Using the definitions

  foldr f b []     = b
  foldr f b (x:xs) = x f foldr f b xs
  

and

  foldl f b []     = b
  foldl f b (x:xs) = foldl f (b f x) xs
  

The expression foldr (-) 0 [1,2,3] can be evaluated as

  foldr (-) 0 [1,2,3]
    = 1 - foldr (-) 0 [2,3]
    = 1 - (2 - foldr (-) 0 [3])
    = 1 - (2 - (3 - foldr (-) 0 []))
    = 1 - (2 - (3 - 0))
    = 2
  

and the expression foldl (-) 0 [1,2,3] as

  foldl (-) 0 [1,2,3]
    = foldl (-) (0 - 1) [2,3]
    = foldl (-) ((0 - 1) - 2) [3]
    = foldl (-) (((0 - 1) - 2) - 3) []
    = ((0 - 1) - 2) - 3
    = - 6
  

See Grep.