Hello, I recently had a problem where GHC couldn't deduce some type classes from my constraints. I would get error messages like: Could not deduce (Conflict (PatchInfoAnd p), Patchy (PatchInfoAnd p)) from the context (RepoPatch p) But, I had some instances like this (minus the function definitions): class (Patchy p, Effect p, FromPrims p, Conflict p) => RepoPatch p instance Effect p => Effect (PatchInfoAnd p) instance Conflict (p x y) => Conflict (PatchInfoAnd (p x y)) instance RepoPatch (p x y) => Patchy (PatchInfoAnd (p x y)) The x and y in the last two are just phantom types on p. When I noticed that the phantom types were missing from the instance definition of Effect (PatchInfoAnd p), I removed them from the last two definitions. This made the "Could not deduce..." error message disappear. Why do the phantom types not belong in the instance definition? I'd like to understand the problem more fully, but I'm not sure what the problem is or how to get started researching it, so if someone could point me in the right direction I'd be very happy! Thanks! Jason