[Haskell-cafe] implementing a class using superclasses?

[Mark McConnell]

 Thank you to everyone who replied.  The thing I was misunderstanding was, to quote Jade, "This attempts to define `Ord a` in general, for *every* type `a` at once.  The `Lex2 a` constraint is only checked *after* the compiler has decided this is the instance to use."
I appreciate seeing how a Proxy constructor, or DerivingVia, would solve the problem.  However, I decided just to document carefully what I was doing.

    On Wednesday, March 26, 2025 at 06:14:00 PM EDT, Jade Hagborg <jhagborg at quasicoherent.solutions> wrote:  
 
 On Wed, 2025-03-26 at 18:08 -0400, Jade Hagborg via Haskell-Cafe wrote:
>     instance (Primary a, Secondary a) => Ord (Lex2 a) where
>         compare (MkLex2 x) (MkLex2 y) = comp1 x y <> comp2 x y

My bad (copy-paste error), should be

    instance (Primary a, Secondary a) => Ord (UsePrimaryAndSecondary a) where
        compare (MkUsePrimaryAndSecondary x) 
                (MkUsePrimaryAndSecondary y) = comp1 x y <> comp2 x y

Sorry if that caused confusion.

  
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