Published on April 2, 2016; tags: Haskell
When confronted with a problem of sorting a list in descending order in Haskell, it is tempting to reach for a “lazy” solution
reverse . sort.
An obvious issue with this is efficiency. While sorting in descending order should in theory be exactly as efficient as sorting in ascending order, the above solution requires an extra list traversal after the sorting itself is done.
This argument can be dismissed on the grounds that
reverse’s run time, \(\Theta(n)\), is, in general, less than
sort’s run time, \(O(n \log n)\), so it’s not a big deal. Additionally, one could argue that, unlike more complex solutions, this one is “obviously correct”.
As the rest of this article explains, neither of these claims holds universally.
Here are the two ways to sort a list in descending order that I am aware of. Both require the more general
sortBy :: (a -> a -> Ordering) -> [a] -> [a]
The first argument to
sortBy is the comparison function. For each pair of arguments it returns a value of type
data Ordering = LT | EQ | GT
which describes the ordering of those arguments.
The “standard” ordering is given by the
compare function from the
Ord typeclass. Thus,
sort is nothing more than
sort = sortBy compare
The first solution to the descending sort problem exploits the fact that, to get the opposite ordering, we can simply swap around the two arguments to
sortDesc = sortBy (flip compare)
The second solution relies on the
comparing function from
Data.Ord, which gives a particular way to compare two values: map them to other values which are then compared using the standard
comparing :: Ord a => (b -> a) -> b -> b -> Ordering
This trick is often used in mathematics: to maximize a function \(x\mapsto f(x)\), it suffices to minimize the function \(x \mapsto -f(x)\). In Haskell, we could write
sortDesc = sortBy (comparing negate)
and it would work most of the time. However,
> sortDesc [1,minBound::Int] [-9223372036854775808,1]
Besides, negation only works on numbers; what if you want to sort a list of pairs of numbers?
Data.Ord defines a
Down newtype which does exactly what we want: it reverses the ordering between values that it’s applied to.
> 1 < 2 True > Down 1 < Down 2 False
Thus, the second way to sort the list in descending order is
sortDesc = sortBy (comparing Down)
Christopher King points out that the last example may be simplified with the help of the
sortOn function introduced in base 4.8 (GHC 7.10):
sortOn :: Ord b => (a -> b) -> [a] -> [a]
Thus, we can write
sortDesc = sortOn Down
Very elegant, but let’s look at how
sortOn is implemented:
sortOn f = map snd . sortBy (comparing fst) . map (\x -> let y = f x in y `seq` (y, x))
This is somewhat more complicated than
sortBy (comparing Down), and it does the extra work of first allocating \(n\) cons cells and \(n\) tuples, then allocating another \(n\) cons cells for the final result.
Thus, we might expect that
sortOn performs worse than
sortBy. Let’s check our intuition:
import Criterion import Criterion.Main import Data.List import Data.Ord list :: [Int] list = [1..10000] main = defaultMain [ bench "sort" $ nf sort (reverse list) , bench "sortBy" $ nf (sortBy (comparing Down)) list , bench "sortOn" $ nf (sortOn Down) list ]
benchmarking sort time 134.9 μs (134.3 μs .. 135.4 μs) 1.000 R² (0.999 R² .. 1.000 R²) mean 134.8 μs (134.2 μs .. 135.7 μs) std dev 2.677 μs (1.762 μs .. 3.956 μs) variance introduced by outliers: 14% (moderately inflated) benchmarking sortBy time 131.0 μs (130.6 μs .. 131.4 μs) 1.000 R² (1.000 R² .. 1.000 R²) mean 131.1 μs (130.8 μs .. 131.4 μs) std dev 965.1 ns (766.5 ns .. 1.252 μs) benchmarking sortOn time 940.5 μs (928.6 μs .. 958.1 μs) 0.998 R² (0.997 R² .. 0.999 R²) mean 950.6 μs (940.9 μs .. 961.1 μs) std dev 34.88 μs (30.06 μs .. 44.19 μs) variance introduced by outliers: 27% (moderately inflated)
As we see,
sortOn is 7 times slower than
sortBy in this example. I also included
sort in this comparison to show that
sortBy (comparing Down) has no runtime overhead.
There is a good reason why
sortOn is implemented in that way. To quote the documentation:
sortOn fis equivalent to
sortBy (comparing f), but has the performance advantage of only evaluating f once for each element in the input list. This is called the decorate-sort-undecorate paradigm, or Schwartzian transform.
f performed any non-trivial amount of work, it would be wise to cache its results — and that’s what
Down is a newtype constructor — it performs literally no work at all — so the caching effort is wasted.
Yuriy Syrovetskiy points out that there is also
sortWith defined in
GHC.Exts, which has the same type as
sortOn but does no caching. So if you want to abbreviate
sortBy (comparing Down), you can say
sortWith Down — but you need to import
sortWith lives in
GHC.Exts and not in
Data.Ord? It was originally added to aid writing SQL-like queries in Haskell, although I haven’t seen it used a single time in my career.
Thanks to Haskell’s laziness in general and the careful implementation of
sort in particular,
sort can run in linear time when only a fixed number of first elements is requested.
So this function will return the 10 largest elements in \(\Theta(n)\) time:
take 10 . sortBy (comparing Down)
While our “lazy” solution
take 10 . reverse . sort
(which, ironically, turns out not to be lazy enough — in the technical sense of the word “lazy”) will run in \(O(n \log n)\) time. This is because it requests the last 10 elements of the sorted list, and in the process of doing so needs to traverse the whole sorted list.
This may appear paradoxical if considered outside of the context of lazy evaluation. Normally, if two linear steps are performed sequentially, the result is still linear. Here we see that adding a linear step upgrades the overall complexity to \(O(n \log n)\).
As I mentioned in the beginning, the simplicity of
reverse . sort may be deceptive. The semantics of
reverse . sort and
sortBy (comparing Down) differ in a subtle way, and you probably want the semantics of
sortBy (comparing Down).
This is because
sortBy are stable sorting functions. They preserve the relative ordering of “equal” elements within the list. Often this doesn’t matter because you cannot tell equal elements apart anyway.
It starts to matter when you use
comparing to sort objects by a certain feature. Here we sort the list of pairs by their first elements in descending order:
> sortBy (comparing (Down . fst)) [(1,'a'),(2,'b'),(2,'c')] [(2,'b'),(2,'c'),(1,'a')]
According to our criterion, the elements
(2,'c') are considered equal, but we can see that their ordering has been preserved.
revese-based solution, on the other hand, reverses the order of equal elements, too:
> (reverse . sortBy (comparing fst)) [(1,'a'),(2,'b'),(2,'c')] [(2,'c'),(2,'b'),(1,'a')]
Sort with care!
The original version of this article said that
sort runs in \(\Theta(n \log n)\) time. @obadzz points out that this is not true:
sort is implemented in such a way that it will run linearly when the list is already almost sorted in any direction. Thus I have replaced \(\Theta(n \log n)\) with \(O(n \log n)\) when talking about