[GHC] #12594: DeriveAnyClass fails to derive some classes

13 Sep 2016

      #12594: DeriveAnyClass fails to derive some classes
-------------------------------------+-------------------------------------
           Reporter:  ivanm          |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 {{{#!hs
 {-# LANGUAGE DeriveAnyClass, DeriveGeneric #-}

 import Database.PostgreSQL.Simple
 import GHC.Generics

 data Foo = Foo { bar :: Int }
   deriving (Generic, ToRow)
 }}}

 This succeeds using `ghci`, but fails when trying to compile it using
 `ghc` with the following error message:

 {{{
     • No instance for (Database.PostgreSQL.Simple.ToField.ToField Char)
         arising from the first field of ‘Foo’ (type ‘Int’)
       Possible fix:
         use a standalone 'deriving instance' declaration,
           so you can specify the instance context yourself
     • When deriving the instance for (ToRow Foo)
 }}}

 However, it works if I use `instance ToRow Foo` instead of relying upon
 `DeriveAnyClass`.

 I've tried this with other types instead of `Int`, using `newtype` instead
 of `data` and having multiple fields in the datatype.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12594
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler