Determine how lazy LazyList should be

[NthPortal]

Background

(quoted from #11105)

Currently, LazyList has two forms of laziness:

1. laziness in determining whether or not it is empty (and by extension, how many elements it has)
2. independent laziness of elements - that is, evaluating a subsequent head or tail of the LazyList does not cause the evaluation of previous heads.

Form 1 is one of the primary reasons for the creation of LazyList, is generally maintained throughout operations on it, and I think is is generally agreed to be of very high value.

Form 2 is less obviously valuable. Historically, it exists because the original implementation of LazyList had a strict state (empty or cons), and lazy head and tail (like Stream, but with a lazy head as well); when the design of LazyList was changed to have a lazy state and strict tail (because lazy tail doesn't add anything once the state is lazy), the lazy head was still kept.

During various discussions on various issues, it has been noted that the fact that LazyList's supports form 2 of laziness

• makes some implementations more complex (though not overly so)
• often adds no value, because the LazyList is created in a fashion (e.g. from an Iterator) that does not have form 2 of laziness
• is subverted by many common operations (such as filter, dropWhile) which by their nature must evaluate elements sequentially in order to determine the next element

This yields the question (quoted from @sjrd at scala/scala#6880 (comment)):

As added food for thought: I wonder whether it is still useful that the head be lazy. Are there use cases where one already knows that the lazy list is non-empty, but doesn't know yet what its head will be (and cannot synchronously compute it)? My experience from programming in Oz (where this lazier LazyList is basically the base data type used everywhere--and optimized by the VM) doesn't support any such use case. [...]

(see also #11089 (comment))

Issue and Investigation Areas

Should LazyList support form 2 of laziness by having a lazy head? There are a few points to think about in this regard:

1. How significant is the implementation cost of supporting form 2 of laziness? Keeping in mind that the implementation supporting form 2 of laziness already exists, but must be maintained.
2. How significant is the performance cost of having a lazy head?
3. Are there use cases where, despite the fact that the creation of the LazyList does not support form 2 of laziness, the fact that some methods (such as map) support form 2 of laziness is valuable because it allows performing independently lazy expensive operations?
4. Are there use cases for which form 2 of laziness is valuable in the creation of the LazyList (in contrast to what is discussed in the previous question)? I and several others have yet to come up with one.

I think that if we cannot come up any compelling use cases for form 2 of laziness, taking into account how frequently it is not present anyway, then LazyList should be changed to have a strict head and not support form 2 of laziness. I say this despite the fact that I have invested significant effort into writing comprehensive tests which ensure that all methods which can preserve/support form 2 of laziness do so. Once 2.13 is out, it will be difficult to change LazyList's behaviour even in subsequent releases of Scala because it would be a significant change in semantics (much like the way Stream cannot be a deprecated type alias for LazyList).

Consensus

Form 2 of laziness should be removed, and head made strict.

---

I don't know if this issue actually needs to be resolved by 2.13.0-RC1, but it definitely needs to be resolved by the final release of 2.13.0, and it would probably be better to resolve it at least a few release candidates beforehand.

[NthPortal]

Regarding point 1: I am happy to maintain LazyList for the foreseeable future, but that's obviously not a guarantee forever

[lrytz]

Thank you very much for writing this up! The design definitely needs to be finalized before RC1, which is scheduled for end of January.

It sounds to me like going for a strict head is a better choice for the standard library. The goal of the standard library is to find the "right" balance between simplicity, productivity, performance. Since we're not aware of a good use case for a lazy head right now, we should go for the simpler solution. If the need shows up, people can implement that collection outside the standard library, and if that really takes off we'll reconsider.

[NthPortal]

@texasbruce I don't understand what you mean - how does that give you form 2 of laziness with a strict head?

I also don't understand why it is inconsistent with other collection types per se - the whole point of it is to be lazy.

[NthPortal]

I am not suggesting that calling head somehow does not evaluate the element. I am saying that calling tail.head does not cause the evaluation of head

[NthPortal]

@texasbruce it is already implemented (see scala/scala#7355)

[sjrd]

I believe my opinion on this question is already clear to anyone who's read my various comments in the various issues, but I'll state it unambiguously here.

I think LazyList should not have a lazy head.

A lazy head clearly has more overhead and a higher implementation complexity, and as was mentioned we don't have any use case.

[NthPortal]

I believe I have come up with a use case (point 3).

You have a text file, and you read the lines into a LazyList (which does not have form 2 of laziness). You then perform an expensive analysis on each line of the file (via map).

val lines: LazyList[String] = ???
val analysed = lines map doExpensiveAnalysis

However, the result of the analysis may tell you that the following n lines are of little value to you (for whatever reason), so you want to drop them. With a lazy head, you can avoid the expensive analysis for the lines you are skipping using drop.

val filtered = {
  def next(elems: LazyList[Elem]): LazyList[Elem] =
    if (elems.isEmpty) elems
    else {
      val head = elems.head
      val skip = head.skip
      head #:: next(elems.drop(skip + 1))
    }
  next(analysed)
}

Without a lazy head, it is more difficult to avoid the analysis (because drop evaluates elements), and makes the code less clear (and requires an impure function updating a var).

var skip = 0
val filtered = lines flatMap { line =>
  if (skip > 0) {
    skip -= 1
    Nil
  } else {
    val elem = doExpensiveAnalysis(line)
    skip = elem.skipCount
    elem :: Nil
  }
}

If doExpensiveAnalysis is not its own method, the code becomes even less clear as the filtering (done via flatMap) and analysis are now mingled together.

Or perhaps you would like to do something with the last analysed line in the file matching some predicate, and then proceed to perform other operations (perhaps like the one above) starting from the front of the LazyList.

// side note: why isn't there a `findLast`?
val lastMatchingIdx = analysed.lastIndexWhere(somePredicate)
val lastMatching = analysed(lastMatchingIdx)

Without a lazy head, doing that search would require analysing all lines, whereas with a lazy head, it might analyse only a single line (if the predicate is true for the last line). It still needs to read all the lines, but it doesn't need to perform the expensive analysis for all of them.

[lrytz]

Thanks, good to have an example! An alternative is to keep a reference to the original non-mapped lines and call drop on that.

I think as long as a lazy head "only" provides some convenience (and that doesn't seem to come up too often), it would be better to opt for the simpler model. If there was a more serious limitation like the one we had with 2.12 Stream (#10696), that would be a much stronger argument.

[odersky]

I also tend to make head strict. It seems to be simpler conceptually.

[shawjef3]

I've run into a scenario where I wanted head to be lazy. Sometimes I want to use Iterator as control structure rather than a collection, where I get to accumulate results if I like. I use Stream (<= Scala 2.12) to lazily run each iteration and get the result. In these cases, when I call Iterator#toStream, I don't want the Iterator to execute until I explicitly start walking the Stream.

Here's an example of what happens for Scala 2.12. I don't want side effects when calling toStream, but there are.

scala> :paste
// Entering paste mode (ctrl-D to finish)

val i = new Iterator[Int] {
  var x = 0
  override def next(): Int = {
    val currentX = x
    x += 1
    println(currentX)
    currentX
  }

  override def hasNext: Boolean = true
}

// Exiting paste mode, now interpreting.

i: Iterator[Int]{def x: Int; def x_=(x$1: Int): Unit} = <iterator>

scala> i.toStream
0
res7: scala.collection.immutable.Stream[Int] = Stream(0, ?)

[sjrd]

@shawjef3 What you're describing is "form 1" of laziness as defined in the first post of this thread, and that is a given for LazyList. We're only considering dropping form 2 of laziness.