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.

常数

常量是为模块或脚本内的共享静态值命名的一种方式。

常量必须在编译时知道。常量的值存储在编译的模块或脚本中。每次使用该常量时，都会生成该值的新副本。

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

声明

常量声明以 const 关键字开头，后跟名称、类型和值。它们可以存在于脚本或模块中

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

For example

例如

script {

    const MY_ERROR_CODE: u64 = 0;

    fun main(input: u64) {
        assert!(input > 0, MY_ERROR_CODE);
    }

}

address 0x42 {
module 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.

命名

常量必须以大写字母 A 到 Z 开头。在第一个字母之后，常量名称可以包含下划线 _、字母 a 到 z、字母 A 到 Z 或数字 0 到 9。

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.

即使您可以在常数中使用字母 a 到 z。一般的风格准则是只使用大写字母 A 到 Z，每个单词之间用下划线 _。

这种以 A 到 Z 开头的命名限制是为了给未来的语言特性留出空间。以后可能会或可能不会删除它。

Visibility

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

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

有效表达式

目前，常量仅限于基本类型 bool、u8、u64、u128、address 和向量 u8。未来对其他向量值的支持（除了“字符串”样式的文字）将在稍后提供。

Values

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

值

通常，为 const 分配其类型的简单值或文字。例如

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.

复杂表达式

除了文字之外，常量还可以包含更复杂的表达式，只要编译器能够在编译时将表达式简化为一个值即可。

目前，可以使用相等运算、所有布尔运算、所有位运算和所有算术运算。

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 EQUAL: bool = 1 == 1;

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

如果操作会导致运行时异常，编译器将给出无法生成常量值的错误

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. 请注意，常量当前不能引用其他常量。将来会添加此功能以及对其他表达式的支持。

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