I think the gain comes from two things:
- You can take an intermediate form of seq_foreach_from_spec() and pass it around, similar to lexical filehandles. (Remember how cool that was when 5.6 added it?)
- You can extend it a lot more easily than for-loops. In the trivial case, which is what you're describing, it's very easy to see how the for-for version is easier to handle than the FP version. What if you're working 5-10 nested loops? What if you're working a loop with a bunch of double-nested pairs, all of which are similar?
Let's say that you can create an intermediate representation that allows you to pass in N lists and do XYZ to it. What XYZ does is irrelevant, except to say that you need it done to more than one group of lists. Think about it as templates for algorithms and I think you'll see the gain.
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