[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 request   |               Status:  new
        Priority:  normal            |            Milestone:  8.2.1
       Component:  Compiler          |              Version:  7.10.2
      Resolution:                    |             Keywords:  newcomer,
                                     |  TypeFamilies
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Changes (by bgamari):

 * milestone:  8.0.2 => 8.2.1


@@ -3,1 +3,1 @@
- {{{
+ {{{#!hs

New description:

 Consider this contrived example:

 {{{#!hs
 {-# 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.

--

Comment:

 This won't happen for 8.0.2.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10789#comment:9>
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