Implicit parameters
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The GHC Users Guide has an Implicit Parameters section.
Working example:
{-# LANGUAGE ImplicitParams #-}
import Data.List (sortBy)
sortBy' :: (?cmp :: a -> a -> Ordering) => [a] -> [a]
sortBy' = sortBy ?cmp
sort :: Ord a => [a] -> [a] sort = let ?cmp = compare in sortBy' main = putStrLn (show (sort [3,1,2]))