# OCaml Higher Order Functions Composition operators

## Example

Two useful higher-order functions are the binary application (`@@`) and reverse-application or "pipe" (`|>`) operators. Although since 4.01 they're available as primitives, it might still be instructive to define them here:

``````let (|>) x f = f x
let (@@) f x = f x
``````

Consider the problem of incrementing the square of 3. One way of expressing that computation is this:

``````(* 1 -- Using parentheses *)
succ (square 3)
(* - : int = 10 *)

(* where `square` is defined as: *)
let square x = x * x
``````

Note that we couldn't simply do `succ square 3` because (due to left-associativity) that would reduce to the meaningless `(succ square) 3`. Using application (`@@`) we can express that without the parentheses:

``````(* 2 -- Using the application operator *)
succ @@ square 3
(* - : int = 10 *)
``````

Notice how the last operation to be performed (namely `succ`) occurs first in the expression? The reverse-application operator (`|>`) allows us to, well, reverse this:

``````(* 3 -- Using the reverse-application operator *)
3 |> square |> succ
(* - : int = 10 *)
``````

The number 3 is now "piped" through `square` and then `succ`, as opposed to being applied to `square` to yield a result that `succ` is applied to.