TypeScript Traits Concept

traits.ts

/********************************************
 * Base trait system
 *
 * Author: Torben Köhn <t.koehn@outlook.com>
 * License: MIT
 *
 *******************************************/

/**
 * Symbol to keep implemented trait references on the instance.
 *
 * Required for instanceof checks.
 */
const TraitSymbol = Symbol('traits')

/**
 * A class that uses traits.
 */
export type TraitConsumer = { readonly [TraitSymbol]: Trait[] }

export const isTraitConsumer = (value: unknown): value is TraitConsumer => {
  return (
    typeof value === 'object' &&
    value !== null &&
    TraitSymbol in value &&
    Array.isArray((value as TraitConsumer)[TraitSymbol])
  )
}

/**
 * A trait is a class that can be mixed into other classes to provide additional functionality.
 *
 * See it as an interface with implementation.
 *
 * Traits MUST be abstract classes and cannot be instantiated directly.
 * Traits MUST NOT have a constructor with parameters.
 *
 * Abstract methods can be used and will be propagated to the consuming class properly.
 */
export abstract class Trait {
  /**
   * Parameterless standard constructor for traits.
   */
  constructor() {}

  /**
   * Allows instanceof checks for traits.
   *
   * @param instance The instance to check
   * @returns True if the instance implements the trait
   */
  static [Symbol.hasInstance](instance: unknown) {
    return (
      isTraitConsumer(instance) &&
      instance[TraitSymbol].some((traitInstance) => traitInstance.constructor === this)
    )
  }
}

/**
 * A constructor type for traits.
 *
 * Notice traits MUST be abstract classes. See {@link Trait}.
 */
export type TraitConstructor = abstract new () => Trait

/**
 * Type helper to convert a union type to an intersection type.
 *
 * It will turn `A | B | C` into `A & B & C`.
 */
type UnionToIntersection<U> = (U extends any ? (k: U) => void : never) extends (k: infer I) => void
  ? I
  : never

/**
 * Allows to create a class that implements one or more traits.
 * 
 * Arguments need to be {@link TraitConstructor} types.
 *
 * Example:
 * ```ts
 * class Clone<T> extends Trait {
 *   abstract clone(): T
 * }
 * 
 * class Vector2 extends traits(Clone<Vector2>) {
 *   constructor(
 *     public x: number = 0,
 *     public y: number = 0
 *   ) {
 *     super()
 *   }
 
 *   override clone(): Vector2 {
 *     return new Vector2(this.x, this.y)
 *   }
 * }
 * ```
 *
 * @param traits The trait constructors to implement
 * @returns A base class implementing the given traits
 */
export const traits = <const T extends readonly TraitConstructor[]>(...traits: T) => {
  return class implements TraitConsumer {
    readonly [TraitSymbol]: Trait[] = []

    constructor() {
      for (const CurrentTrait of traits) {
        const traitInstance = Reflect.construct(CurrentTrait, [])
        const properties = Object.getOwnPropertyDescriptors(Object.getPrototypeOf(traitInstance))
        for (const [key, descriptor] of Object.entries(properties)) {
          if (key === 'constructor') {
            continue
          }

          Object.defineProperty(this, key, descriptor)
        }
        this[TraitSymbol].push(traitInstance)
      }
    }
  } as {
    new (): UnionToIntersection<InstanceType<T[number]>>
  }
}

/***************************
 * Example traits and usage
 ***************************/

abstract class Serialize extends Trait {
  // Abstract methods work properly
  abstract serialize(): unknown
}

abstract class Deserialize extends Trait {
  abstract deserialize(data: unknown): void
}

abstract class Clone<T> extends Trait {
  abstract clone(): T
}

abstract class Stringify extends Trait {
  // Full implementations are no problem
  stringify(): string {
    return `[Object ${this.constructor.name}]`
  }
}

abstract class Print extends Trait {
  printId(): void {
    // Other traits can be checked via instanceof
    if (this instanceof Stringify) {
      console.log(`ID: stringify ${this.stringify()}`)
      return
    }

    console.log(`ID: toString ${this.toString()}`)
  }
}

// Using traits(A, B, C, ...) to "implement" traits
class Foo extends traits(Serialize, Deserialize, Clone<Foo>, Stringify, Print) {
  override serialize(): unknown {
    return {}
  }

  override deserialize(data: unknown): void {
    // no-op
  }

  override clone(): Foo {
    return new Foo()
  }
}

const foo = new Foo()
foo.printId() // ID: stringify [Object Foo]

console.log(`foo instanceof Serialize: ${foo instanceof Serialize}`) // true
console.log(`foo instanceof Deserialize: ${foo instanceof Deserialize}`) // true
console.log(`foo instanceof Clone: ${foo instanceof Clone}`) // true
console.log(`foo instanceof Print: ${foo instanceof Foo}`) // true

/***************************
 * Stateful example
 ***************************/

abstract class Counter extends Trait {
  // Fields/state is always propagated to the consuming class
  abstract count: number

  increment(): number {
    this.count++
    return this.count
  }

  decrement(): number {
    this.count--
    return this.count
  }
}

class StatefulFoo extends traits(Counter) {
  count = 0

  logAndIncrement(): void {
    console.log(`Current count: ${this.count}`)
    this.increment()
  }

  logAndDecrement(): void {
    console.log(`Current count: ${this.count}`)
    this.decrement()
  }
}

const a = new StatefulFoo()
a.logAndIncrement() // Current count: 0
a.logAndIncrement() // Current count: 1
a.logAndIncrement() // Current count: 2

const b = new StatefulFoo()
b.logAndDecrement() // Current count: 0
b.logAndDecrement() // Current count: -1
b.logAndDecrement() // Current count: -2

