I've looked at different folds and folding in general as well as a few others and they explain it fairly well.
I'm still having trouble on how a lambda would work in this case.
foldr (\y ys -> ys ++ [y]) [] [1,2,3]
Could someone go through that step-by-step and try to explain that to me?
And also how would foldl work?
Using
foldr f z [] = z
foldr f z (x:xs) = x `f` foldr f z xs
And
k y ys = ys ++ [y]
Let's unpack:
foldr k [] [1,2,3]
= k 1 (foldr k [] [2,3]
= k 1 (k 2 (foldr k [] [3]))
= k 1 (k 2 (k 3 (foldr k [] [])))
= (k 2 (k 3 (foldr k [] []))) ++ [1]
= ((k 3 (foldr k [] [])) ++ [2]) ++ [1]
= (((foldr k [] []) ++ [3]) ++ [2]) ++ [1]
= ((([]) ++ [3]) ++ [2]) ++ [1]
= (([3]) ++ [2]) ++ [1]
= ([3,2]) ++ [1]
= [3,2,1]