#13267: Constraint synonym instances -------------------------------------+------------------------------------- Reporter: ezyang | Owner: Type: bug | Status: new Priority: normal | Milestone: Component: Compiler (Type | Version: 8.1 checker) | Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC accepts | Unknown/Multiple invalid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by ezyang: @@ -70,1 +70,1 @@ - MonadSample (Impl t) a` you might prefer writing `instance MonadSample t + MonadSample (Impl t) a` you might prefer writing `instance MonadSample2 t New description: Something funny happens when you try to declare an instance of a constraint synonym: {{{ {-# LANGUAGE ConstraintKinds #-} module F where type ShowF a = Show (a -> Bool) instance ShowF Int where show _ = "Fun" }}} I get: {{{ F.hs:8:5: error: ‘show’ is not a (visible) method of class ‘ShowF’ | 8 | show _ = "Fun" | ^^^^ }}} OK, but it gets weirder. Look at: {{{ {-# LANGUAGE ConstraintKinds, FlexibleContexts, FlexibleInstances #-} module F where type ShowF a = (Show (a -> Bool)) instance ShowF Int where }}} This is accepted (with a complaint that `show` is not implemented.) It gets even more awful: {{{ {-# LANGUAGE ConstraintKinds, FlexibleContexts, FlexibleInstances, MultiParamTypeClasses #-} module F where type ShowF a = (Show Bool, Show Int) instance ShowF Int where }}} This is awful: GHC treats `Show Bool` and `Show Int` as if they were constraints, and then emits the following DFun: {{{ df9d1b4635f2a752f29ff327ab66d1cb $f(%,%)ShowShow :: (Show Bool, Show Int) DFunId {- Strictness: m, Inline: CONLIKE, Unfolding: DFun: @ a @ b. @ (Show Bool) @ (Show Int) $fShowBool $fShowInt -} }}} I don't even know what this is supposed to mean. OK, so what should we do? I think there are a few possibilities: 1. Completely outlaw instance declarations on constraint synonyms. 2. Allow instance declarations on constraint synonyms, but only if after desugaring the synonym, you end up with a single class head. I would find this useful in a few cases, for example, if you are writing `instance MonadSample (Impl t) MyMonad`, if you had `type MonadSample2 t a = MonadSample (Impl t) a` you might prefer writing `instance MonadSample2 t MyMonad` instead 3. Figure out what to do with instance declarations with multiple class heads, and proceed accordingly. -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13267#comment:2 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler