[GHC] #9612: Use functional dependencies to give more specific error messages

[GHC]

#9612: Use functional dependencies to give more specific error messages
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              Reporter:  rwbarton    |            Owner:
                  Type:  feature     |           Status:  new
  request                            |        Milestone:
              Priority:  normal      |          Version:  7.8.3
             Component:  Compiler    |         Keywords:
  (Type checker)                     |     Architecture:  Unknown/Multiple
            Resolution:              |       Difficulty:  Unknown
      Operating System:              |       Blocked By:
  Unknown/Multiple                   |  Related Tickets:
       Type of failure:              |
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:              |
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Comment (by Simon Peyton Jones <simonpj@…>):

 In [changeset:"20632d37b5bcb68bb0ca34238f1ed49c7be3a8f7/ghc"]:
 {{{
 #!CommitTicketReference repository="ghc"
 revision="20632d37b5bcb68bb0ca34238f1ed49c7be3a8f7"
 Do not discard insoluble Derived constraints

 This is preparing for a fix to Trac #9612. The idea is that insoluble
 constraints are nice solid errors that we should not discard before
 we have a chance to report them.  So TcRnTypes.dropDerivedWC now
 keeps insoluble Derived constrains, and instead TcSimplify.solve_wanteds
 filters them out

 We get somewhat better error message for kind-equality failures too.

 A slight downside is that to avoid *duplicate* kind-equality failures
 when we float a kind-incompatible equality (e.g.  alpha:* ~ Int#),
 I've disabled constraint-floating when there are insolubles.  But that
 in turn makes a handful of error messages a little less informative;
 good examples are mc21, mc22, mc25.  But I am re-jigging the
 constraint floating machinery in another branch, which will make this
 go back to the way it was before.
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9612#comment:3>
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