[GHC] #15191: Deriving via DeriveAnyClass not behaving the same as an emply instance declaration

28 May 2018

      #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
           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:
-------------------------------------+-------------------------------------
 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

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15191
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