On Wed, 26 Mar 2025, Mark McConnell via Haskell-Cafe wrote:
Here is a simplified, self-contained version of some code I'm working on. I have a Ord type Foo that I want to be ordered primarily by comp1, and, in case of ties, secondarily by comp2. It is important to my users to know that the primary sorting will always be by comp1. Therefore, I am trying to use to the type system to articulate that Foo must be ordered in a two-fold lexicographic way. My code is
module Lex2Test where
class Ord a => Primary a where comp1 :: a -> a -> Ordering comp1 = compare
class Ord a => Secondary a where comp2 :: a -> a -> Ordering comp2 = compare
class (Primary a, Secondary a) => Lex2 a
instance Lex2 a => Ord a where compare x y = comp1 x y <> comp2 x y
This instance declaration means, that you want to define an Ord instance universally for all types 'a'. This includes all the types that already have Ord instances, like Int, Char, Bool etc. Did you want to define an instance on Foo instead?