Add lazyZip to ArrayOps and StringOps

[julienrf]

The lazyZip operation is currently only available on Iterable. To use it on an Array or a String you have to first convert them to iterables. The lazyZip operation should be available on Array and String without requiring this conversion.

[szeiger]

A more generally applicable definition would be to put toLazyZipOps into Predef and make use of IsIterableLike. I was going to make the argument that it is unnecessary overhead for Array and String because they are strict but actually that doesn't even matter. LazyZipOps forces its argument, there are no by-name tricks to prevent that, so why does it exist in the first place? Shouldn't lazyZip be defined as a method directly on Iterable?

[szeiger]

Context for lazyZip: scala/collection-strawman#223

[julienrf]

Shouldn't lazyZip be defined as a method directly on Iterable?

The problem is that you would then be able to chain lazyZip too many times. Consider the following expressions:

def example(xs: List[Int]): Unit = {
  (xs lazyZip xs): LazyZip2[Int, Int, List[Int]]
  // `LazyZipN` types are special lazy collection types
  // We can go up to LazyZip4
  (xs lazyZip xs lazyZip xs lazyZip xs): LazyZip4[Int, Int, Int, Int, List[Int]]
  // After that, you get a compilation error
  xs lazyZip xs lazyZip xs lazyZip xs lazyZip xs
  // error: method lazyZip is not a member of LazyZip4
}

However, note that there is an implicit conversion from LazyZipN[E1, E2, ...] to Iterable[(E1, E2, …)]. If lazyZip was defined as a method on Iterable, it would become possible to call lazyZip on a LazyZip4 instance:

(xs lazyZip xs lazyZip xs lazyZip xs lazyZip xs): LazyZip2[(Int, Int, Int, Int), Int, List[(Int, Int, Int, Int)]]
// We would get a LazyZip*2* instance instead of an expected LazyZip5 (which does not exist)

In order to make it impossible to call lazyZip on a LazyZip4 instance, the lazyZip method is available via an implicit conversion from Iterable. As a consequence, it is possible to call it on an Iterable instance, but not on something that can be converted into an Iterable (such as a LazyZip4 instance).

[julienrf]

A more generally applicable definition would be to put toLazyZipOps into Predef and make use of IsIterableLike

Yes, that’s a good suggestion!