On 13 Dec 2016, at 16:01, Erlend Hamberg <erlend@hamberg.no> wrote:

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 <fa-ml@ariis.it> wrote:

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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--

Erlend Hamberg

erlend@hamberg.no

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