Haskell: summing up the elements of a list

Question

I have to sum up the elements of a list like this for example: if your input is ['a','b','c'] , the output should be ["a","ab","abc"] . But I have no idea how to code this... could somebody give me a hint? I would be very grateful!

Answer 1

all those hints are fine but you probably struggle with those too

I would suggest you start from here:

sumList [] = []
sumList [x] = [[x]]
sumList (x:xs) = ... : map (...) (sumList xs)

and try to figure out what you have to put into both ... ( hint not the same ;))

the first line is only there to give some reasonable result for an empty list as input - you could remove it (recursion should not hit it)

The second one will do [1] -> [[1]]

Now you have to figure out what to do with more - here is an additional hint:

sumList [1,2]
{ 3. line - x = 1, xs = [2] }
= ... : map (...) (sumList [2])
{ 2. line }
= ... : map (...) [[2]]

now you want

= [[1],[1,2]]

so it look like you can do this with

first ... = [1]
second ... = map (prepend 1) to every list in [[2]]