Add a"same" function to Data.Eq

Add a"same" function to Data.Eq that parallels the "comparing" function in Data.Ord. For example:

-- | 
-- > same p x y = (p x) == (p y)
--
-- Useful combinator for use in conjunction with the @xxxBy@ family
-- of functions from "Data.List", for example:
--
-- >   ... groupBy (same fst) ...
same :: (Eq a) => (b -> a) -> b -> b -> Bool
same p x y = (p x) == (p y)

For a closer parallel, I suppose "same" could return "Equality" (similar to "Ordering"), and the functions "nubBy", "deleteBy", "deleteFirstsBy", "unionBy", "intersectBy" and "groupBy" should take (a -> a -> Equality) (a -> a -> Bool). But I don't know if there's any benefit to that.

changed weight to 3

added Tfeature request Trac import core libraries labels

changed the description

What's the benefit of same p over (==) on p?

((==) `on`) :: Eq b => (a -> b) -> a -> a -> Bool

Trac metadata

Trac field	Value
Component	Prelude → libraries/base

Additions to base should go through the http://www.haskell.org/haskellwiki/Library_submissions process.

Also, see #979 (closed) which suggests adding the same function under the name equating.

I agree that this should go through the libraries@ process.

For the record I'm also on the equating bandwagon, but keep in mind comparing was created before on was discovered, so when this has come up for discussion before comparing was being left around as a historical artifact than as a model to copy.

closed

Also see #9330 (closed) and http://www.haskell.org/pipermail/libraries/2014-July/023283.html.

Trac metadata

Trac field	Value
Resolution	Unresolved → ResolvedDuplicate

added Plow label