"Implement the reverse function using the sort function" from https://bindthegap.news/issues/BindTheGap-01Nov2020.pdf

SortAsReverse.hs

{-# LANGUAGE ScopedTypeVariables #-}
{-# LANGUAGE TypeApplications #-}

module SortAsReverse where

-- challenge from bindthegap.news issue 1, Nov 2020
--
-- implement reverse using sort

import Data.Coerce (coerce)
import Data.List (sort)

-- | reverse a list
--
-- >>> sortAsReverse "0123456789"
-- "9876543210"
--
-- >>> sortAsReverse  "9876543210"
-- "0123456789"
--
-- Generates equivalent results (not runtime) to Prelude.reverse
--
-- prop> reverse xs == sortAsReverse xs
--
-- Highly dependent upon the implementation of sort
sortAsReverse :: forall a. [a] -> [a]
sortAsReverse = coerce (sort @(Rev a))

newtype Rev a = Rev a

instance Ord (Rev a) where
  compare _ _ = GT

instance Eq (Rev a) where
  _ == _ = False