Ordering a list of lists in Haskell

Question

Say if I had a list of lists like [[1,3,4],[1,5,6,7],[2,8,0]] or ["QQQ", "RRRR", "TTTTT"] , is there a function that will order them by the number of elements in inner lists i.e. so in the Int list, the 4 element list goes to the front and in the Strings list, the Ts go to the front and then the Rs?

Answer 1

Use sortBy with a custom predicate:

Prelude> import Data.List
Prelude Data.List> let l = [[1,3,4],[1,5,6,7],[2,8,0]]
Prelude Data.List> sortBy (\e1 e2 -> compare (length e2) (length e1))  l
[[1,5,6,7],[1,3,4],[2,8,0]]

Edit: Thanks @JJJ for a more beautiful variant

Prelude Data.List> import Data.Ord
Prelude Data.List Data.Ord> sortBy (flip $ comparing length) l
[[1,5,6,7],[1,3,4],[2,8,0]]

Answer 2

sortBy from Data.List and comparing from Data.Ord will help you.

foo = sortBy (comparing (negate . length))

bar = foo ["QQQ", "RRRR", "TTTTT"]