operation will be of the same type as the operands. To perform operations on different types, the operands need to be cast to the same type.

Casting with as

Move supports explicit casting between integer types. The syntax for it is:

<expression> as <type>

Note, that it may require parentheses around the expression to prevent ambiguity.

let x: u8 = 42;
let y: u16 = x as u16;
let z = 2 * (x as u16); // ambiguous, requires parentheses

A more complex example, preventing overflow:

let x: u8 = 255;
let y: u8 = 255;
let z: u16 = (x as u16) + ((y as u16) * 2);

Overflow

Move does not support overflow / underflow, an operation that results in a value outside the range of the type will raise a runtime error. This is a safety feature to prevent unexpected behavior.

let x = 255u8;
let y = 1u8;

// This will raise an error
let z = x + y;