[GHC] #12088: Type/data family instances in kind checking

[GHC]

#12088: Type/data family instances in kind checking
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        Reporter:  alexvieth         |                Owner:
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.2.1
       Component:  Compiler (Type    |              Version:  8.1
  checker)                           |
      Resolution:                    |             Keywords:  TypeInType
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #11348, #12239    |  Differential Rev(s):  Phab:D2272
       Wiki Page:                    |
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Comment (by simonpj):

 Here's another example of the phasing problem, this one posted by
 [https://mail.haskell.org/pipermail/glasgow-haskell-
 users/2016-September/026399.html Dave Menendez]:
 {{{
 {-# LANGUAGE TemplateHaskell, TypeInType, TypeFamilies #-}

 module DH where

 import Data.Kind (Type)

 type family K t :: Type
 type family T t :: K t -> Type

 data List

 type instance K List = Type

 type instance T List = []   -- Error on this line
 -- Error is:
 --   *   Expected kind K List -> Type, but [] has kind `* -> *`
 --         In the type `[]`
 --       In the type instance declaration for `T`
 }}}
 The reasson is the we need teh `K List` instance to typecheck the `T List`
 instance.

 Adding a separator just before the `instance T List` declaration makes it
 work

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12088#comment:30>
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