[GHC] #10836: Better error reporting for closed type families

[GHC]

#10836: Better error reporting for closed type families
-------------------------------------+-------------------------------------
        Reporter:  jstolarek         |                   Owner:
            Type:  bug               |                  Status:  closed
        Priority:  normal            |               Milestone:
       Component:  Compiler          |                 Version:  7.11
      Resolution:  fixed             |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  Incorrect         |               Test Case:
  warning at compile-time            |  typecheck/should_fail/T10836
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
-------------------------------------+-------------------------------------

Comment (by jstolarek):

 Simon, this change is a huge improvement over the previous situation, but
 not everything is yet perfect. Consider this:

 {{{#!hs
 type family FClosed a b c = (result :: *) | result -> a b c where
     FClosed Int  Char Bool = Bool
     FClosed Char Bool Int  = Int
     FClosed Bool Int  Char = Int

 type family Bar a = r | r -> a where
     Bar Int  = Bool
     Bar Bool = Int
     Bar Bool = Char

 bar :: Bar a -> Bar a
 bar x = x

 barapp :: Char
 barapp = bar 'c'
 }}}
 I would expect to get two errors:
 {{{
 T6018.hs:8:5: error:
     Type family equations violate injectivity annotation:
       FClosed Char Bool Int = Int
       FClosed Bool Int Char = Int
     In the equations for closed type family ‘FClosed’
     In the type family declaration for ‘FClosed’

 T6018.hs:20:14: error:
     Couldn't match expected type ‘Bar a0’ with actual type ‘Char’
     The type variable ‘a0’ is ambiguous
     In the first argument of ‘bar’, namely ‘'c'’
     In the expression: bar 'c'
     In an equation for ‘barapp’: barapp = bar 'c'
 }}}
 But I only get the first one. Is this expected?

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10836#comment:4>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler