[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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Comment (by goldfire):

 No worries about the noise -- and sorry if my initial response was a bit
 curt. This is confusing territory to be sure!

 For me, the key that unlocked all of this was to realize that instance
 selection always ignores contexts. So GHC will choose an instance based
 only on the instance head (that is, the bit to the right of `=>`). Only
 then will it try to satisfy the context.

 More elaborate instance selection control is possible with type families,
 but I'm afraid I can't find a decent, introductory example of how to do
 this.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12787#comment:4>
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