Optimise NonEmptyTraverse implementation

[gagandeepkalra]

resolves #3381

depends on #3375

[diesalbla]

There is a bit of LISP here. This code may be more readable as follows:

Suggested change

	tail.headOption.fold(Eval.now(Apply[G].map(f(head))(NonEmptyLazyList(_)))) { h =>
	Apply[G].map2Eval(f(head), Eval.defer(loop(h, tail.tail)))((b, acc) => b +: acc)
	}
	val fh = f(head)
	tail match {
	case _ if tail.isEmpty =>
	Eval.now(Apply[G].map(fh)(NonEmptyLazyList(_)))
	case h #:: ttail =>
	val ftail = Eval.defer(loop(h, ttail))
	Apply[G].map2Eval(fh, ftail)(_ +: _)
	}

The changes are:

• Extract f(head), which is eagerly evaluated in both branches, as a val.
• Replace the Option.fold by a pattern-match.
• Replace the cases of headOption by two cases on the tail lazy list itself. For the first case, there seems not to be any case-object, like Nil, for the empty list. Discussion.
• In the second case, abbreviate (b, acc) => b +: acc by underscores _ +: _.

[gagandeepkalra]

Hello, I see what you mean there.

On my part, usually I try to avoid pattern match (it's slow)

[codecov-commenter]

Codecov Report

Merging #3382 into master will decrease coverage by 0.02%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #3382      +/-   ##
==========================================
- Coverage   91.63%   91.60%   -0.03%     
==========================================
  Files         382      382              
  Lines        8304     8311       +7     
  Branches      232      226       -6     
==========================================
+ Hits         7609     7613       +4     
- Misses        695      698       +3     

[LukaJCB]

Thanks @gagandeepkalra :)

[travisbrown]

This looks good to me! Do you mind rebasing to resolve the conflicts, @gagandeepkalra? I can merge it after that.

[gagandeepkalra]

Thank you @travisbrown @LukaJCB, this was fun. I'm happy knowing short-circuiting is already useful (I see foldable there).

[LukaJCB]

Thanks a ton!

[travisbrown]

Thanks again, @gagandeepkalra!