Question

So, I’m really frying my brain trying do understand the foldl.foldr composition.
Here is a example:

(foldl.foldr) (+) 1 [[1,2,3],[4,5,6]]

The result is 22, but what’s really happening here?

To me it looks like this is what is happening: foldl (+) 1 [6,15].
My doubt is related to the foldr part. Shouldn’t it add the 1 to all the sub-lists? Like this: foldr (+) 1 [1,2,3].
In my head the 1 is added just one time, is it right? (probably not, but I want to know how/why!).

I’m very confused (and perhaps making all the confusion, haha).
Thank you!

Answer 1

(foldl.foldr) (+) 1 [[1,2,3],[4,5,6]]

becomes

foldl (foldr (+)) 1 [[1,2,3],[4,5,6]]

So you get

foldl (foldr (+)) (foldr (+) 1 [1,2,3]) [[4,5,6]]

after the first step of foldl, or

foldl (foldr (+)) 7 [[4,5,6]]

if we evaluate the applied foldr (unless the strictness analyser kicks in, it would in reality remain an unevaluated thunk until the foldl has traversed the entire list, but the next expression is more readable with it evaluated), and that becomes

foldl (foldr (+)) (foldr (+) 7 [4,5,6]) []

and finally

foldl (foldr (+)) 22 []
~> 22