[GHC] #10789: Notify user when a kind mismatch holds up a type family reduction

[GHC]

#10789: Notify user when a kind mismatch holds up a type family reduction
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              Reporter:  goldfire    |             Owner:
                  Type:  feature     |            Status:  new
  request                            |
              Priority:  normal      |         Milestone:
             Component:  Compiler    |           Version:  7.10.2
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
             Test Case:              |        Blocked By:
              Blocking:              |   Related Tickets:
Differential Revisions:              |
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 Consider this contrived example:

 {{{
 {-# LANGUAGE TypeFamilies, PolyKinds, UndecidableInstances #-}

 module Bug where

 import Data.Proxy

 type family F (a :: k) :: k
 type instance F a = G a

 type family G a
 type instance G a = a

 foo :: Proxy (F Maybe) -> Proxy Maybe
 foo = id
 }}}

 This (correctly) fails to compile. The error message is

 {{{
 Bug.hs:14:7:
     Couldn't match type ‘F Maybe’ with ‘Maybe’
     Expected type: Proxy (F Maybe) -> Proxy Maybe
       Actual type: Proxy Maybe -> Proxy Maybe
     In the expression: id
     In an equation for ‘foo’: foo = id
 Failed, modules loaded: none.
 }}}

 But this is peculiar, but it surely looks like `F` should be a type-level
 identity function! Of course, upon further inspection, we see that `F` is
 partial. It reduces only at kind `*`. This is quite hard to figure out,
 though, especially given that we're using the "default to `*`" behavior of
 open type families to arrange for this kind restriction.

 Thus, I propose: figure out when a type family reduction is held up due to
 a kind mismatch, and inform the user.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10789>
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