[GHC] #979: Adding on

[GHC]

#979: Adding on
-------------------------------+--------------------------------------------
    Reporter:  nad             |       Owner:         
        Type:  proposal        |      Status:  new    
    Priority:  normal          |   Milestone:         
   Component:  libraries/base  |     Version:  6.6    
    Severity:  normal          |    Keywords:         
  Difficulty:  Unknown         |    Testcase:         
Architecture:  Unknown         |          Os:  Unknown
-------------------------------+--------------------------------------------
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.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/979>
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