Constants

Constants are a way of giving a name to shared, static values inside of a module or script.

The constant’s must be known at compilation. The constant’s value is stored in the compiled module or script. And each time the constant is used, a new copy of that value is made.

Declaration

Constant declarations begin with the const keyword, followed by a name, a type, and a value. They can exist in either a script or module

const <name>: <type> = <expression>;

For example

script {
  const MY_ERROR_CODE: u64 = 0;
 
  fun main(input: u64) {
    assert!(input > 0, MY_ERROR_CODE);
  }
}
 
module 0x42::example {
  const MY_ADDRESS: address = @0x42;
 
  public fun permissioned(s: &signer) {
    assert!(std::signer::address_of(s) == MY_ADDRESS, 0);
  }
}

Naming

Constants must start with a capital letter A to Z. After the first letter, constant names can contain underscores _, letters a to z, letters A to Z, or digits 0 to 9.

script {
  const FLAG: bool = false;
  const MY_ERROR_CODE: u64 = 0;
  const ADDRESS_42: address = @0x42;
}

Even though you can use letters a to z in a constant. The general style guidelines are to use just uppercase letters A to Z, with underscores _ between each word.

This naming restriction of starting with A to Z is in place to give room for future language features. It may or may not be removed later.

Visibility

public constants are not currently supported. const values can be used only in the declaring module.

Valid Expressions

Currently, constants are limited to the primitive types bool, u8, u16, u32, u64, u128, u256, address, and vector<u8>. Future support for other vector values (besides the “string”-style literals) will come later.

Values

Commonly, consts are assigned a simple value, or literal, of their type. For example

script {
  const MY_BOOL: bool = false;
  const MY_ADDRESS: address = @0x70DD;
  const BYTES: vector<u8> = b"hello world";
  const HEX_BYTES: vector<u8> = x"DEADBEEF";
}

Complex Expressions

In addition to literals, constants can include more complex expressions, as long as the compiler is able to reduce the expression to a value at compile time.

Currently, equality operations, all boolean operations, all bitwise operations, and all arithmetic operations can be used.

script {
  const RULE: bool = true && false;
  const CAP: u64 = 10 * 100 + 1;
  const SHIFTY: u8 = {
    (1 << 1) * (1 << 2) * (1 << 3) * (1 << 4)
  };
  const HALF_MAX: u128 = 340282366920938463463374607431768211455 / 2;
  const REM: u256 = 57896044618658097711785492504343953926634992332820282019728792003956564819968 % 654321;
  const EQUAL: bool = 1 == 1;
}

If the operation results in a runtime exception, the compiler will give an error that it is unable to generate the constant’s value

script {
  const DIV_BY_ZERO: u64 = 1 / 0; // error!
  const SHIFT_BY_A_LOT: u64 = 1 << 100; // error!
  const NEGATIVE_U64: u64 = 0 - 1; // error!
}

Note that constants cannot currently refer to other constants. This feature, along with support for other expressions, will be added in the future.

Enum Types Generics