Nils Anders Danielsson
(*) `on` f = \x y -> f x * f y
The main question seems to be which module to put the function in. Based on previous discussion on the libraries list I suggest creating a new module Control.Function containing on plus (initially) some other combinators (working solely on and with functions) from the Prelude.
I support the addition of `on`. I also agree it should be placed with other combinators that operate purely on functions. However, the "Control." hierarchy is certainly the wrong place for it. The very name Control is intended to suggest things that are slightly outside the usual functional domain - more in the region of imperative control structures. Here are some alternative suggestions: Data.Function Data.Functional Prelude.Functional Function.Combinators Prelude.Combinators Regards, Malcolm