Re: [GHC] #12503: Template Haskell regression: GHC erroneously thinks a type variable is also a kind

20 Aug 2016

      #12503: Template Haskell regression: GHC erroneously thinks a type variable is also
a kind
-------------------------------------+-------------------------------------
        Reporter:  RyanGlScott       |                Owner:
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:
       Component:  Template Haskell  |              Version:  8.0.1
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 The above program might seem somewhat contrived, but I actually ran into
 this problem in real code. The `generic-deriving` library does something
 very similar to derive `Generic1` instances using Template Haskell (that
 is, it re-uses the kind information it gets from `reify`). Here's some
 code that triggers the same error:

 {{{#!hs
 {-# LANGUAGE DataKinds #-}
 {-# LANGUAGE PackageImports #-}
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE TemplateHaskell #-}
 {-# LANGUAGE TypeFamilies #-}
 {-# OPTIONS_GHC -ddump-splices #-}
 module Regression where

 -- Using generic-deriving-1.11
 import "generic-deriving" Generics.Deriving.TH

 data T k a
 $(deriveAll1 ''T)
 }}}

 {{{
 $ /opt/ghc/8.0.1/bin/ghc Regression.hs
 [1 of 1] Compiling Regression       ( Regression.hs, Regression.o )
 Regression.hs:13:3-16: Splicing declarations
     deriveAll1 ''T
   ======>
     instance GHC.Generics.Generic1 (T (k_avv :: k_avx) :: GHC.Types.Type
                                                           ->
 GHC.Types.Type) where
       type GHC.Generics.Rep1 (T (k_avv :: k_avx) :: GHC.Types.Type
                                                     -> GHC.Types.Type) =
 GHC.Generics.D1 (GHC.Generics.MetaData "T" "Regression" "main" False)
 GHC.Generics.V1
       GHC.Generics.from1
         = \ val_a3ph
             -> case val_a3ph of {
                  y_a3pi
                    -> GHC.Generics.M1
                         (case y_a3pi of {
                            _ -> error "No generic representation for empty
 datatype T" }) }
       GHC.Generics.to1
         = \ val_a3pj
             -> case val_a3pj of {
                  GHC.Generics.M1 y_a3pk
                    -> case y_a3pk of { _ -> error "No values for empty
 datatype T" } }

 Regression.hs:13:3: error:
     Variable ‘k_avv’ used as both a kind and a type
     Did you intend to use TypeInType?

 Regression.hs:13:3: error:
     Variable ‘k_avv’ used as both a kind and a type
     Did you intend to use TypeInType?
 }}}

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12503#comment:1
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