Coole implementierung, vor allem mit dem isinstance. Was ich noch hinzufügen würde, ist die Möglichkeit, traits zu extenden. Also z.B.

abstract class Print extends Stringify {
  override stringify(): string {
    return "only gets called if the final class does not override stringify"
  }

  printId(): void {
    console.log(`ID: stringify ${this.stringify()}`)
  }
}

Dazu die umgekehrte prototype chain

const prototypeChain = (obj: object): object[] => obj === null ? [] : [...prototypeChain(Object.getPrototypeOf(obj)), obj]

und dann in Zeile 55 stattdessen

instance[TraitSymbol].flatMap(prototypeChain).some((traitInstance) => traitInstance.constructor === this)

und in Zeile 111 dann die method resolution order implementieren

const properties: Readonly<Record<keyof any, PropertyDescriptor>> = Object.assign({}, ...prototypeChain(traitInstance).map(Object.getOwnPropertyDescriptors))

Außerdem ist es vielleicht sauberer so einen Typen hier zu benutzen

type Intersect<L extends readonly any[]> = L extends readonly [infer Head, ...infer Tail] ? Head & Intersect<Tail> : unknown

Um in Zeile 123 dann stattdessen

new (): Intersect<{[Index in keyof T]: InstanceType<T[Index]>}>

zu schreiben.

Um Trait-Requirements zu implementieren, könnte man ein Symbol RequiredTraitsSymbol hinzufügen und dem Typ trait einen static Member geben

static [RequiredTraitsSymbol] = [] as readonly TraitConstructor[]

dann ein Typ-Prädikat bauen

type RequirementsSatisfied<Requirements extends readonly TraitConstructor[], Existing> =
  Requirements extends readonly [infer Head, ...infer Tail]
    ? Head extends TraitConstructor & Existing
      ? RequirementsSatisfied<
            Tail extends readonly TraitConstructor[] ? Tail : never, 
            Existing
        >
      : false
    : true

type SatisfiesTraitOrder<Traits extends readonly TraitConstructor[], Existing = never> =
  Traits extends readonly [infer Head, ...infer Tail]
    ? Head extends TraitConstructor
      ? RequirementsSatisfied<
            Head[typeof RequiredTraitsSymbol] extends readonly TraitConstructor[] ? Head[typeof RequiredTraitsSymbol] : never, 
            Existing
        > extends true
        ? SatisfiesTraitOrder<
          Tail extends readonly TraitConstructor[] ? Tail : never,
          Existing | Head
        >
        : false
      : false
    : true

und dann in Zeile 122 - 124 stattdessen so was wie

  } as SatisfiesTraitOrder<T> extends true ? {
    new (): Intersect<{[Index in keyof T]: InstanceType<T[Index]>}>
  } : never

schreiben. Wenn man dann z.B.

abstract class Print extends Stringify {
  static [RequiredTraitsSymbol] = [Stringify] as const
  ...
}

hat und dann versucht, von trait(Print) zu extenden, bekommt man einen Type-Error. Bei trait(Stringify, Print) aber nicht. Man kann dann auch zur Runtime Reflection über die Abhängigkeiten machen, indem man einfach die RequiredTraitsSymbol-Property ausliest.

Ein Fix für das Typpredikat, damit subtypes von Requirements auch akzeptiert werden:

type RequirementSatisfied<Requirement extends TraitConstructor, Existing extends readonly TraitConstructor[]> =
  Existing extends readonly [infer Head, ...infer Tail]
    ? Head extends Requirement
      ? true
      : RequirementSatisfied<
            Requirement,
            Tail extends readonly TraitConstructor[] ? Tail : never
        >
    : false

type RequirementsSatisfied<Requirements extends readonly TraitConstructor[], Existing extends readonly TraitConstructor[]> =
  Requirements extends readonly [infer Head, ...infer Tail]
    ? Head extends TraitConstructor
      ? RequirementSatisfied<Head, Existing> extends true
        ? RequirementsSatisfied<
              Tail extends readonly TraitConstructor[] ? Tail : never, 
              Existing
          >
        : false
      : false
    : true

type SatisfiesTraitOrder<TraitsSuffix extends readonly TraitConstructor[], TraitsPrefix extends readonly TraitConstructor[] = []> =
  TraitsSuffix extends readonly [infer Head, ...infer Tail]
    ? Head extends TraitConstructor
      ? RequirementsSatisfied<
            Head[typeof RequiredTraitsSymbol] extends readonly TraitConstructor[] ? Head[typeof RequiredTraitsSymbol] : never, 
            TraitsPrefix
        > extends true
        ? SatisfiesTraitOrder<
          Tail extends readonly TraitConstructor[] ? Tail : never,
          readonly [...TraitsPrefix, Head]
        >
        : false
      : false
    : true

Damit kann man dann auch sowas hier machen

abstract class Print extends traits(Stringify, Serialize) {
  static [RequiredTraitsSymbol] = [Stringify, Serialize] as const
  printId(): void {
    console.log(`ID: stringify ${this.stringify()} and ${this.serialize()}`)
  }
}