.. and I am positive there must be a way of beautifying it, but I am struggling. I bet there is just some lovely way of making this all shrink to three lines.. So here's the problem. I have two lists of tuples: (timestamp, value) What I would like to do in do a kind of 'zip' on two of these lists to make a list of (timestamp, (value1, value2)) with the following rules: - If the timestamps are equal it's easy - make your new element an move on - If one of the lists has a timestamp that the other doesn't, repeat an old value from the other list - If we don't have an old value yet, then don't create an element in the new list. e.g. if I ran my algorithm on these two lists d1 = [ (1,"a"), (2,"b"), (3,"c") ] d2 = [ (2,"b'"), (4,"d'") ] I would like to get result = [ (2, (b,b')), (3, (c,b')), (4, (c,d')) ] e.g. there was no data in d2 for our first element so we skipped it. Okay, so here is my code.. It works, but makes me feel a bit dirty. To explain my nomenclature 't' is 'timestamp of', 'v' is 'value of'. vx' and vy' are the 'old' values from the previous iteration in case a repeat is needed. They are Maybes because at the beginning there may be no old value. d1 = [ (1,"a"), (2,"b"), (3,"c") ] d2 = [ (2,"b'"), (4,"d'") ] t (x,y) = x v (x,y) = y js vx' vy' (x:xs) (y:ys) | t x == t y = ( (t x), (v x, v y) ) : js (Just (v x)) (Just (v y)) xs ys | t x < t y = maybe (js (Just (v x)) Nothing xs (y:ys)) (\z -> ( t x, (v x, z ) ) : ( js (Just (v x)) (Just z) xs (y:ys))) vy' | t x > t y = maybe (js Nothing (Just (v y)) (x:xs) ys) (\z -> ( t y, (z, v y ) ) : ( js (Just z) (Just (v y)) (x:xs) ys)) vx' js vx' vy' (x:xs) [] = maybe [] (\z -> ( t x, (v x, z ) ) : ( js (Just (v x)) (Just z) xs [])) vy' js vx' vy' [] (y:ys) = maybe [] (\z -> ( t y, (z, v y ) ) : ( js (Just z) (Just (v y)) [] ys )) vx' js _ _ [] [] = [] You call it with the first two arguments as Nothing to kick it off (I have a trivial wrapper function to do this) It works fine:
:t js js :: (Ord t) => Maybe a1 -> Maybe a -> [(t, a1)] -> [(t, a)] -> [(t, (a1, a))]
js Nothing Nothing d1 d2 [(2,("b","b'")),(3,("c","b'")),(4,("c","d'"))]
But it just feels gross. Any advice on how to tame this beast would be greatly appreciated :) All the best, Philip