Problem when mixing type classes and type families

Question

This code compiles fine:

{-# LANGUAGE MultiParamTypeClasses, FunctionalDependencies, FlexibleInstances,
  UndecidableInstances, FlexibleContexts, EmptyDataDe         


        

Answer 1

Rec is not a type constructor; it's a type function. Maybe you can use it only in the type of a value of type definition, not in a class declaration? I'm guessing wildly here; I don't understand all the rules for type families.

I don't know how to fix it, but some things to try include:

• Get rid of class Sel and just define type family Res a s b :: *. Use type instance instead of the class mechanism.

• It's barely possible that making type Rec injective using data will help, but I don't think so.

• Cut back to the smallest number of language extensions that could possibly work—it will make it easier for others to help you, and it might help the compiler as well.