ZipRev.hs

zipRev :: [a] -> [b] -> [(a,b)]
zipRev xs ys = fr where
  (fr, allbs) = go [] allbs xs ys
  
  go acc ~(b':bs') (a:as) (b:bs) = ((a,b') : res, bss)
    where
      (res, bss) = go (b:acc) bs' as bs
  go acc _ _ _ = ([], acc)

I'm too tired to think about this now. Have you tried the puzzle I set a couple months ago, to write an optimal incremental (<*) for Data.Sequence?

…

On Tue, May 14, 2019, 11:33 AM WillNess ***@***.***> wrote: wrong test case, huh. compare results for zipRev [1..4] [10..12] with your code and theirs -- I tested my re-write, at the top of my answer, with the added clause .... g x c ([],t) = t Yours produces [(1,12),(2,11),(3,10)] but the augmented theirs -- [(2,12),(3,11),(4,10)]. Not wrong then. :) — You are receiving this because you authored the thread. Reply to this email directly, view it on GitHub <https://gist.github.com/3e149df5a03c3ae2e51c51277919e189?email_source=notifications&email_token=AAOOF7KUH4XVIVFD5RQIXBDPVLLUBA5CNFSM4HM2VFKKYY3PNVWWK3TUL52HS4DFVNDWS43UINXW23LFNZ2KUY3PNVWWK3TUL5UWJTQAFSBIW#gistcomment-2917003>, or mute the thread <https://github.com/notifications/unsubscribe-auth/AAOOF7J6JI55YSSEHHCCRIDPVLLUBANCNFSM4HM2VFKA> .

It just aligns the lists at the wrong end:

   1   2   3   4
       12  11  10  -- OP's code
   12  11  10      -- your code

About that, could you remind me, with the link to the SO entry? I remembered something vaguely about this, recently, but couldn't remember exactly what it was...

I don't know about the SO comment, but here's the GitHub issue. Remember: the function must be incremental/lazy the way liftA2 is, and must be time and space-optimal. You will almost certainly need to mash up ideas from the internal functions aptyMiddle (which I created to implement (<*>) and later liftA2) and applicativeTree (which was created, I believe by Wasserman, to implement replicateA and thereby replicate) to accomplish this mission.

thanks, will try to see what I can do.