iterables_utilities.js

const compose = (fn, ...funcs) => {
  return (...args) => {
    return funcs.reduce((acc, func) => func(acc), fn(...args))
  }
}

const generate = (fn, dependency = LazyIterable, descriptor) => {
  return Object.create(
    Object.assign({ [Symbol.iterator]: fn }, dependency),
    descriptor
  )
}

const map = (fn, iterable) => {
  return generate(() => {
    const iterator = iterable[Symbol.iterator]()

    return {
      next: () => {
        const { done, value } = iterator.next()

        return ({ done, value: done ? undefined : fn(value) })
      }
    }
  })
}

const reduce = (fn, seed, iterable) => {
  const iterator = iterable[Symbol.iterator]()
  let iterationResult
  let accumulator = seed
  do {
    iterationResult = iterator.next()
    accumulator = iterationResult.done ? accumulator : fn(accumulator, iterationResult.value)
  } while (!iterationResult.done)
  return accumulator
}

const filter = (fn, iterable) => {
  return generate(() => {
    const iterator = iterable[Symbol.iterator]()

    return {
      next: () => {
        do {
          var { done, value } = iterator.next()
        } while (!done && !fn(value))
        return { done, value }
      }
    }
  })
}

const until = (fn, iterable) => {
  return generate(() => {
    const iterator = iterable[Symbol.iterator]()

    return {
      next: () => {
        let { done, value } = iterator.next()
        done = done || fn(value)
        return ({ done, value: done ? undefined : value })
      }
    }
  })
}

const first = (iterable) => {
  return iterable[Symbol.iterator]().next().value
}

const rest = (iterable) => {
  return generate(() => {
    const iterator = iterable[Symbol.iterator]()

    iterator.next()
    return iterator
  })
}

const take = (numberToTake, iterable) => {
  return generate(() => {
    const iterator = iterable[Symbol.iterator]()
    let remainingElements = numberToTake

    return {
      next: () => {
        let { done, value } = iterator.next()

        done = done || remainingElements-- <= 0

        return ({ done, value: done ? undefined : value })
      }
    }
  })
}

const getValue = function (iterable) {
  return Array.from(iterable)
}