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

[GHC]

#8828: allow fully applied type families in instance heads
-------------------------------------+------------------------------------
        Reporter:  aavogt            |            Owner:  goldfire
            Type:  feature request   |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.6.2
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
-------------------------------------+------------------------------------
Changes (by simonpj):

 * owner:   => goldfire


Comment:

 Seems very reasonable.   The rule would be this: you normalise the
 instance type, and check that there are no remaining type family calls.
 After all, with `-XFlexibleInstances` you can use type synonyms in
 instance heads, and this is just an extension of the same idea.

 c.f. http://www.haskell.org/ghc/docs/latest/html/users_guide/type-class-
 extensions.html#flexible-instance-head

 Richard will work on this in due course.

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8828#comment:1>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler