[GHC] #11357: Regression when deriving Generic1 on poly-kinded data family

5 Jan 2016

      #11357: Regression when deriving Generic1 on poly-kinded data family
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:
               Type:  bug            |            Status:  new
           Priority:  high           |         Milestone:
          Component:  Compiler       |           Version:  8.1
  (CodeGen)                          |
           Keywords:  Generics,      |  Operating System:  Unknown/Multiple
  TypeInType                         |
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 On GHC 7.10.3, the following code compiles:

 {{{#!hs
 {-# LANGUAGE DeriveGeneric #-}
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE TypeFamilies #-}
 module ProxyFam where

 import GHC.Generics (Generic1)

 data family   ProxyFam (a :: k)
 data instance ProxyFam (a :: k) = ProxyCon deriving Generic1
 }}}

 But on GHC 8.1, it fails:

 {{{
 $ /opt/ghc/head/bin/ghc ProxyFam.hs
 [1 of 1] Compiling ProxyFam         ( ProxyFam.hs, ProxyFam.o )

 ProxyFam.hs:10:53: error:
     • Can't make a derived instance of ‘Generic1 ProxyFam’:
         ProxyFam must not be instantiated; try deriving `ProxyFam k a'
 instead
     • In the data instance declaration for ‘ProxyFam’
 }}}

 I'm not sure what exactly is going on here, but I have a hunch. The
 `Generic1` typeclass is of kind `* -> *`, which means that in a `Generic1
 ProxyFam` instance, the kind of `a` is instantiated to `*`.

 Curiously, though, the same error does ''not'' appear when `deriving
 Generic` for a normal datatype (e.g., `data ProxyFam (a :: k) = ProxyCon
 deriving Generic1`).

 Richard, I'm stuck as to how to fix this. I suspect this was triggered by
 `-XTypeInType`-related changes, specifically,
 [http://git.haskell.org/ghc.git/blobdiff/6e56ac58a6905197412d58e32792a04a63b9...
 this change]:

 {{{#!diff
 diff --git a/compiler/typecheck/TcGenGenerics.hs
 b/compiler/typecheck/TcGenGenerics.hs
 index 2c5b80e..fb18517 100644 (file)
 --- a/compiler/typecheck/TcGenGenerics.hs
 +++ b/compiler/typecheck/TcGenGenerics.hs
 @@ -147,7 +146,7 @@ canDoGenerics tc tc_args
            --
            -- Data family indices can be instantiated; the `tc_args` here
 are
            -- the representation tycon args
 -              (if (all isTyVarTy (filterOut isKind tc_args))
 +              (if (all isTyVarTy (filterOutInvisibleTypes tc tc_args))
                  then IsValid
                  else NotValid (tc_name <+> text "must not be
 instantiated;" <+>
                                 text "try deriving `" <> tc_name <+>
 tc_tys <>
 }}}

 What exactly does `filterOutInvisibleTypes` do?

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11357
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