[GHC] #9118: Can't eta-reduce representational coercions

[GHC]

#9118: Can't eta-reduce representational coercions
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       Reporter:  goldfire          |             Owner:
           Type:  feature request   |            Status:  new
       Priority:  normal            |         Milestone:
      Component:  Compiler          |           Version:  7.8.2
       Keywords:                    |  Operating System:  Unknown/Multiple
   Architecture:  Unknown/Multiple  |   Type of failure:  None/Unknown
     Difficulty:  Unknown           |         Test Case:
     Blocked By:                    |          Blocking:
Related Tickets:                    |
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 When I say

 {{{
 import Data.Type.Coercion
 import Data.Coerce
 import Data.Proxy

 eta2 :: Coercible (f a) (g a) => Proxy a -> Coercion f g
 eta2 _ = Coercion
 }}}

 I get

 {{{
     Could not coerce from ‘f’ to ‘g’
       because ‘f’ and ‘g’ are different types.
       arising from a use of ‘Coercion’
     from the context (Coercible (f a) (g a))
       bound by the type signature for
                  eta2 :: Coercible (f a) (g a) => Proxy a -> Coercion f g
       at /Users/rae/temp/Roles.hs:5:9-56
     In the expression: Coercion
     In an equation for ‘eta2’: eta2 _ = Coercion
 }}}

 Unlike the case for #9116, this one is ''not'' expressible in Core. (At
 least, I don't see a way to do it.) So, to do this, we would have to
 update Core and then update the constraint solver. I ''do'' think this
 would be type safe. But, I also think it's reasonable to wait for someone
 with a real use case to shout before doing this. The only use case I have
 is to be able to do this:

 {{{
 -- says that a's parameter is representational
 class Rep (a :: k1 -> k2) where
   co :: Coercible x y => Coercible (a x) (a x)

 instance Rep f => Rep (Compose f) where ...
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9118>
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