[GHC] #9334: Implement "instance chains"

[GHC]

#9334: Implement "instance chains"
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        Reporter:  diatchki          |                Owner:  diatchki
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  7.9
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by Iceland_jack):

 Replying to [comment:9 aavogt]:
 > In some sense this can already be done:
 > {{{
 > class FailHasNoInstances a => Fail a
 > class FailHasNoInstances a -- not exported to ban Fail instances
 >
 > instance Fail "Char may not have a Num instance" => Num Char
 > main = print $ '1' + '1'
 > }}}

 Is there a difference between using `Fail` and Proposal/CustomTypeErrors?
 (see #11967)

 {{{#!hs
 instance TypeError (Text "Boo-urns!") => Num Char
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9334#comment:14>
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