Duncan Coutts schrieb:
So start a new thread and lets try and find out if there is a consensus.
As an example variation from a program I wrote:
-- mergeBy cmp xs ys = (only_in_xs, in_both, only_in_ys)
mergeBy :: (a -> b -> Ordering) -> [a] -> [b] -> ([a], [(a, b)], [b]) mergeBy cmp = merge [] [] [] where merge l m r [] ys = (reverse l, reverse m, reverse (ys++r)) merge l m r xs [] = (reverse (xs++l), reverse m, reverse r) merge l m r (x:xs) (y:ys) = case x `cmp` y of GT -> merge l m (y:r) (x:xs) ys EQ -> merge l ((x,y):m) r xs ys LT -> merge (x:l) m r xs (y:ys)
It's rare of course that you need the generality of merging lists of different types. Though of course this also covers the slightly more common case of comparing where == is only an equivalence relation and you want to keep or combine both elements.
I've found the following generalization sufficient enough: merge :: (a -> a -> Ordering) -> (a -> a -> [a] -> [a]) -> [a] -> [a] -> [a] merge cmp jn l1 l2 = case l1 of [] -> l2 x1 : r1 -> case l2 of [] -> l1 x2 : r2 -> let recmerge = merge cmp jn in case cmp x1 x2 of LT -> x1 : recmerge r1 l2 EQ -> jn x1 x2 $ recmerge r1 r2 GT -> x2 : recmerge l1 r2 Add a parameter to join equal elements with the recursively merged list tails. This allows to keep both or only one elements or to add occurrences counts of multi-sets. Cheers Christian