[Haskell-beginners] A better way to "integrate"

[Dimitri DeFigueiredo]

Awesome haskellers,

I am coding up a little function that aggregates "ask orders" in a 
currency exchange.
Another way to look at it, is that the function takes as input a 
histogram or fdf (in list format) and outputs the cumulative 
distribution cdf (also in list format). So we are kind of "integrating" 
the input list.

When given a list of asks in order of increasing price, the function 
returns a list of points in the graph of the total supply curve.

Here's an example:

asks:                           returned list:

[ (Price 42, Volume 0.5),      [ (Price 21,         Volume 0.5),
   (Price 50, Volume  1 ),        (Price 21+50=71,   Volume 1.5),
   (Price 55, Volume 0.2)]        (Price 21+50+11=82,Volume 1.7)]

the returned list gives us the total supply curve (price = y-axis, 
quantity/volume = x-axis, so the order is flipped)

Summarizing

* We're adding up the volumes. The last volume on the list is the total 
volume available for sale.
* We calculate the total amount to be paid to buy the current volume 
(for each item in the list).

I have written up a simple function to do this:

aggregate :: Num a => [(a,a)] -> [(a,a)]
aggregate xs = aggregate' 0 0 xs

aggregate' :: Num a => a -> a -> [(a,a)] -> [(a,a)]
aggregate' _ _ [] = []
aggregate' accX accY ((x,y):ls) = let accX' = accX + x * y
                                       accY' = accY +     y

                                       in  (accX',accY') : aggregate' 
accX' accY' ls


main = print $ aggregate [(42,0.5),(50,1),(55,0.2)]

However, this does not look very good to me and it feels like I'm 
reinventing the wheel.

Question: Is there a better Haskell way to do this? I'm really anal 
about making it easy to read.

Thanks!

Dimitri