[GHC] #9112: support for deriving Vector/MVector instances

[GHC]

#9112: support for deriving Vector/MVector instances
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        Reporter:  jwlato            |            Owner:
            Type:  feature request   |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.8.2
      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 rwbarton):

 Replying to [comment:6 bitemyapp]:
 > I am trying to build hackage-server on Mac OS X with GHC 7.8 and I was
 told this ticket is relevant to my problems.
 >
 > I've included the build error in this comment. [...]

 These ''specific'' errors are easy to fix. `VecBase.MVector` (hereafter
 abbreviated `MVector`) is a standalone data family. There is an instance
 `MVector s Word32`, but no instance for the new type `MVector s DocId`.
 GHC 7.6.3 did not care at all about this when producing an instance
 `VecMut.MVector MVector DocId` with generalized newtype deriving, which is
 totally bogus because the derived instance has methods like `basicLength`
 of type `MVector s DocId -> Int`, even though there is no type `MVector s
 DocId` at all! A client can then define a mismatched instance `data
 instance MVector s DocId = Foo Char`, which will effectively be unsafely
 coerced to `MVector s Word32`, leading to undefined behavior (most likely
 a segfault).

 GHC 7.8 actually pays attention to what is going on here. To satisfy it
 all you need to do is define an instance for the data family in a way such
 that `MVector s DocId` is actually coercible to `MVector s Word32`. For
 example, `newtype instance MVector s DocId = DocVector (MVector s Word32)`
 will do the job. (Then you will want an instance for `VecBase.Vector`,
 too.)

 However, you will then immediately run into the other error involving the
 role of the parameter to `m` in the types of the other methods of
 `VecMut.MVector`, as discussed above.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9112#comment:10>
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