#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: | -------------------------------------+------------------------------------- Comment (by Zemyla): The instance for `(<|>)` would probably be more efficient as: {{{ #!haskell ZipList xs <|> ZipList ys = ZipList $ go (0 :: Int) xs where go n _ | n `seq` False = undefined go n [] = drop n ys go n (x:xs) = x:go (n + 1) xs }}} This way, it only has to iterate over xs once. Or, you could do it in `build`/`foldr` style: {{{ #!haskell ZipList xs <|> ZipList ys = ZipList (build $ zipListPlusBF xs ys) zipListPlusBF :: [a] -> [a] -> (a -> b -> b) -> b -> b zipListPlusBF xs ys c z = foldr goX endX xs 0# where goX x r n# = c x $ r (n# +# 1#) endX n# = foldr goY endY ys n# goY y r n# | isTrue# (n# ># 0#) = r (n# -# 1#) | otherwise = c y $ r 0# endY _ = z }}} This would have the advantage that it should ideally realize, for instance, that pure x <|> anything = pure x, or at the very least doesn't reference the right-hand side at all. We could probably use both and switch between them with `RULES` like is done for most of the algorithms in `Data.List`. -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13520#comment:9 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler