[GHC] #8828: allow fully applied type families in instance heads

[GHC]

#8828: allow fully applied type families in instance heads
------------------------------------+-------------------------------------
       Reporter:  aavogt            |             Owner:
           Type:  feature request   |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.6.2
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
------------------------------------+-------------------------------------
 Hi, it would be nice if the following example were acceptable:
 {{{
 {-# LANGUAGE FlexibleInstances, TypeFamilies, TypeSynonymInstances #-}
 data X a = X a
 type family TX a
 type instance TX a = X a

 instance Show (TX Int)
 }}}

 But ghc complains that it cannot substitute the instance of TX:
 {{{
 ts.hs:6:10:
     Illegal type synonym family application in instance: TX Int
     In the instance declaration for ‛Show (TX Int)’
 }}}

 A more practical (but not self-contained) example involving extensible
 records looks http://lpaste.net/100436

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8828>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler