Re: [Haskell-cafe] Foldl

zacara:

Hello, i'd like to write a function that given a list like [1,2,3,4...] returns a list of couple where the first element is the corresponding element of the string, and the second is the sum of the previous elements. An example: input: [1,2,3,4] output: [(1,0)(2,1)(3,3)(4,6)]

Looks like a scanl (a prefix fold) -- the "of the previous elements" is a clue that you're describing a scan: Prelude> :t scanl scanl :: (a -> b -> a) -> a -> [b] -> [a] Prelude> scanl (+) 0 [1..4] [0,1,3,6,10] You should be able to work out the rest from here (or rewrite it as a fold). -- Don