Re: [GHC] #13154: Standalone-derived anyclass instances aren't as permissive as empty instances

22 Jan 2018

      #13154: Standalone-derived anyclass instances aren't as permissive as empty
instances
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        Reporter:  RyanGlScott       |                Owner:  RyanGlScott
            Type:  bug               |               Status:  patch
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             Keywords:  deriving
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D3337
       Wiki Page:                    |
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Comment (by RyanGlScott):

 Another component of this issue which Iceland_jack noticed is that one
 cannot standalone-derive any instances for primitive types, among which
 include `(->)`:

 {{{#!hs
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE DeriveAnyClass #-}
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE MagicHash #-}
 {-# LANGUAGE StandaloneDeriving #-}
 module Bug where

 import GHC.Exts

 class C1 a
 deriving instance C1 (a -> b)

 class C2 (a :: TYPE 'IntRep)
 deriving instance C2 Int#
 }}}
 {{{
 $ ghc/inplace/bin/ghc-stage2 Bug.hs
 [1 of 1] Compiling Bug              ( Bug.hs, Bug.o )

 Bug.hs:11:1: error:
     • Can't make a derived instance of ‘C1 (a -> b)’:
         The last argument of the instance must be a data or newtype
 application
     • In the stand-alone deriving instance for ‘C1 (a -> b)’
    |
 11 | deriving instance C1 (a -> b)
    | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

 Bug.hs:14:1: error:
     • Can't make a derived instance of ‘C2 Int#’:
         The last argument of the instance must be a data or newtype
 application
     • In the stand-alone deriving instance for ‘C2 Int#’
    |
 14 | deriving instance C2 Int#
    | ^^^^^^^^^^^^^^^^^^^^^^^^^
 }}}

 I'll lump this bug under this ticket.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13154#comment:8
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