There is a really nice solution that takes advantage of Ordering's Monoid
instance (see https://wiki.haskell.org/Monoid).
The imports you need:
import Data.List (sortBy)
import Data.Ord (Down(..), comparing)
import Data.Monoid ((<>)) -- the “mappend” operator
You can then combine two calls to `comparing`
sortBy (comparing (Down . snd) <> comparing fst) xs
(`Down` is just a newtype that reverses the ordering, since you wanted the
first element in descending order and the second in ascending order.)
On Tue, 13 Dec 2016 at 16:30 Francesco Ariis
On Tue, Dec 13, 2016 at 02:36:39PM +0000, mike h wrote:
Hi,
I’m trying to sort a list of tuples. A char and a count of that char (Char , Int) e.g.
[ ('r',2), ('c',2),('a', 2), ('b',3), ('f',2)]
e.g. ‘r’ occurs twice etc. The order should be based on the count first and then ties broken by the natural ordering of char.
You should provide sortBy with an appropriate compare function, e.g.
comp (a,b) (c,d) | a > c = GT | -- etc etc.
or go with the manky but working hack:
λ> :m Data.List λ> sortOn (\(a, b) -> b*(-100) + fromEnum a) [('r',2), ('c',2),('a', 2), ('b',3), ('f',2)] [('b',3),('a',2),('c',2),('f',2),('r',2)]
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