尝试实现通用规范和访问者模式时，类型不满足约束并错误地扩展了接口

Question

我正在尝试同时实现通用规范模式和通用访问者模式。这是我的基本接口。

export interface Specification<T, TVisitor extends SpecificationVisitor<TVisitor, T>> {
  accept(visitor: TVisitor): void;
  isSatisfiedBy(candidate: T): boolean;
  and(other: Specification<T, TVisitor>): Specification<T, TVisitor>;
  andNot(other: Specification<T, TVisitor>): Specification<T, TVisitor>;
  or(other: Specification<T, TVisitor>): Specification<T, TVisitor>;
  orNot(other: Specification<T, TVisitor>): Specification<T, TVisitor>;
  not(): Specification<T, TVisitor>;
}

export interface SpecificationVisitor<TVisitor extends SpecificationVisitor<TVisitor, T>, T> {
  visit(specification: AndSpecification<T, TVisitor>): void;
  visit(specification: AndNotSpecification<T, TVisitor>): void;
  visit(specification: OrSpecification<T, TVisitor>): void;
  visit(specification: OrNotSpecification<T, TVisitor>): void;
  visit(specification: NotSpecification<T, TVisitor>): void;
}

为方便起见，我为基本布尔运算符实现了一些基础 classes 和抽象 class。

export abstract class CompositeSpecification<T, TVisitor extends SpecificationVisitor<TVisitor, T>> implements Specification<T, TVisitor> {
  abstract isSatisfiedBy(candidate: T): boolean;
  abstract accept(visitor: TVisitor): void;

  and(other: Specification<T, TVisitor>): Specification<T, TVisitor> {
    return new AndSpecification<T, TVisitor>(this, other);
  }
  andNot(other: Specification<T, TVisitor>): Specification<T, TVisitor> {
    return new AndNotSpecification<T, TVisitor>(this, other);
  }
  or(other: Specification<T, TVisitor>): Specification<T, TVisitor> {
    return new OrSpecification<T, TVisitor>(this, other);
  }
  orNot(other: Specification<T, TVisitor>): Specification<T, TVisitor> {
    return new OrNotSpecification<T, TVisitor>(this, other);
  }
  not(): Specification<T, TVisitor> {
    return new NotSpecification<T, TVisitor>(this);
  }
}

export class AndSpecification<T, TVisitor extends SpecificationVisitor<TVisitor, T>> extends CompositeSpecification<
  T,
  TVisitor
> {
  constructor(readonly left: Specification<T, TVisitor>, readonly right: Specification<T, TVisitor>) {
    super();
  }

  accept(visitor: TVisitor): void {
    visitor.visit(this);
  }

  isSatisfiedBy(candidate: T): boolean {
    return this.left.isSatisfiedBy(candidate) && this.right.isSatisfiedBy(candidate);
  }
}

export class AndNotSpecification<T, TVisitor extends SpecificationVisitor<TVisitor, T>> extends CompositeSpecification<T, TVisitor> {
  constructor(readonly left: Specification<T, TVisitor>, readonly right: Specification<T, TVisitor>) {
    super();
  }

  accept(visitor: TVisitor): void {
    visitor.visit(this);
  }

  isSatisfiedBy(candidate: T): boolean {
    return this.left.isSatisfiedBy(candidate) && !this.right.isSatisfiedBy(candidate);
  }
}

export class OrSpecification<T, TVisitor extends SpecificationVisitor<TVisitor, T>> extends CompositeSpecification<
  T,
  TVisitor
> {
  constructor(readonly left: Specification<T, TVisitor>, readonly right: Specification<T, TVisitor>) {
    super();
  }

  accept(visitor: TVisitor): void {
    visitor.visit(this);
  }

  isSatisfiedBy(candidate: T): boolean {
    return this.left.isSatisfiedBy(candidate) || this.right.isSatisfiedBy(candidate);
  }
}

export class OrNotSpecification<T, TVisitor extends SpecificationVisitor<TVisitor, T>> extends CompositeSpecification<
  T,
  TVisitor
> {
  constructor(readonly left: Specification<T, TVisitor>, readonly right: Specification<T, TVisitor>) {
    super();
  }

  accept(visitor: TVisitor): void {
    visitor.visit(this);
  }

  isSatisfiedBy(candidate: T): boolean {
    return this.left.isSatisfiedBy(candidate) || !this.right.isSatisfiedBy(candidate);
  }
}

export class NotSpecification<T, TVisitor extends SpecificationVisitor<TVisitor, T>> extends CompositeSpecification<
  T,
  TVisitor
> {
  constructor(readonly other: Specification<T, TVisitor>) {
    super();
  }

  accept(visitor: TVisitor): void {
    visitor.visit(this);
  }

  isSatisfiedBy(candidate: T): boolean {
    return !this.other.isSatisfiedBy(candidate);
  }
}

