[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       |           Version:  8.0.1
  Haskell                            |
           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:
-------------------------------------+-------------------------------------
 The following program compiles without issue on GHC 7.6.3 through GHC
 7.10.3, but fails to compile on GHC 8.0.1 and GHC HEAD. (I added a CPP
 guard simply because the `DataD` constructor changed between 7.10 and
 8.0.)

 {{{#!hs
 {-# LANGUAGE CPP #-}
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE TemplateHaskell #-}
 {-# OPTIONS_GHC -ddump-splices #-}
 module Regression where

 import Language.Haskell.TH

 data T k
 class C a

 $(do TyConI (DataD [] tName [ KindedTV kName kKind]
 #if __GLASGOW_HASKELL__ >= 800
                    _
 #endif
                    _ _) <- reify ''T
      d <- instanceD (cxt []) (conT ''C `appT` (conT tName `appT` sigT
 (varT kName) kKind)) []
      return [d])
 }}}

 {{{
 $ /opt/ghc/8.0.1/bin/ghc Regression.hs
 [1 of 1] Compiling Regression       ( Regression.hs, Regression.o )
 Regression.hs:(13,3)-(19,15): Splicing declarations
     do { TyConI (DataD []
                        tName_a2RT
                        [KindedTV kName_a2RU kKind_a2RV]
                        _
                        _
                        _) <- reify ''T;
          d_a31Y <- instanceD
                      (cxt [])
                      (conT ''C
                       `appT` (conT tName_a2RT `appT` sigT (varT
 kName_a2RU) kKind_a2RV))
                      [];
          return [d_a31Y] }
   ======>
     instance C (T (k_avB :: k_avC))

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

 Somewhat confusingly, you can use `instance C (T (k_avB :: k_avC))` on its
 own, and it will compile without issue. Also, quoting it doesn't seem to
 trip up this bug, as this also compiles without issue:

 {{{#!hs
 {-# LANGUAGE CPP #-}
 {-# LANGUAGE KindSignatures #-}
 {-# LANGUAGE PolyKinds #-}
 {-# LANGUAGE TemplateHaskell #-}
 {-# OPTIONS_GHC -ddump-splices #-}
 module WorksSomehow where

 import Language.Haskell.TH

 data T k
 class C a

 $([d| instance C (T k) |])
 }}}

 The original program has several workarounds:

 1. Turn off `PolyKinds` (OK, this isn't a very good workaround...)
 2. Add an explicit kind signature to `T`, e.g., `data T (k :: k1)`
 3. Change the type variable of `T` to some other letter, e.g., `data T p`
 or `data T k1`

 Given that (3) is a successful workaround, I strongly suspect that GHC is
 confusing the type variable `k` (which gets `ddump-splice`d as `k_avB`)
 with the kind variable `k` (which gets `ddump-splice`d as `k_avC`) due to
 their common prefix `k`.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12503
GHC http://www.haskell.org/ghc/
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