[GHC] #16027: Cannot use DefaultSignatures for TypeFamiles

[GHC]

#16027: Cannot use DefaultSignatures for TypeFamiles
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        Reporter:  Ericson2314       |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.6.3
       Component:  Compiler          |              Version:  8.6.2
      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 RyanGlScott):

 Until constrained type families are implemented (which likely won't be for
 a good while), you can do this:

 {{{#!hs
 class Monad m => MyMonad m where
   type AssociatedMonad m :: Type -> Type
   type AssociatedMonad m = DefaultAssociatedMonad m

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

 type family DefaultAssociatedMonad (m :: Type -> Type) :: Type -> Type
 where
   DefaultAssociatedMonad (t m') = t (AssociatedMonad m')
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/16027#comment:1>
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