[GHC] #12787: Weird type constraint with undecidable instances

[GHC]

#12787: Weird type constraint with undecidable instances
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        Reporter:  nome              |                Owner:
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |             Keywords:
      Resolution:  invalid           |  UndecidableInstances
Operating System:  Linux             |         Architecture:  x86_64
 Type of failure:  GHC rejects       |  (amd64)
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by nome):

 * status:  new => closed
 * resolution:   => invalid


Comment:

 After some more head banging and scratching, I'm heading towards the
 conclusion that I simply can't use the context of an instance declaration
 to put a restriction on the types it applies to. It looks like it should
 be possible in principle, but if that's not how the language works,
 there's little point in arguing. Unlike the minimal example, the extended
 one given above doesn't work in Hugs either. I'm still not entirely sure
 about all of this stuff, particularly the overlapping instances bit, but
 it appears that the bug was really in my interpretation.

 Sorry for the noise... I've never met a language that's both so beautiful
 and so liable to tying knots into my brain.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12787#comment:3>
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