[GHC] #10592: Allow cycles in class declarations

[GHC]

#10592: Allow cycles in class declarations
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              Reporter:              |             Owner:
  MikeIzbicki                        |            Status:  new
                  Type:  feature     |         Milestone:
  request                            |           Version:  7.10.1
              Priority:  normal      |  Operating System:  Unknown/Multiple
             Component:  Compiler    |   Type of failure:  None/Unknown
  (Type checker)                     |        Blocked By:
              Keywords:              |   Related Tickets:
          Architecture:              |
  Unknown/Multiple                   |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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 In particular, I would like to have a class hierarchy that looks like:

 {{{
 {-# LANGUAGE TypeFamilies #-}
 {-# LANGUAGE FlexibleContexts #-}

 type family Scalar x

 class Field (Scalar x) => Vector x

 class Vector x => Field x
 }}}

 It is the case that every field is a vector, and the scalar associated
 with every vector is a field.  But we can't currently enforce that.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10592>
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