Why hooks and forks in the J language?
I recently said:
I don't know why [Ken] Iverson thought the hook was the thing to embed in
the [J] language.
And I think I now recall that the name of the language itself, J, is
intended to showcase the hook, so he must have thought it was pretty
wonderful.
A helpful Hacker News
comment pointed me to
the explanation. Here Iverson explains why the “hook”
feature: it is actually the
S combinator in disguise. Recall that
$${\bf S} x y z = x z (y z).$$ This is exactly what J's hook computes
when you write (x y) z. For instance, if I understand correctly, in
J (+ !) means the one-place operation that takes an argument !!z!!
to !!z + z! !!.
As McBride and Paterson point
out, S
is also the same as the <*> operator in the Reader instance of
Applicative.
Since in J the only possible inputs to a hook are functions, it is
operating in the Reader idiom and in that context its hook is doing
the same thing as Haskell's <*>. Similarly, J's “fork” feature can
be understood as essentially the same as the Reader insance of
Haskell's liftA2.
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