[GHC] #10675: GHC does not check the functional dependency consistency condition correctly

[GHC]

#10675: GHC does not check the functional dependency consistency condition
correctly
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        Reporter:  simonpj           |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.10.1
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by goldfire):

 Simon says:
 > I'm not sure what the right thing here is.

 I say that we should reject these programs. They're bogus! This might make
 some affected people complain, but the situation seems similar to what
 happened between 7.6 and 7.8 where the FD check was tightened (most
 recently). Some people's programs stopped working, but it was all for a
 good reason.

 The examples all look like an attempt to encode instance chains. GHC has
 hitherto failed to notice the overlap, but it's been there all along.
 Happily, each of these examples could straightforwardly (if verbosely and
 unpleasantly) be refactored to use closed type families.

 Or, if we ever get around to finishing the work for #8634, folks who want
 the old behavior can just call their functional dependency to be
 dysfunctional (which it really is) and get on with it.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10675#comment:6>
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