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

[GHC]

#15008: Type synonyms with hidden, determined type variables
-------------------------------------+-------------------------------------
           Reporter:  nomeata        |             Owner:  (none)
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:  Research needed
          Component:  Compiler       |           Version:  8.5
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 I found myself wanting to write (simplified)

 {{{
 type MyMonad m = (MonadReader r m, Show r)
 }}}
 but GHC does not accept this, it wants `r` as part of the type synonym. So
 why did I think this could work? Because `MonadReader` has a functional
 dependency:
 {{{
 class Monad m => MonadReader r m | m -> r
 }}}

 so if `m` determines `r`, why do I have to write `MyMonad r m` and not
 just `MyMonad m`? Can I not “hide” the fact that, somewhere internal to
 the `MyMonad` synonym, there is an additional variable `r` around?

 Clearly supporting this would need a language proposal, but I wanted to
 open a ticket here first to hear if this would even be possible to
 implement.

 ([https://stackoverflow.com/questions/29686920/is-it-possible-to-have-
 forgetful-type-synonyms-in-haskell Others have wanted this before as
 well.])

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