以上所有工作和编译都没有错误。但是，当我运行尝试创建一个扩展基本 SpecificationVisitor 接口的接口并实现一个扩展抽象 CompositeSpecification 的 class 时，编译器出现问题。

export interface NumberComparatorVisitor extends SpecificationVisitor<NumberComparatorVisitor, number> {
  visit(specification: GreaterThan): void;
}

export class GreaterThan extends CompositeSpecification<number, NumberComparatorVisitor> {
  constructor(readonly value: number) {
    super();
  }

  accept(visitor: NumberComparatorVisitor): void {
    visitor.visit(this);
  }

  isSatisfiedBy(candidate: number): boolean {
    return candidate > this.value;
  }
}

我收到以下错误：

Type 'NumberComparatorVisitor' does not satisfy the constraint 'SpecificationVisitor<NumberComparatorVisitor, number>'.ts(2344)

Interface 'NumberComparatorVisitor' incorrectly extends interface 'SpecificationVisitor<NumberComparatorVisitor, number>'.
  Types of property 'visit' are incompatible.
    Type '(specification: GreaterThan) => void' is not assignable to type '{ (specification: AndSpecification<number, NumberComparatorVisitor>): void; (specification: AndNotSpecification<number, NumberComparatorVisitor>): void; (specification: OrSpecification<...>): void; (specification: OrNotSpecification<...>): void; (specification: NotSpecification<...>): void; }'.
      Types of parameters 'specification' and 'specification' are incompatible.
        Type 'AndSpecification<number, NumberComparatorVisitor>' is not assignable to type 'GreaterThan'.ts(2430)

Type 'NumberComparatorVisitor' does not satisfy the constraint 'SpecificationVisitor<NumberComparatorVisitor, number>'.
  Types of property 'visit' are incompatible.
    Type '(specification: GreaterThan) => void' is not assignable to type '{ (specification: AndSpecification<number, NumberComparatorVisitor>): void; (specification: AndNotSpecification<number, NumberComparatorVisitor>): void; (specification: OrSpecification<...>): void; (specification: OrNotSpecification<...>): void; (specification: NotSpecification<...>): void; }'.
      Types of parameters 'specification' and 'specification' are incompatible.
        Property 'value' is missing in type 'AndSpecification<number, NumberComparatorVisitor>' but required in type 'GreaterThan'.ts(2344)

我不太明白它为什么抱怨。我需要更改什么才能使它按我预期的方式工作？

Answer 1

哇，里面有很多代码。让我们将其缩减为显示相同问题的 minimal reproducible example：

interface Foo {
  ovld(x: string): number;
  ovld(x: number): boolean;
}

interface BadBar extends Foo {  // error!
  ovld(x: boolean): string;
}

这里有一个 Foo 接口和一个名为 ovld 的 overloaded 方法。此方法有两个调用签名；一个需要 string，另一个需要 number。我们尝试创建扩展它的 BadBar 接口。我们的目的是向 ovld 添加第三个重载，它接受 boolean。但它不起作用！为什么？

答案是您不能在扩展接口时简单地添加重载。接口扩展是不是interface merging。通过在扩展接口中重新声明 ovld，您告诉编译器用 BadBar 中的版本完全覆盖 Foo 中 ovld 的类型。并且因为 BadBar 中的单个调用签名不能分配给 Foo 中的调用签名，这是一个错误，就像这是一个错误：

interface XXX {
  prop: string;
}
interface YYY extends XXX { // error!
  prop: boolean; 
}

在这两种情况下，您都在通过采用现有的属性或方法并进行无效更改来错误地扩展接口。唯一可接受的更改是缩小更改，例如将 string 更改为 "a" | "b"。

那么，如果我们不能仅通过在 BadBar 中重新声明 ovld() 来添加重载，我们如何做到这一点？一种方法是将 indexed access type to get the existing call signatures from the parent interface, and then intersect 与新的调用签名一起使用：

interface GoodBar extends Foo {
  ovld: ((x: boolean) => string) & Foo["ovld"]
} 
/* (property) GoodBar.ovld: ((x: boolean) => string) & {
  (x: string): number;
  (x: number): boolean;
} */

交集 X & Y 将被视为 X 的有效缩小，因此编译器错误消失。此外，TypeScript 认为函数的交集等同于重载。因此，上面为您提供了新的 (x: boolean) => string 调用签名作为第一个重载，然后是来自 Foo:

的现有两个重载签名

declare const goodBar: GoodBar;
goodBar.ovld(false).toUpperCase();
goodBar.ovld("hello").toFixed();
goodBar.ovld(123) === true;

万岁！

这意味着您的 NumberComparatorVisitor 可能需要更改为如下内容：

export interface NumberComparatorVisitor extends
  SpecificationVisitor<NumberComparatorVisitor, number> {
  visit: (
    ((specification: GreaterThan) => void) &
    SpecificationVisitor<NumberComparatorVisitor, number>["visit"]
  );
}

如果我进行更改，错误就会消失，因为现在 NumberComparatorVisitor 确实是其父接口的有效扩展。

Playground link to code

尝试实现通用规范和访问者模式时，类型不满足约束并错误地扩展了接口

Type does not satisfy the constraint and incorrectly extends interface when trying to implement generic Specification and Visitor patterns

typescript

generics

types

visitor-pattern

specification-pattern