[GHC] #14978: GADTs don't seem to unpack properly

26 Mar 2018

      #14978: GADTs don't seem to unpack properly
-------------------------------------+-------------------------------------
           Reporter:  dfeuer         |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.6.1
          Component:  Compiler       |           Version:  8.2.2
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  Runtime
  Unknown/Multiple                   |  performance bug
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 Consider this simple code:

 {{{#!hs
 type family Foo a b

 data Goof a b where
   Goof :: !(Foo a b :~: Int) -> Goof a b
 }}}

 `(:~:)` has just one constructor (`Refl`) with no arguments, so I'd expect
 it to unpack to a void representation:

 {{{#!hs
 GoofConstr :: Foo a b ~N# Int => Goof a b
 WGoof :: Foo a b :~: Int -> Goof a b
 WGoof eq = case eq of Refl -> GoofConstr
 }}}

 But it doesn't seem to unpack at all.

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/14978
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