[GHC] #8630: Kind inference fails to account for associated types

[GHC]

#8630: Kind inference fails to account for associated types
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        Reporter:  goldfire                 |            Owner:
            Type:  bug                      |           Status:  new
        Priority:  normal                   |        Milestone:
       Component:  Compiler (Type checker)  |          Version:  7.7
      Resolution:                           |         Keywords:
Operating System:  Unknown/Multiple         |     Architecture:
 Type of failure:  None/Unknown             |  Unknown/Multiple
       Test Case:                           |       Difficulty:  Unknown
        Blocking:                           |       Blocked By:
                                            |  Related Tickets:
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Comment (by simonpj):

 I suppose you are right here.  But I think it'll take quite a bit of code
 to do the Right Thing here.  The Right Thing is, I suppose, to walk over
 the associated-type declarations of the instance, and gather any kind
 constraints they generate.  But consider something like this:
 {{{
 instance C (T a) where
   type F (T a) b = b -> a
 }}}
 Here we must bring 'b' into scope before doing kind-checking on the RHS of
 the type instance.  And then there are data families.  This all duplicates
 the code we subsequently use for typechecking those type/data instances.
 Fiddly.

 Moreover, you could argue the same for ''value'' declarations too:
 {{{
 instance C (T a) where
    op x y = [x, y::a -> a]
 }}}
 There's a type signature deep in the term that constrains the kind of 'a'.
 It would arguably be inconsistent to look at ''type'' instances but not
 ''value'' instances.  And we really really are not going to look into the
 value terms!

 So I'm not sure it's worth fixing this.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8630#comment:1>
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