The most common kind of primitive recursive function I find myself writing these days is a variation on the theme of merging two sorted lists. You can see some examples in my Ranged Sets library at http://ranged-sets.sourceforge.net/. For instance: -- | Set union for ranged sets. Infix precedence is left 6. rSetUnion, (-\/-) :: DiscreteOrdered v => RSet v -> RSet v -> RSet v -- Implementation note: rSetUnion merges the two lists into a single -- sorted list and then calls normalise to combine overlapping ranges. rSetUnion (RSet ls1) (RSet ls2) = RSet $ normalise $ merge ls1 ls2 where merge ls1 [] = ls1 merge [] ls2 = ls2 merge ls1@(h1:t1) ls2@(h2:t2) = if h1 < h2 then h1 : merge t1 ls2 else h2 : merge ls1 t2 -- | Set intersection for ranged sets. Infix precedence is left 7. rSetIntersection, (-/\-) :: DiscreteOrdered v => RSet v -> RSet v -> RSet v rSetIntersection (RSet ls1) (RSet ls2) = RSet $ filter (not . rangeIsEmpty) $ merge ls1 ls2 where merge ls1@(h1:t1) ls2@(h2:t2) = rangeIntersection h1 h2 : if rangeUpper h1 < rangeUpper h2 then merge t1 ls2 else merge ls1 t2 merge _ _ = [] Union also has its own merge function. The worst case I've come across was at work. I can't talk about the details, but it involved manipulating two functions of time represented by lists of samples. So I had a type TimeFunc = [(Value, Time)], and the job was to compare two TimeFuncs with samples at different times. Step 1 was to interpolate each TimeFunc with the values for the times in the other TimeFunc, giving a result of type [(Value, Value, Time)] for the union of all the times in both original TimeFuncs. I wrote a truly hairy zipTimeFunc function with guards to match each possible case. It worked, but it must have been 100 lines if you include the comments to explain each case and demonstrate totality. So I'm wondering if anyone has a more general pattern. Unfold? Some variation on a theme of zip? I once tried writing a generalised merge, but it needed half a dozen functions as arguments to handle all the various cases. It was kludgier than just rolling a new merge routine every time. And I don't think it would have handled the TimeFunc case. Paul.