[GHC] #11484: Type synonym using -XTypeInType can't be spliced with TH

23 Jan 2016

      #11484: Type synonym using -XTypeInType can't be spliced with TH
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           Reporter:  RyanGlScott    |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Template       |           Version:  8.0.1-rc1
  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:
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 The following code compiles:

 {{{#!hs
 {-# LANGUAGE TemplateHaskell #-}
 {-# LANGUAGE TypeInType #-}
 {-# OPTIONS_GHC -ddump-splices #-}
 module Foo where

 import Data.Kind

 type TySyn (k :: *) (a :: k) = ()

 -- $([d| type TySyn2 (k :: *) (a :: k) = () |])
 }}}

 But uncomment the last line, and it doesn't compile:

 {{{
 $ /opt/ghc/head/bin/ghc Foo.hs
 [1 of 1] Compiling Foo              ( Foo.hs, Foo.o )
 Foo.hs:10:3-43: Splicing declarations
     [d| type TySyn2_aBH (k_aBI :: *) (a_aBJ :: k_aBI) = () |]
   ======>
     type TySyn2_a4BF (k_a4BG :: Type) (a_a4BH :: k_a4BG) = ()

 Foo.hs:10:3: error:
     • Couldn't match expected kind ‘GHC.Prim.Any’ with actual kind ‘k1’
     • In the type declaration for ‘TySyn2’

 Foo.hs:10:3: error:
     • Kind variable ‘k_a4BG’ is implicitly bound in datatype
       ‘TySyn2’, but does not appear as the kind of any
       of its type variables. Perhaps you meant
       to bind it (with TypeInType) explicitly somewhere?
       Type variables with inferred kinds: k_a4BG (a_a4BH :: GHC.Prim.Any)
     • In the type declaration for ‘TySyn2’
 }}}

 There are two issues here:

 1. The error message claims that `TySyn2` is a datatype when it is
 actually a type synonym. This should be easy enough to fix; just change
 [http://git.haskell.org/ghc.git/blob/89bdac7635e6ed08927d760aa885d3e7ef3edb81...
 the code] that throws the error message to invoke
 [http://git.haskell.org/ghc.git/blob/89bdac7635e6ed08927d760aa885d3e7ef3edb81...
 tyConFlavour].
 2. For some reason, the type variable `a` in `TySyn2` fails to kind-check.
 Somehow, an `Any` got in there, but I'm not sure where it snuck in.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11484
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