[GHC] #10185: Coercible solver incomplete with non-variable transitivity

23 Mar 2015

      #10185: Coercible solver incomplete with non-variable transitivity
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              Reporter:  goldfire    |             Owner:
                  Type:  bug         |            Status:  new
              Priority:  low         |         Milestone:  ⊥
             Component:  Compiler    |           Version:  7.11
              Keywords:              |  Operating System:  Unknown/Multiple
          Architecture:              |   Type of failure:  None/Unknown
  Unknown/Multiple                   |        Blocked By:
             Test Case:              |   Related Tickets:
              Blocking:              |
Differential Revisions:              |
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 If I say

 {{{
 import Data.Coerce
 import Data.Proxy

 foo :: (Coercible (a b) (c d), Coercible (c d) (e f)) => Proxy (c d) -> a
 b -> e f
 foo _ = coerce
 }}}

 I get an error, because GHC isn't smart enough to figure out transitivity
 in this case. It ''can'' do it with bare variables, but not variable
 applications. It's possible there's a way to do this, but we'll wait for
 someone to shout before investing effort.

 Do shout if this lack of functionality is ruining your day!

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Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/10185
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