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

21 Jan 2016

      #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 Iceland_jack):

 Should this be rejected?

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

 instance C (Either a b) (Maybe c) where
   type T (Either a b) (Maybe b) = a
 }}}

 GHC 8.1.20160117 gives:

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

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/11450#comment:9
GHC http://www.haskell.org/ghc/
The Glasgow Haskell Compiler