[GHC] #8177: Roles for type families

[GHC]

#8177: Roles for type families
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              Reporter:  simonpj     |            Owner:  goldfire
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.6.3
            Resolution:              |         Keywords:
      Operating System:              |     Architecture:  Unknown/Multiple
  Unknown/Multiple                   |       Difficulty:  Unknown
       Type of failure:              |       Blocked By:
  None/Unknown                       |  Related Tickets:
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by dmcclean):

 Hmmm.

 I was concerned there for a second, because actually `Quantity'` won't be
 in scope for my users. Its not exported from the module because you can
 use it to do nefarious things.

 So I thought that my test in GHCi had been insufficiently thorough,
 because GHCi doesn't respect the export list and lets you get at
 unexported things for convenience.

 So I made a new module to try it.

 {{{
 import Numeric.Units.Dimensional.DK.Prelude
 import Data.Coerce

 main = do
          let x = 3.7 :: Double
          let y = (coerce x) :: Mass Double
          putStrLn . show $ y
          -- putStrLn . show $ Quantity' x -- doesn't compile with this
 line here
 }}}

 It works just fine and prints "3.7 m". With the commented line it doesn't
 compile, complaining that `Quantity'` is (as I intended) not in scope. (If
 it had been in scope, it would still fail because the dimension is
 ambiguous, but that's not the point.)

 So it seems that whether or not the newtype constructor is in scope is not
 the determining factor here. If it was intended to be, than this may be a
 bug?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8177#comment:22>
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