J executes statements right to left so your ~. >./\ a
is equivalent to:
~. (((>./) \) a)
It would be a hook if it was parenthesized like:
(~. ((>./) \)) a
Some relevant discussions: function composition and how to take the train.
Edit: To make this clearer, f g y
is always f (g y)
. If you want the hook, you have to write (f g) y
.