[GHC] #15008: Type synonyms with hidden, determined type variables

[GHC]

#15008: Type synonyms with hidden, determined type variables
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        Reporter:  nomeata           |                Owner:  (none)
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  Research
                                     |  needed
       Component:  Compiler          |              Version:  8.5
      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 Iceland_jack):

 Did you have something like that in mind?

 {{{#!hs
 -- user writes
 type MyMonad m = (MonadReader r m, Show r)
 }}}

 but GHC knows that `r` is determined by `m` so silently

 {{{#!hs
 -- GHC transforms it into
 type MyMonad m = (forall xx. (MonadReader xx m, Show xx))
 }}}

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15008#comment:3>
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