Hi Twan,
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
All good, apart from deleteFirstsOn. I honestly don't know what that function does, I was thinking possibly something like the first component of tuples? But no real idea.
When f is a slow function the call "sortOn f xs" recomputes f more than necessary. A version that caches the result of f is easy to define:
sortOn' f = map fst . sortOn snd . map (\x -> (x,f x))
In Hoogle I use sortOn to be the non-cacheing version, and sortWith to be the cacheing version. However, I think we can just ignore the non-cacheing version - if your extraction function is expensive you probably want to think a bit anyway.
To be able to sort lists in reverse order, a simple newtype should be added to Data.Ord:
newtype Down a = Down { getDown :: a }
instance Ord a => Ord (Down a) where Down x < Down y = y < x
Now "sortOn Down xs == reverse (sort xs)". The name Down comes from [2], maybe Desc is a better name?
I prefer Down, and strongly support this proposal as well.
1. should sortOn be added to Data.List?
Yes.
2. should all other *On functions be added as well?
Yes (apart from deleteFirstsOn)
3. what name should these functions get?
*On, as you have given them.
4. should the sortOn' variations be added? What about the naming?
No.
5. should Down be added to Data.Ord?
Yes. Thanks Neil ============================================================================== Please access the attached hyperlink for an important electronic communications disclaimer: http://www.credit-suisse.com/legal/en/disclaimer_email_ib.html ==============================================================================