[zaptheimpaler]

I don't understand one thing about this notation:

   fun :: A -> B -> C -> D

can be bracketed in a bunch of different ways -

   fun1 :: (A -> B) -> (C -> D)
   fun2 :: A -> (B -> C -> D)

etc. and don't these all mean different things?

fun1 would take a function and return a function, whereas fun2 takes an A and returns a curried function of B,C -> D.

[Muphrid]

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.

[dwenzek]

Read:

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

[kryptiskt]

Yes, functions need to be bracketed, as they are single arguments with a complex type.