mike h mike_k_houghton at yahoo.co.uk
Tue Dec 13 16:15:18 UTC 2016

```Thanks folks.

Francesco
Cool - I just came to that conclusion too and did

tupleOrder :: (Char, Int) -> (Char, Int) -> Ordering
tupleOrder (c1, x1) (c2, x2)
-- char compared by ord and a is less than b!
| x1 == x2 && c1 <= c2 = GT
| x1 == x2 && c1 >= c2 = LT
| x1 < x2 = LT
| x1 > x2 = GT

and then did sortBy.

Erlend

I’ll try that - Monoids have such an understated elegance. :)

> On 13 Dec 2016, at 16:01, Erlend Hamberg <erlend at hamberg.no> wrote:
>
> There is a really nice solution that takes advantage of Ordering's Monoid instance (see https://wiki.haskell.org/Monoid <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 at ariis.it <mailto:fa-ml at 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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