Question

I am being asked to make an haskell function that computes something like

1^2 + 2^2 + 3^2 ...

While I find it quite easy to implement with list comprehensions

sum [ k^2 | k <- [1..100]]

or maps

sum (map (\x -> x*x) [1..100])

I’m having some hard time getting how to achieve it with foldls.

If I am not wrong, one needs no less than 3 parameters in a recursive function to achieve a result with this:

1. The current position (1… up to n)
2. The current sum
3. Where to stop

Even if I define this function, it will still return a tuple, not a number (like I need it to!).

Could anyone be kind enough to give me some clues on what I may be missing?

Thanks

Answer 1

The “current position” (actually the next item in the list, just like in your map and list comprehension versions) and where to stop are implicit in the list being folded over. The current sum is the “accumulator” parameter of the fold. So, fill in the blank:

foldl (\runningSum nextNumber -> ____) 0 [1..100]