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.