[GHC] #7487: Deriving Generic1 for a type containing Either

[GHC]

#7487: Deriving Generic1 for a type containing Either
-----------------------------+----------------------------------------------
Reporter:  spl               |          Owner:  dreixel         
    Type:  bug               |         Status:  new             
Priority:  normal            |      Component:  Compiler        
 Version:  7.6.1             |       Keywords:                  
      Os:  Unknown/Multiple  |   Architecture:  Unknown/Multiple
 Failure:  None/Unknown      |      Blockedby:                  
Blocking:                    |        Related:                  
-----------------------------+----------------------------------------------
 With the following code:

 {{{
 {-# LANGUAGE DeriveGeneric #-}
 import GHC.Generics
 data T a = C (Either Int a) deriving Generic1
 }}}

 I get the error message:

 {{{
 Main.hs:3:38:
     Can't make a derived instance of `Generic1 T':
       must not apply type constructors that cannot be represented with
 `Rep1'
       (such as `Either') to arguments that involve the last type parameter
     In the data declaration for `T'
 }}}

 But when I look at the instances of `Generic1`, I see the following
 (summarized):

 {{{
 *Main> :i Generic1
 ...
 instance Generic1 (Either a) -- Defined in `GHC.Generics'
 ...
 }}}

 So, it seems that `Either a` can be represented with `Rep1`. Consequently,
 the error message is a bit confusing.

 Similarly, I can define my own version of `Either` -- e.g. `data U a b = A
 a | B b deriving Generic1` -- replace `Either` with `U` and still get the
 message. I also get this problem using `U` as `data U a b = U a b deriving
 Generic1` but not with `T` as `data T a = C (Int, a) deriving Generic1`.

 Lastly, I try this:

 {{{
 data T a = C (Either Int (T a)) deriving Generic1
 }}}

 but the error is even stranger here:

 {{{
 Main.hs:3:38:
     Can't make a derived instance of `Generic1 T':
       must not apply type constructors that cannot be represented with
 `Rep1'
       (such as `T') to arguments that involve the last type parameter
     In the data declaration for `T'
 }}}

 What's the general problem with these examples?

  * Can the error message be improved?
  * Can the deriving mechanism be extended for these types?

 I'm assuming I can write my own `Generic1` instance, though I haven't
 actually tried.

-- 
Ticket URL: <http://hackage.haskell.org/trac/ghc/ticket/7487>
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