Re: [GHC] #15649: Errors about ambiguous untouchable variable when using constraint variable in RankN type

16 Sep 2018

      #15649: Errors about ambiguous untouchable variable when using constraint variable
in RankN type
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        Reporter:  infinity0         |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  Compiler          |              Version:  8.4.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by infinity0):

 Note that the error also goes away if we specialise `r` to be of kind `*`
 rather than `* -> Constraint`, for example:

 {{{#!haskell
 -- as the first snippet, but add:
 {-# LANGUAGE FlexibleContexts #-}

 -- and then:

 instance DynPS ((~) ()) ()

 loadAll3
   :: forall a r . (DynPS ((~) r) a)
   => (forall ref . r ~ ref => ref -> PSAny ((~) r))
   -> a -> Maybe a
 loadAll3 loader r = loadAll (DynLoad' loader :: DynLoad' ((~) r)) r

 testCallable3 :: IO (Maybe ())
 testCallable3 = return $ loadAll3 undefined ()
 }}}

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15649#comment:1
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