The order of operations here is "right associative". So

  A -> B -> C -> D

is equivalent to any of the following

  A -> (B -> C -> D)
  A -> (B -> (C -> D))
  A -> B -> (C -> D)

They're equivalent through currying. But you can only add or remove parentheses for stuff that is on the right of a function arrow. (A -> B) -> (C -> D) is a different thing.