[GHC] #8884: Reifying closed type families is broken

[GHC]

#8884: Reifying closed type families is broken
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        Reporter:  goldfire          |            Owner:  goldfire
            Type:  bug               |           Status:  merge
        Priority:  normal            |        Milestone:  7.8.1
       Component:  Template Haskell  |          Version:  7.8.1-rc2
      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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Changes (by goldfire):

 * status:  new => merge


Comment:

 The fix affects ''open'' type families as well as closed. Both families
 had exactly the same mis-behavior, upon inspection of the code.

 Given that, it's unclear to me whether this should be merged. The old
 behavior was clearly wrong -- for example, reifying a type family and then
 trying to splice it would end in failure. But, the old behavior was also
 quite reliable, and it's conceivable that users have noticed the poor
 behavior and have coded against it. (In fact, a co-author of one of my
 libraries has done just that, which is how I discovered the GHC bug!)

 So, we must choose between
 * Retaining the incorrect behavior (which has been around for some time),
 while not surprising users in the final 7.8 release, or
 * Having correct behavior.

 I slightly favor the latter, believing that this error was so egregious
 that no one (other than me, evidently) is using this feature of GHC. If
 anyone else were using the feature, we would have seen the bug report!

 So, I'm setting this ticket to `merge` but am happy to have others
 disagree about that decision.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8884#comment:2>
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