#12731: Generic type class has type family; leads to big dep_finsts -------------------------------------+------------------------------------- Reporter: ezyang | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler | Version: 8.1 Resolution: | Keywords: Generics Operating System: Unknown/Multiple | Architecture: | Unknown/Multiple Type of failure: None/Unknown | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Changes (by RyanGlScott): * keywords: => Generics * cc: RyanGlScott (added) Comment: As you've discovered, it's possible to manually define `Generic` instances (as long as Safe Haskell isn't enabled, I might point out). But even if we restrict `Generic` instances to only be derived, I don't think that would guarantee they don't overlap. After all, you can do this (requires GHC 8.0 or later): {{{#!hs {-# LANGUAGE DeriveGeneric #-} {-# LANGUAGE FlexibleInstances #-} {-# LANGUAGE StandaloneDeriving #-} import GHC.Generics data Foo a = Foo a deriving instance {-# OVERLAPPABLE #-} Generic (Foo a) deriving instance {-# OVERLAPPING #-} Generic (Foo Int) }}} We used to forbid `Generic` instances like `Generic (Foo Int)` (see #5939), but reverted that decision when we found out that it wreaked havoc with sufficiently poly-kinded `Generic1` instances that require kinds to be instantiated with `*` (see #11732). -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12731#comment:3 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler