[GHC] #15546: Display coaxiom branch incompatibilities in GHCi

[GHC]

#15546: Display coaxiom branch incompatibilities in GHCi
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        Reporter:  mniip             |                Owner:  mniip
            Type:  feature request   |               Status:  new
        Priority:  normal            |            Milestone:  8.6.1
       Component:  GHCi              |              Version:  8.4.3
      Resolution:                    |             Keywords:  TypeFamilies,
                                     |  GHCi
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):  Phab:D5097
       Wiki Page:                    |
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Comment (by goldfire):

 Compatibility is a user-facing issue. See
 [https://downloads.haskell.org/~ghc/latest/docs/html/users_guide/glasgow_exts.html
 #compatibility-and-apartness-of-type-family-equations the manual]. Here is
 an example:

 {{{#!hs
 type family b1 && b2 where
   True  && b2    = b2
   False && b2    = False
   b1    && True  = b1
   b2    && False = False
   b     && b     = b
 }}}

 {{{
 λ> :kind! forall b. b && True
 forall b. b && True :: Bool
 = b
 }}}

 Note that the first two equations unify with the target `b && True`.
 Normally, this means that any later equations wouldn't apply. But because
 the first two equations are both compatible with the third, we don't let
 the fact that they unify hold up reduction via the third equation, as we
 can see.

 I thus agree that the ability to print out incompatibilities is sometimes
 helpful for users, not just implementors.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15546#comment:9>
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