Transforming a List of 2-Tuples in Haskell

Question

(Background: Trying to learn Haskell, very new to functional programming. Typically used to Python.)

Suppose I have a list of 2-tuples, a histogram:

let h = [(1,2),(3,5),(4,6),(5,3),(6,7),(7,4),(8,6),(9,1)]

In imperative terms, I want to change the second term of each pair to be the sum of all the previous second pairs. In Python, the following (admittedly complex) list comprehension could do it:

[(p[0], sum( [p[1] for p in histogram[:i+1]] ))
    for i, p in enumerate(histogram)]

assuming histogram refers to a list of 2-tuples like h above.

Here's what I have so far in Haskell:

zip [fst p | p <- h] (scanl1 (+) [snd k | k <- h])

This works, but I wonder:

• Is this reading through the list once, or twice?
• Can it be expressed better? (I expect so.)

In case it isn't clear, this is the expected output for the above:

[(1,2),(3,7),(4,13),(5,16),(6,23),(7,27),(8,33),(9,34)]

Answer 1

You could use this function

accumulate = scanl1 step
        where step (_,acc) (p1,p2) = (p1,acc+p2)

Here is the result on your sample data:

*Main> accumulate h
[(1,2),(3,7),(4,13),(5,16),(6,23),(7,27),(8,33),(9,34)]

Answer 2

If you're new to Haskell this might be a little too early, but lens offers a nice succinct way:

> scanl1Of (traverse . _2) (+) h
[(1,2),(3,7),(4,13),(5,16),(6,23),(7,27),(8,33),(9,34)]

You can easily accumulate only the first one by switching to _1:

> scanl1Of (traverse . _1) (+) h
[(1,2),(4,5),(8,6),(13,3),(19,7),(26,4),(34,6),(43,1)]

Or accumulate all values as a sort of nested list:

> scanl1Of (traverse . both) (+) h
[(1,3),(6,11),(15,21),(26,29),(35,42),(49,53),(61,67),(76,77)]