[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
      Resolution:                    |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:  #9117
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Description changed by simonpj:

Old description:

> 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 ...
> }}}

New description:

 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 #9117, 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 ...
 }}}

--

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9118#comment:2>
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