[GHC] #13262: Allow type synonym family application in instance head if it has no free variables

[GHC]

#13262: Allow type synonym family application in instance head if it has no free
variables
-------------------------------------+-------------------------------------
        Reporter:  ezyang            |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  low               |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:  backpack
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:  12680
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
-------------------------------------+-------------------------------------

Comment (by simonpj):

 Let's not do this unless there is a really compelling reason.  More
 complication.  And soon you'll want
 {{{
 type instance F (G [a]) = blah
 }}}
 to work, assuming that `G [a]` can reduce to (say) `[Tree a]`.

 This way lies madness.  Let's not complicate the language of type
 patterns.  Yet anyway.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13262#comment:10>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler