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

[Bertram Felgenhauer]

Twan van Laarhoven wrote:
> Hello list,
>
>
> 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

Too many functions, and too little added functionality, IMHO.

>     newtype Down a = Down { getDown :: a }

I like this one.

Bertram