[GHC] #13901: Lift the "Illegal polymorphic type" restriction on type families

[GHC]

#13901: Lift the "Illegal polymorphic type" restriction on type families
-------------------------------------+-------------------------------------
           Reporter:  RyanGlScott    |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
  (Type checker)                     |
           Keywords:  TypeFamilies   |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #11962
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 Currently, I can do this:

 {{{#!hs
 type Foo = forall a. Maybe a
 }}}

 But if I try to refactor this to an equivalent type family:

 {{{#!hs
 type family Foo where
   Foo = forall a. Maybe a
 }}}

 Then GHC complains:

 {{{
     • Illegal polymorphic type: forall a. Maybe a
     • In the equations for closed type family ‘Foo’
       In the type family declaration for ‘Foo’
 }}}

 We should lift this restriction. This would be necessary, for instance, to
 implement #11962.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/13901>
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