[GHC] #9223: Type equality makes type variable untouchable

[GHC]

#9223: Type equality makes type variable untouchable
-------------------------------------+-------------------------------------
              Reporter:  Feuerbach   |            Owner:
                  Type:  bug         |           Status:  new
              Priority:  normal      |        Milestone:
             Component:  Compiler    |          Version:  7.8.2
  (Type checker)                     |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:              |  Related Tickets:
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
-------------------------------------+-------------------------------------

Comment (by tvynr):

 Ah ha!  This makes perfect sense; thank you for the excellent example and
 explanation.  In particular, this points out that whether a principled
 type exists is dependent upon some pretty incidental factors, like whether
 the output type and the input type happen to have anything in common.  I
 can see why GHC conservatively rejects both of these, since getting it to
 work would result in a somewhat fragile system.

 I had previously supposed that the problem was somehow related to the fact
 that I had two data -- the `TokenType` and the payload -- and that the `a`
 in the first parameter of `SomeToken` (`t a`) was not being unified with
 the `a` in the second parameter (`a`) for the purposes of inference.  I
 think I had gotten than impression by the names `a` and `a1` without
 realizing that one of them arose merely due to the use of `+` in the body.

 Thanks again!

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9223#comment:9>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler