You can transfer a fold into an infix operator notation (writing in between):
This example fold using the accumulator function x
fold x [A, B, C, D]
thus equals
A x B x C x D
Now you just have to reason about the associativity of your operator (by putting parentheses!).
If you have a left-associative operator, you’ll set the parentheses like this
((A x B) x C) x D
Here, you use a left fold. Example (haskell-style pseudocode)
foldl (-) [1, 2, 3] == (1 - 2) - 3 == 1 - 2 - 3 // - is left-associative
If your operator is right-associative (right fold), the parentheses would be set like this:
A x (B x (C x D))
Example: Cons-Operator
foldr (:) [] [1, 2, 3] == 1 : (2 : (3 : [])) == 1 : 2 : 3 : [] == [1, 2, 3]
In general, arithmetic operators (most operators) are left-associative, so foldl is more widespread. But in the other cases, infix notation + parentheses is quite useful.