infinite-lists.js

class List {
  static fromArray(arr) {
    if (arr.length == 0) {
      return new EmptyList()
    }
    return new FullList(
      arr[0],
      () => List.fromArray(arr.slice(1))
    )
  }
  forEach(f) {
    if (this instanceof EmptyList) {
      return
    }
    f(this.head)
    this.tail().forEach(f)
  }
  take(n) {
    if (n == 0) {
      return new EmptyList()
    }
    if (this instanceof EmptyList) {
      return this
    }
    return new FullList(this.head, () => this.tail().take(n - 1))
  }
  print() {
    this.forEach(x => console.log(x))
  }
  map(f) {
    if (this instanceof EmptyList) {
      return this
    }
    return new FullList(f(this.head), () => this.tail().map(f))
  }
  append(x) {
    if (this instanceof EmptyList) {
      return new FullList(x, () => new EmptyList())
    }
    return new FullList(this.head, () => this.tail().append(x))
  }
  filter(f) {
    if (this instanceof EmptyList) {
      return new EmptyList()
    }
    if (!f(this.head)) {
      return this.tail()
    }
    return new FullList(this.head, () => this.tail().filter(f))
  }
  length() {
    return this.reduce((a, _) => a + 1, 0)
  }
  sum() {
    return this.reduce((a, x) => a + x, 0)
  }
  reduce(f, a) { // this is [b], f is (a, b) -> a
    if (this instanceof EmptyList) {
      return a
    }
    return this.tail().reduce(f, f(a, this.head))
  }
}

class FullList extends List {
  constructor(head, tail) {
    super()
    this.head = head
    this.tail = tail
  }
}

class EmptyList extends List {}

function range(start, end) {
  if (start > end) {
    return new EmptyList()
  }
  return new FullList(start, () => range(start + 1, end))
}

const naturalNumbers = range(1, Math.infinity)
const evenNumbers = naturalNumbers.map(x => x * 2)
const evenNumbersDivisibleByThree = evenNumbers.filter(x => x % 3 == 0)

evenNumbersDivisibleByThree.take(5).print()