Haskell: How to put multiple instances in the same module?

Question

Let\'s say that I have the following code:

import Data.List.Ordered

data Person = Person String String
     deriving (Show, Eq)

main :: IO ()
main = print          


        

Answer 1

It seems that all the functions in Data.List.Ordered have "by" variants that let you provide a comparison function rather than using an Ord instance. If there's no single obvious "standard" ordering, that's not a bad option.

It then becomes your responsibility to ensure any given list is always used with a consistent comparison function, however, which is a potential source of all kinds of fun bugs.

If I needed to work with many ordered lists using a variety of orderings, rather than fussing with newtype wrapping and unwrapping--which is dreadfully tedious--I'd consider a data type to bundle a list with its comparison function, then define proxy functions that use the "by" functions with the associated comparison.