Question

Currently I’m messing around with Haskell. My knowledge about Haskell (and functional languages in general) is still low but I’m working on it.
What really bothers me is a (as I thought) simple task: folding nested lists with one fold per depth.

fcalc = foldr (\x y -> (foldr (**) 1 x) * (foldr (**) 1 y)) [1.0, 1.0] [[2.0, 3.0], [4.0, 5.0]]

What it should do: 2^3 * 4^5 where the ^ is done by the lambda’d inner folds. Sadly it does not work.

Occurs check: cannot construct the infinite type: t0 = [t0]
In the third argument of `foldr', namely `y'

I’ve read a bit about the given “infinite type” error mainly indicating that a variable is used as e.g. element while it was a list instead. This made me thinking about the second param of the outer foldr as the problem but without success.
I just don’t get it. :/

Answer 1

Your problem is that you’re trying to do the a^b calculation on your accumulator inside the fold, while also doing it on each element. What you really want is something like

fcalc = foldr (\x y -> (foldr (**) 1 x) * y) 1 [[1.0, 1.0], [2.0, 3.0], [4.0, 5.0]]

Remember, the output of each step of foldr is plugged in as the second argument of the next step (e.g. the y variable in this case). Since your foldr step is returning a number, the y variable is already a number, and therefore you can’t fold over it.