您如何在TypeScript中模拟名义键入？ [英] How do you emulate nominal typing in TypeScript?

问题描述

我的项目中有很多函数，这些函数都以数字作为参数。此数字的一半时间是数组的索引，另一半时间是光标位置（数组中两个条目之间的点）。即使使用命名约定，这也会引起混乱。

I have many functions in my project that take an number as parameter; half the time this number is an index into an array, the other half of the time, it is a cursor position (a point between two entries in an array). This causes confusion, even with naming conventions.

我要强制以下功能采用预期的名义类型。

I would like to enforce that the functions below are taking the intended nominal types.

class Index extends Number {}
class CursorPosition extends Number {}

function getElement(i: Index) {}
function getRange(p1: CursorPosition, p2: CursorPosition) {}

const myIndex: Index = 6;
const myPosition: CursorPosition = 6;

getElement(1); // would like this to fail at compile time
getRange(2, 3); // would like this to fail at compile time
getElement(myPosition); // would like this to fail at compile time
getRange(myIndex, myIndex); // would like this to fail at compile time

getElement(myIndex); // would like this to pass at compile time
getRange(myPosition, myPosition); // would like this to pass at compile time

我知道打字稿使用结构化打字，这是为什么没有开箱即用呢？

I understand that typescript uses structural typing, and this is why this does no occur "out of the box".

此外，我也考虑过将变量装箱并添加仲裁属性：

Also, I have considered both boxing my variables and adding a arbitray propery:

class myNum extends Number { 
  l: "1";
}

或使用强制转换。

class myNum { 
  arb: "arbitrary property value";
}

const mn2: myNum = <any>8;

function getElement2(a: any[], i: myNum) {
  return a[<any>i];
}

getElement2([], mn2);
getElement2([], 6);

还有更好的主意吗？

推荐答案

您可以使用品牌类型：

type Index =  Number & { __type: 'Index'}
type CursorPosition =  Number & { __type: 'CursorPosition'}


function getElement(i: Index) {}
function getRange(p1: CursorPosition, p2: CursorPosition) {}

function indexFromNumber(n: number) :Index {
    return n as any;
}

function cursorPositionFromNumber(n: number): CursorPosition {
    return n as any;
}

const myIndex: Index = indexFromNumber(6);
const myPosition: CursorPosition = cursorPositionFromNumber(6);

getElement(1); // error
getRange(2, 3);  // error
getElement(myPosition);  // error
getRange(myIndex, myIndex); // error

getElement(myIndex); // ok 
getRange(myPosition, myPosition); //ok 

您需要定义一个辅助函数来创建类型实例或使用类型断言（ const myIndex = 1等于索引），但是如果您传递一个简单的数字，则呼叫站点将给出错误。

You need to define a helper function to create instances of the type or use type assertions ( const myIndex = 1 as any as Index), but the call site will give errors if you pass a simple number.

此文章还有更多内容关于该主题的讨论。打字稿编译器还将这种方法用于路径

This article has a bit more of a discussion on the topic. Also the typescript compiler uses this approach for paths

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