Proposal #2659: Add sortOn and friends to Data.List

[Henning Thielemann]

On Sun, 5 Oct 2008, Twan van Laarhoven wrote:

> Almost all uses of sortBy in user code use 'comparing', 'on' or a similar 
> construction [1]. I think we should add a function that makes this common 
> behavior more convenient:
>
>    sortOn :: Ord b => (a -> b) -> [a] -> [a]
>
> For consistency we should also add *On for the other *By functions in 
> Data.List:
>
>    nubOn           :: Eq b => (a -> b) -> [a] -> [a]
>    deleteOn        :: Eq b => (a -> b) -> a -> [a] -> [a]
>    deleteFirstsOn  :: Eq b => (a -> b) -> [a] -> [a] -> [a]
>    unionOn         :: Eq b => (a -> b) -> [a] -> [a] -> [a]
>    intersectOn     :: Eq b => (a -> b) -> [a] -> [a] -> [a]
>    groupOn         :: Eq b => (a -> b) -> [a] -> [[a]]
>    sortOn          :: Ord b => (a -> b) -> [a] -> [a]
>    insertOn        :: Ord b => (a -> b) -> a -> [a] -> [a]
>    maximumOn       :: Ord b => (a -> b) -> [a] -> a
>    minimumOn       :: Ord b => (a -> b) -> [a] -> a
>   (nubSortOn       :: Ord b => (a -> b) -> [a] -> [a]) -- see #2629

I also prefer these functions and have called them '*Key', because they 
work on a key:
    http://darcs.haskell.org/htam/src/Useful.hs

> Questions:
>  1. should sortOn be added to Data.List?

yes

>  2. should all other *On functions be added as well?

yes

>  3. what name should these functions get?

'On' is ok for me

>  4. should the sortOn' variations be added? What about the naming?

Both variants differ only in efficiency, where none is superior over the 
other. I don't think that the prime is a good way to indicate the 
difference. Maybe you can use 'sortKey' for the variant for selector 
functions and 'sortOn' for the caching variant.

>  5. should Down be added to Data.Ord?

I find it useful.