hack枚举介绍

2018-11-08 17:22 更新

如果您熟悉C#,C ++或Java等语言中的枚举(enumerations),那么Hack枚举将让您感到宾至如归。枚举封装了一组相关的常量,与使用全局常量或类常量不同。枚举实际上创建了一个新的类型,可以通过名称进行注释。

注意:在这一点上,hack只支持int和string枚举。

语法

枚举的语法相对简单,与其他支持枚举的语言类似。

enum <NameOfEnum>: <string | int> {
  <Member1> = <value>;
  <Member2> = <value>;
  :
  :
  <MemberN> = <value>;
} 

枚举的名称必须是名称空间唯一的,并且枚举的成员也必须唯一命名。

<?hh

namespace Hack\UserDocumentation\Enums\Intro\Examples\Simple;

enum Size: int {
  SMALL = 0;
  MEDIUM = 1;
  LARGE = 2;
  X_LARGE = 3;
}

Output

枚举成员的值

枚举成员的值必须与枚举的类型相匹配。如果你的枚举被标记为一个int枚举,那么成员值必须是int。而且,这些值必须能够被静态评估。换句话说,没有变量等可以用作枚举成员值的一部分。

要访问成员值,可以使用类常量类似的语法

<NameOfEnum> :: <MemberN>
<?hh

namespace Hack\UserDocumentation\Enums\Intro\Examples\MemberValues;

enum Size: string {
  SMALL = "small" ;
  MEDIUM = "medium";
  LARGE = "large";
  X_LARGE = "x-large";
}

function say_all_sizes(): void {
  echo Size::SMALL . PHP_EOL . Size::MEDIUM . PHP_EOL .
       Size::LARGE . PHP_EOL . Size::X_LARGE;
}

say_all_sizes();

Output

small
medium
large
x-large

注意:枚举成员没有顺序的隐式值的概念。例如,如果将第一个成员设置为0,则默认情况下,下一个成员的值为1。每个枚举成员必须有一个明确的值分配给它。

铸造到底层类型

枚举有底层的类型,但它们是不可互换的。枚举是一个独特的类型。和其他任何类型一样,例如,你不能int把一个函数传递给一个枚举,或者一个枚举值给一个函数string。

但是,您可以使用简单的转换来将枚举的成员值转换为其基础类型,并且可以使用Hack的特殊枚举函数 assert()coerce()从基础类型转换为枚举类型。

注:您可以使用一些语言结构,echo而不需要转换为基础类型。

<?hh

namespace Hack\UserDocumentation\Enums\Intro\Examples\Casting;

enum Size: string {
  SMALL = "small" ;
  MEDIUM = "medium";
  LARGE = "large";
  X_LARGE = "x-large";
}

function say_greeting_with_size(string $size): void {
  echo "Hello. We have size " . $size . PHP_EOL;
}

function give_shirt(Size $size): void {
  echo "Here is your shirt of size " . $size . PHP_EOL;
}

function sales(): void {
  // We need a cast here because say_greeting_with_size is expecting a string
  // and it is incompatible with the value of an enum (even though it is a
  // string underlying).
  say_greeting_with_size((string) Size::SMALL);
  // To go from the underlying type to the enum type, Hack provides builtin
  // enum functions to help you. The assert function basically tells the
  // typechecker that you are NOT going to give it a value that doesn't exist
  // in the enum.
  give_shirt(Size::assert("small"));
}

sales();

Output

Hello. We have size small
Here is your shirt of size small

隐式转换

通过使用as约束运算符,可以使其转换为隐式的基础类型。

<?hh

namespace Hack\UserDocumentation\Enums\Intro\Examples\ImplicitCasting;

// We are now telling the typechecker that this enum is constrained to the
// underlying string type, and that implicit casts should be allowed.
enum Size : string as string {
  SMALL = "small" ;
  MEDIUM = "medium";
  LARGE = "large";
  X_LARGE = "x-large";
}

function say_greeting_with_size(string $size): void {
  echo "Hello. We have size " . $size . PHP_EOL;
}

function give_shirt(Size $size): void {
  echo "Here is your shirt of size " . $size . PHP_EOL;
}

function sales(): void {
  // Implict casting at work
  say_greeting_with_size(Size::SMALL);
  // No implicit casting this way though.
  // To go from the underlying type to the enum type, Hack provides builtin
  // enum functions to help you. The assert function basically tells the
  // typechecker that you are NOT going to give it a value that doesn't exist
  // in the enum.
  give_shirt(Size::assert("small"));
}

sales();

Output

Hello. We have size small
Here is your shirt of size small
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