[Haskell-beginners] Sorting

[Erlend Hamberg]

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 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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-- 
Erlend Hamberg
erlend at hamberg.no
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