#15191: Deriving via DeriveAnyClass not behaving the same as an emply instance declaration -------------------------------------+------------------------------------- Reporter: Darwin226 | Owner: (none) Type: bug | Status: new Priority: normal | Milestone: 8.6.1 Component: Compiler | Version: 8.2.2 Resolution: | Keywords: Operating System: Unknown/Multiple | Architecture: Type of failure: GHC rejects | Unknown/Multiple valid program | Test Case: Blocked By: | Blocking: Related Tickets: | Differential Rev(s): Wiki Page: | -------------------------------------+------------------------------------- Description changed by Darwin226: Old description:

I've opened [https://stackoverflow.com/questions/50557019/deriving-via- deriveanyclass-not-behaving-the-same-as-an-emply-instance-declarati a question on StackOverflow] describing the issue. I'll copy it here:

----

I have the following code

{{{#!hs {-# LANGUAGE PolyKinds, DefaultSignatures, FlexibleContexts, DeriveAnyClass, DeriveGeneric #-} {-# LANGUAGE FlexibleInstances, MultiParamTypeClasses, UndecidableInstances #-} module DeriveTest where

import GHC.Generics

class GenericClass a m where instance GenericClass f m => GenericClass (M1 i c f) m instance Condition a m => GenericClass (K1 i a) m

class Condition (a :: k) (m :: * -> *) where instance (Condition a m, Condition b m) => Condition (a b) m instance {-# OVERLAPPABLE #-} Condition (a :: k) m

class Class (e :: (* -> *) -> *) where classF :: e m -> () default classF :: GenericClass (Rep (e m)) m => e m -> () classF = undefined }}}

It defines the class Class of types that have a higher-kinded type as a parameter. It also defines a generic way to derive an instance of that class. Now if I declare a new datatype like this, and try to derive an instance of Class

{{{#!hs data T a m = T { field :: a } deriving (Generic, Class) }}}

I get the following error:

{{{ * Overlapping instances for Condition a m arising from the 'deriving' clause of a data type declaration Matching instances: instance [overlappable] forall k (a :: k) (m :: * -> *). Condition a m instance forall k1 k2 (a :: k1 -> k2) (m :: * -> *) (b :: k1). (Condition a m, Condition b m) => Condition (a b) m (The choice depends on the instantiation of `a, m' To pick the first instance above, use IncoherentInstances when compiling the other instance declarations) * When deriving the instance for (Class (T a)) | 22 | deriving (Generic, Class) | ^^^^^ }}}

The error sort of makes sense because I guess. The instance really does depend on the instantiation of a. However, if I just write an empty instance like this:

{{{#!hs data T a m = T { field :: a } deriving (Generic) instance Class (T a) -- works }}}

It works. Why? And how can I make it behave the same with the deriving statement?

----

Ryan Scott suggested I open a ticket and that the issue probably isn't with the deriving mechanisms. Still, I chose to keep the title because that's what the original problem was and I've seen [https://github.com/GetShopTV/swagger2/issues/144 similar issues] before

New description: I've opened [https://stackoverflow.com/questions/50557019/deriving-via- deriveanyclass-not-behaving-the-same-as-an-emply-instance-declarati a question on StackOverflow] describing the issue. I'll copy it here: ---- I have the following code {{{#!hs {-# LANGUAGE PolyKinds, DefaultSignatures, FlexibleContexts, DeriveAnyClass, DeriveGeneric #-} {-# LANGUAGE FlexibleInstances, MultiParamTypeClasses, UndecidableInstances #-} module DeriveTest where import GHC.Generics class GenericClass a m where instance GenericClass f m => GenericClass (M1 i c f) m instance Condition a m => GenericClass (K1 i a) m class Condition (a :: k) (m :: * -> *) where instance (Condition a m, Condition b m) => Condition (a b) m instance {-# OVERLAPPABLE #-} Condition (a :: k) m class Class (e :: (* -> *) -> *) where classF :: e m -> () default classF :: GenericClass (Rep (e m)) m => e m -> () classF = undefined }}} It defines the class Class of types that have a higher-kinded type as a parameter. It also defines a generic way to derive an instance of that class. Now if I declare a new datatype like this, and try to derive an instance of Class {{{#!hs data T a m = T { field :: a } deriving (Generic, Class) }}} I get the following error: {{{ * Overlapping instances for Condition a m arising from the 'deriving' clause of a data type declaration Matching instances: instance [overlappable] forall k (a :: k) (m :: * -> *). Condition a m instance forall k1 k2 (a :: k1 -> k2) (m :: * -> *) (b :: k1). (Condition a m, Condition b m) => Condition (a b) m (The choice depends on the instantiation of `a, m' To pick the first instance above, use IncoherentInstances when compiling the other instance declarations) * When deriving the instance for (Class (T a)) | 22 | deriving (Generic, Class) | ^^^^^ }}} The error sort of makes sense I guess. The instance really does depend on the instantiation of a. However, if I just write an empty instance like this: {{{#!hs data T a m = T { field :: a } deriving (Generic) instance Class (T a) -- works }}} It works. Why? And how can I make it behave the same with the deriving statement? ---- Ryan Scott suggested I open a ticket and that the issue probably isn't with the deriving mechanisms. Still, I chose to keep the title because that's what the original problem was and I've seen [https://github.com/GetShopTV/swagger2/issues/144 similar issues] before -- -- Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15191#comment:1 GHC http://www.haskell.org/ghc/ The Glasgow Haskell Compiler