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

[GHC]

#9190: Iface type variable out of scope:  s
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        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):

 And finally, the last dependency not in GHC reduced to this module
 {{{
 {-# LANGUAGE TemplateHaskell #-}
 {-# LANGUAGE TypeFamilies #-}

 module T9190b (derivingUnbox) where

 import Control.Applicative
 import Data.Vector.Unboxed.Base (MVector (..))
 import Language.Haskell.TH

 derivingUnbox :: String -> TypeQ -> DecsQ
 derivingUnbox name argsQ = do
     let mvName = mkName $ "MV_" ++ name
     args <- argsQ
     (_, typ, rep) <- case args of
         ForallT _ cxts (ArrowT `AppT` typ `AppT` rep) -> return (cxts,
 typ, rep)
         ArrowT `AppT` typ `AppT` rep -> return ([], typ, rep)
         _ -> fail "Expecting a type of the form: cxts => typ -> rep"

     s <- VarT <$> newName "s"
     let newtypeMVector = NewtypeInstD [] ''MVector [s, typ]
             (NormalC mvName [(NotStrict, ConT ''MVector `AppT` s `AppT`
 rep)]) []

     return [ newtypeMVector]
 }}}
 to be used with
 {{{
 {-# LANGUAGE MultiParamTypeClasses, TemplateHaskell, TypeFamilies #-}
 module T9190a where

 import T9190b

 data KBNSum = KBNSum Double Double

 derivingUnbox "KBNSum"
     [t| KBNSum -> (Double, Double) |]
 }}}

 There we see a new variable `s` being introduced and put in the type of
 the newtype declaration. Is it TH’s responsibility to abstract over `s`
 here, or GHCs

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9190#comment:8>
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