a - b + c - d == foldr (-) 0 [a..d]

module Fold where                                   
                                                    
import Test.QuickCheck                              
                                                    
op xs = go xs True 0 where                          
    go [] _ acc = acc                               
    go xs@(a:res) True  acc = go res False (acc + a)
    go xs@(a:res) False acc = go res True  (acc - a)
                                                    
prop_iso xs = foldr (-) 0 xs ≡ op xs                

*Fold.Test> foldl (flip (-)) 1 $ reverse [2..3]
0
*Fold.Test> foldr (-) 1 [2..3]
0