[GHC] #9190: Iface type variable out of scope: s

[GHC]

#9190: Iface type variable out of scope:  s
-------------------------------------+------------------------------------
        Reporter:  nomeata           |            Owner:
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.9
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by nomeata):

 Further reduced the problem:

 {{{
 {-# LANGUAGE TemplateHaskell #-}
 {-# LANGUAGE TypeFamilies #-}
 module T9190b where

 import Language.Haskell.TH

 data family Family a b

 foo :: DecsQ
 foo = do
     s <- VarT `fmap` newName "s"
     return [
         NewtypeInstD [] ''Family [s, ConT ''Double]
             (NormalC (mkName "Foo") [(NotStrict, TupleT 0) ])
             []
         ]
 }}}
 and
 {{{
 {-# LANGUAGE TypeFamilies, TemplateHaskell #-}
 module T9190a where
 import T9190b
 foo
 }}}

 yields

 {{{
 b179ce4def4d6d8b892ce82aab2d2a37
   newtype instance T9190b.Family s GHC.Types.Double = Foo ()
     RecFlag: Recursive
 b179ce4def4d6d8b892ce82aab2d2a37
   axiom TFCo:R:FamilysDouble::
       T9190b.Family s0 GHC.Types.Double = T9190a.R:FamilysDouble s0
 family instance T9190b.Family [.], [GHC.Types.Double]
   = T9190a.TFCo:R:FamilysDouble
 }}}

 I’m off traveling to OPSLL, but I hope that this makes it easier for
 someone else to pick up the issue.

 (But maybe I’m completely off the track here, and the interface is
 actually fine – I just discovered that with `-fprint-explicit-foralls`
 this reads
 {{{
 b179ce4def4d6d8b892ce82aab2d2a37
   axiom TFCo:R:FamilysDouble::
     forall s0.
       T9190b.Family s0 GHC.Types.Double = T9190a.R:FamilysDouble s0
 }}}
 and this hiding of `forall` is new in 7.8 or 7.9 compared to 7.6, which
 had printed the `s0` directly. :-( )

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9190#comment:9>
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