Re: [GHC] #12430: TypeFamilyDependencies accepts invalid injectivity annotation

15 Oct 2018

      #12430: TypeFamilyDependencies accepts invalid injectivity annotation
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        Reporter:  vagarenko         |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:
       Component:  Compiler (Type    |              Version:  8.0.1
  checker)                           |
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC accepts       |  Unknown/Multiple
  invalid program                    |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by monoidal):

 Here's a simpler version:

 {{{
 #!hs
 {-# LANGUAGE TypeFamilies, PolyKinds, KindSignatures,
 TypeFamilyDependencies,
 GADTs, ScopedTypeVariables #-}

 module Bug where

 import Data.Kind (Type)

 data Proxy a

 type C a = Int

 type family Family (x :: Type) = (y :: Type) | y -> x where
     Family x  = Proxy (C x)
 }}}

 `Family` shouldn't be allowed to be injective, it's a constant function:
 `Family Int ~ Family Bool`.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12430#comment:2
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