|Reported by:||nad||Owned by:|
|Type of failure:||Test Case:|
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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.