[GHC] #10592: Allow cycles in class declarations

[GHC]

#10592: Allow cycles in class declarations
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        Reporter:  MikeIzbicki       |                Owner:
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler (Type    |              Version:  7.10.1
  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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Description changed by bgamari:

Old description:

> 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.

New description:

 In particular, I would like to have a class hierarchy that looks like:

 {{{#!hs
 {-# 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#comment:10>
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