[GHC] #11450: Associated types at wrong type in instance

[GHC]

#11450: Associated types at wrong type in instance
-------------------------------------+-------------------------------------
        Reporter:  simonpj           |                Owner:
            Type:  bug               |               Status:  new
        Priority:  high              |            Milestone:  8.0.1
       Component:  Compiler          |              Version:  7.10.3
      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:                    |
-------------------------------------+-------------------------------------

Comment (by RyanGlScott):

 > This is bogus, because the equation for `T` has the parameters to
 `Either` reversed.

 Interesting, I was not aware of such a restriction on type family
 instances. Is this documented somewhere in the
 [https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/type-
 families.html users' guide section] on type families? I was under the
 impression that as long as the instantiated type in the associated
 instance was equal to the instance head up to alpha equivalence, then it
 would be accepted, i.e., the above example is a valid type family
 instance, but the following is rejected with an error:

 {{{#!hs
 class C x where
   type T x

 instance C (Either a b) where
   type T (Either b (f a)) = b -> f a
 }}}

 {{{
     • Type indexes must match class instance head
       Found ‘Either b (f a)’ but expected ‘Either a b’
     • In the type instance declaration for ‘T’
       In the instance declaration for ‘C (Either a b)’
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11450#comment:3>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler