[Haskell-beginners] I have created an ugly Haskell program..
Philip Scott
haskell-beginners at foo.me.uk
Sun Nov 1 18:27:42 EST 2009
.. 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
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