[GHC] #12438: DeriveDataTypeable - deriving instance Data (Mu (Const ()))

[GHC]

#12438: DeriveDataTypeable - deriving instance Data (Mu (Const ()))
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        Reporter:  lspitzner         |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      Resolution:                    |             Keywords:  deriving
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 I had to make some changes to your code to get that exact error message.
 Here is what I tried:

 {{{#!hs
 {-# LANGUAGE DeriveDataTypeable, FlexibleInstances, StandaloneDeriving #-}
 module T12438 where

 import Control.Applicative (Const(..))
 import qualified Data.Data

 data Mu f = Mu (f (Mu f))
 deriving instance Data.Data.Data (Mu (Const ()))
 }}}

 Note the addition of `FlexibleInstances`. Also, `Const` wasn't in scope in
 your original program — I assume you are referring to the `Const` from
 `Control.Applicative`? If so, there's a simple explanation for why that
 program won't compile — `Const` doesn't have a `Data` instance. Sure
 enough, this program compiles just fine:

 {{{#!hs
 {-# LANGUAGE DeriveDataTypeable, FlexibleInstances, StandaloneDeriving #-}
 module T12438 where

 import Control.Applicative (Const(..))
 import qualified Data.Data

 deriving instance (Data.Data.Data a, Data.Data.Data b) => Data.Data.Data
 (Const a b)

 data Mu f = Mu (f (Mu f))
 deriving instance Data.Data.Data (Mu (Const ()))
 }}}

 In light of this, there's no bug in `DeriveDataTypeable` - it's just a
 Haskell library design question of whether `Const` should have a `Data`
 instance or not (I wouldn't be opposed to introducing one).

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12438#comment:3>
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