Haskell function which sorts a list of integer lists according to the length of the sublists

Question

I have a list:

[[1,2],[1,2,3],[2,3],[4],[1,2,5,4],[2,3,1]]

I want to sort this list in ascending order and length with respect to sublists

like this:

[[4],[1,2],[2,3],[1,2,3],[2,3,1],[1,2,5,4]]

I found one solution, but that is not what I want

I found this function:

import Data.List

doubleSort :: (Ord a, Num a) => [[a]] -> [[a]]
doubleSort = sortOn sum . map sort

But this sorts the list in a way which I dont want:

[[1,2],[4],[2,3],[1,2,3],[1,2,3],[1,2,4,5]]

Answer 1

sort{[By,On]} are stable sorts, so you can simply sort by the secondary critereon and then by the primary one.

> :m +Data.List
> sortOn length $ sort [[1,2],[1,2,3],[2,3],[4],[1,2,5,4],[2,3,1]]
[[4],[1,2],[2,3],[1,2,3],[2,3,1],[1,2,5,4]]

If it is already sorted lexicographically, then simply sortOn length alone will do.

Answer 2

with one sort

> sortOn (length &&& sum)

&&& is from Control.Arrow