[GHC] #16027: Cannot use DefaultSignatures for TypeFamiles

[GHC]

#16027: Cannot use DefaultSignatures for TypeFamiles
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           Reporter:  Ericson2314    |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:  8.6.3
          Component:  Compiler       |           Version:  8.6.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 I'm surprised there wasn't a ticket for this. Maybe I just couldn't find
 it.

 In short, I want to do something like:
 {{{
 class Monad m => MyMonad m where
     type AssociatedMonad m :: Type -> Type
     default type AssociatedMonad m :: (m ~ t m', MonadTrans t, MyMonad m')
 => Type -> Type
     type AssociatedMonad (t m') = t (AssociatedMonad m')

     fun :: Foo -> m ()
     default fun :: (m ~ t m', MonadTrans t, MyMonad m') => Foo -> m ()
     fun = lift . fun
 }}}

 The syntax is a bit weird, but I hope in the process of implementing
 constrained type families the syntax properly analogous to the term-level
 would shake itself out.

 The use-case, like the normal method original, is to assist with monad
 transformer boilerplate. {{{MonadTrans}}} wouldn't be enough, but
 {{{MFunctor}}} from mmorph might be depending on the method using the
 associated type.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16027>
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