Proposal: Adding on

Duncan Coutts duncan.coutts at
Sat Nov 4 22:17:16 EST 2006

On Sat, 2006-11-04 at 21:25 -0500, Jan-Willem Maessen wrote:
> On Nov 3, 2006, at 11:58 AM, Bulat Ziganshin wrote:
> >
> > while we are here, i also propose to export from Data.List 'merge'
> > function that is useful on its own
> Yes, please.  This qualifies as my most commonly rewritten multi-line  
> Haskell function.
> That said, I've also written most of the variations of merge you  
> might care to imagine (indeed, sorted-list sets and multisets  
> together use pretty much all of them if you include difference and  
> merging of equals).

Yes, there are a number of different options for what we might call
'merge'. It would be good to discuss what the variations are and which
variations might be sensible to include in a standard lib and what they
might be called.

However I think this is a separate issue from the discussion about 'on'.
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.


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