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.