[GHC] #11115: Indicate missing associated type instances

[GHC]

#11115: Indicate missing associated type instances
-------------------------------------+-------------------------------------
           Reporter:  rimmington     |             Owner:
               Type:  feature        |            Status:  new
  request                            |
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  7.10.2
  (Type checker)                     |
           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:
-------------------------------------+-------------------------------------
 If there is no matching instance for a typeclass that defines an
 associated type, GHC cannot further reduce an application of that
 associated type. For nested type family applications, this produces rather
 large error messages, eg:

 {{{
 <interactive>:98:20:
     Couldn't match type ‘units-2.3:Data.Metrology.Factor.Normalize
 (Data.Metrology.Suspicious.Poly.SurfaceUnitFactorsOf
                               (Data.Singletons.Prelude.Tuple.Snd
 (Data.Metrology.Suspicious.Poly.SplitPrefix
                                     (PrefixExp Deci) Deci Core))
                             units-2.3:Data.Metrology.Factor. at -
 '['units-2.3:Data.Metrology.Factor.F
                                                                    Core
 ('Data.Metrology.Z.S
 'Data.Metrology.Z.Zero)])’
                    with ‘'[]’
     In the expression: (25 %> centi Core) `ceilToUnit` deci Core
     In an equation for ‘it’:
         it = (25 %> centi Core) `ceilToUnit` deci Core
 }}}

 The cause of the above error message is that `Deci` is not a member of the
 typeclass `ExpUnitPrefix` that defines the associated type `PrefixExp`.
 Unfortunately this is not clear at all from the error message, and adding
 additional class constraints to the appropriate functions doesn't improve
 things.

 It would be nice if GHC pointed out that there is no `ExpUnitPrefix Deci`
 instance, or at least that `PrefixExp Deci` cannot be further reduced. I
 imagine this would be useful for open type families as well, but I can't
 comment personally.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11115>
GHC <http://www.haskell.org/ghc/>
The Glasgow Haskell Compiler