Why the foldr function gives me a negative result when the list contains a zero?

Question

Here an example to illustrate what I mean

Prelude> foldr (-) 0 [0,1]  
-1

I thought here it would basically do 0 - 0 = 0; 1 - 0 = 1; but the actual result is -1

I tried it with other examples

Prelude> foldr (-) 1 [1,3]   
-1

Prelude> foldr (-) 1 [1,9]
-7

I probably misunderstood how foldr works so I would be happy about an explanation :)

Answer 1

Try foldr (-) 0 [1, 2, 3, 4, 5]. You should get 3. That's because the fold is equivalent to (1 - (2 - (3 - (4 - (5 - 0))))) -- it's starting from the right. If you try foldl, you should get -15. That's because it's starting from the left and is equivalent to (((((0 - 1) - 2) - 3) - 4) - 5).

Your shorter example, foldr (-) 0 [0, 1], is equivalent to 0 - (1 - 0), which reduces to -1.