The function on defined by (*) `on` f = \x y -> f x * f y is convenient when using functions like sortBy: sortBy (compare `on` fst), for example. It also makes the code more readable. Furthermore I consider on to be above the Fairbairn threshold, since * we get rid of two lambdas, * we get rid of the duplication of p, * on has some nice algebraic properties (documented in the patch) * and, most importantly, it is easier at a glance to understand (*) `on` p than to understand \x y -> p x * p y (assuming one knows about on). 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. Deadline for discussion: 2006-11-09. Trac ticket number: 979. -- /NAD