#13520: instance Alternative ZipList -------------------------------------+------------------------------------- Reporter: Iceland_jack | Owner: (none) Type: feature request | Status: closed Priority: normal | Milestone: 8.4.1 Component: libraries/base | Version: 8.0.1 Resolution: fixed | Keywords: newcomer Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Phab:D3416, Wiki Page: | Phab:D4638 -------------------------------------+------------------------------------- Changes (by dfeuer): * differential: Phab:D3416 => Phab:D3416, Phab:D4638 Comment: Zemyla, in an amazing coincidence of timing, I happened to write something very similar to your first instance in Phab:D4638 just now. I think it makes sense to use `foldr` to fold over the first argument. Unfortunately, the `build` is a bit trickier, because it will end up copying the second list if it doesn't fuse away. I'm not sure if it's possible to work around that issue with `RULES`. Appends are generally handled by `augment`, but that mechanism isn't powerful enough for what we're doing here because it leaves no room to use an accumulated count in the tail. As for the hand- unboxing, I'd only go for that if GHC proves incapable of unboxing on its own; it's really quite good at that most of the time. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13520#comment:10 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler