Added Data.Function (Trac ticket #979).

+ A module with simple combinators working solely on and with
  functions.
+ The only new function is "on".
+ Some functions from the Prelude are re-exported.

diff --git a/Data/Function.hs b/Data/Function.hs
new file mode 100644 (file)
index 0000000..b6ccd13
--- /dev/null
+++ b/Data/Function.hs
@@ -0,0 +1,75 @@
+-----------------------------------------------------------------------------
+-- |
+-- Module      :  Data.Function
+-- Copyright   :  Nils Anders Danielsson 2006
+-- License     :  BSD-style (see the LICENSE file in the distribution)
+--
+-- Maintainer  :  libraries@haskell.org
+-- Stability   :  experimental
+-- Portability :  portable
+--
+-- Simple combinators working solely on and with functions.
+
+module Data.Function
+  ( -- * "Prelude" re-exports
+    id, const, (.), flip, ($)
+    -- * Other combinators
+  , on
+  ) where
+
+infixl 0 `on`
+
+-- | @(*) \`on\` f = \\x y -> f x * f y@.
+--
+-- Typical usage: @'Data.List.sortBy' ('compare' \`on\` 'fst')@.
+--
+-- Algebraic properties:
+--
+-- * @(*) \`on\` 'id' = (*)@ (if @(*) ∉ {⊥, 'const' ⊥}@)
+--
+-- * @((*) \`on\` f) \`on\` g = (*) \`on\` (f . g)@
+--
+-- * @'flip' on f . 'flip' on g = 'flip' on (g . f)@
+
+-- Proofs (so that I don't have to edit the test-suite):
+
+--   (*) `on` id
+-- =
+--   \x y -> id x * id y
+-- =
+--   \x y -> x * y
+-- = { If (*) /= _|_ or const _|_. }
+--   (*)
+
+--   (*) `on` f `on` g
+-- =
+--   ((*) `on` f) `on` g
+-- =
+--   \x y -> ((*) `on` f) (g x) (g y)
+-- =
+--   \x y -> (\x y -> f x * f y) (g x) (g y)
+-- =
+--   \x y -> f (g x) * f (g y)
+-- =
+--   \x y -> (f . g) x * (f . g) y
+-- =
+--   (*) `on` (f . g)
+-- =
+--   (*) `on` f . g
+
+--   flip on f . flip on g
+-- =
+--   (\h (*) -> (*) `on` h) f . (\h (*) -> (*) `on` h) g
+-- =
+--   (\(*) -> (*) `on` f) . (\(*) -> (*) `on` g)
+-- =
+--   \(*) -> (*) `on` g `on` f
+-- = { See above. }
+--   \(*) -> (*) `on` g . f
+-- =
+--   (\h (*) -> (*) `on` h) (g . f)
+-- =
+--   flip on (g . f)
+
+on :: (b -> b -> c) -> (a -> b) -> a -> a -> c
+(*) `on` f = \x y -> f x * f y

diff --git a/Data/List.hs b/Data/List.hs
index b006b29..2ce489d 100644 (file)
--- a/Data/List.hs
+++ b/Data/List.hs
@@ -168,6 +168,10 @@ module Data.List
    -- ** The \"@By@\" operations
    -- | By convention, overloaded functions have a non-overloaded
    -- counterpart whose name is suffixed with \`@By@\'.
+   --
+   -- It is often convenient to use these functions together with
+   -- 'Data.Function.on', for instance @'sortBy' ('compare'
+   -- \`on\` 'fst')@.
 
    -- *** User-supplied equality (replacing an @Eq@ context)
    -- | The predicate is assumed to define an equivalence.