That's indeed completely unreadable and I wrote it. But it also isn't typical. In this particular case I replaced an earlier "functional" implementation with this abomination to fix a performance bottleneck. Though I think I might have gone overboard -- unfortunately the commit message isn't very good either and doesn't explain why it was necessary to switch to this style of doing things in addition to what the message actually talks about.

    (--map (list (substring it 3)
                 (aref it 0)
                 (aref it 1))
           (magit-git-items "status" "-z" "--porcelain" "-u" "--ignored"))))

Generally speaking you don't have write code like what you linked too unless you have a very good reason to do so (I doubt that I had in this case, but who knows).