Implements Enum.zip_with and Stream.zip_with

[Adzz]

This is my first stab at this so comments very welcome.

Feature Request thread is here: https://groups.google.com/u/1/g/elixir-lang-core/c/mwBRUILsVHA

My basic approach was that the current zip/1 and zip/2 functions can be implemented by zip_with by doing this:

zip(enums) do
  zip_with(enums, &List.to_tuple/1)
end

To implement zip_with/2 the zip_fun you pass to it will get a list of each of the elements from each enumerable we are zipping. This seemed okay, but it differs from what zip/3 does, where each element from the collection being zipped is a different argument to the zipping fun.

I think that's okay but happy to change them to both be consistent with each other.

[Adzz]

I think this is equivalent, but the Stream code lost me a bit...

[krainboltgreene]

It would be useful to note that this is an endofunctor (the container type you put in is the container type you pull out).

[josevalim]

The following example will fail:

iex> Enum.zip_with([1, 2, 5, 6], 3..4, fn x, y -> x + y end)

That's because we call zip_with and zip_with with pass a list to the fn. You can fix it by calling it like this:

Suggested change

	zip_with([enumerable1, enumerable2], fun)
	zip_with([enumerable1, enumerable2], &apply(fun, &1))

You can also take this approach on Stream.zip_with/3 so it calls fun with two args.

[josevalim]

Btw, please add tests mixing lists with ranges and similar.

[Adzz]

Nice spot! Will do.

What's your thoughts on having zip_with/2's zipping fun receive a list instead?
Essentially turn this line: https://github.com/elixir-lang/elixir/pull/10419/files#diff-6881431a92cd4e3ea0de82bf2338f8eaR3797
into this:

  defp zip_list([h1 | next1], [h2 | next2], fun) do
   # pass a list here to fun
    [fun.([h1, h2]) | zip_list(next1, next2, fun)]
  end

I feel like it will be more consistent

[josevalim]

This looks great, awesome job! Please note that the first paragraph of @doc must always be a one-line summary explaining what the function does. Can you please update them accordingly? Thanks.

[eksperimental]

"then the with the second element"
I think a word or more are missing

[Adzz]

@krainboltgreene I don't think it is an endofunctor... take this example, you zip two maps but get returned a list. Like lots of the enum functions.

Enum.zip_with(%{a: 1}, %{a: 2}, fn {_, left}, {_, right} -> left + right end)
# [3]

[josevalim]

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