[GHC] #9063: Default associated type instances are too general

[GHC]

#9063: Default associated type instances are too general
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        Reporter:  goldfire          |            Owner:
            Type:  bug               |           Status:  new
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.8.2
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by simonpj):

 Can you give an example for instances?

 There is a design question here.  For a start, the associated type should
 really mention at least one of the class variables.  This should be
 prohibited:
 {{{
 class C a where
   type F b :: *
 }}}
 Second, the default instances should mention the same class variable in
 the same position, not some pattern.  For example
 {{{
 class C a where
   type F a b :: *

   type instance F a Int = Int   -- OK
   type instance F a Bool = Char -- OK

   type instance F Int a  = Bool -- Definitely not OK!
   type instance F [a] b = Char  -- Not OK
 }}}
 Reason: the default instances are simply templates, from which you can
 create the real `type instance` in the instance decl. For example
 {{{
 instance C Float
   -- type instance F Float Int  = Int
   -- type instance F Float Bool = Char
 }}}
 How would one instantiate the "not-ok" defaults here??

 We are not currently checking any of this, nor is it documented.
 [http://www.haskell.org/ghc/docs/7.8.2/html/users_guide/type-families.html
 #assoc-decl-defs Current manual entry]

 Before moving to fix it, let's just check that we agree.

 Simon

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9063#comment:2>
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