[GHC] #8470: "Fix" spurious Unused do-bind warnings

[GHC]

#8470: "Fix" spurious Unused do-bind warnings
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        Reporter:  2piix             |            Owner:
            Type:  feature request   |           Status:  closed
        Priority:  normal            |        Milestone:
       Component:  Compiler          |          Version:  7.6.3
      Resolution:  worksforme        |         Keywords:
Operating System:  Unknown/Multiple  |     Architecture:  Unknown/Multiple
 Type of failure:  None/Unknown      |       Difficulty:  Unknown
       Test Case:                    |       Blocked By:
        Blocking:                    |  Related Tickets:
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Comment (by 2piix):

 Yes, now that I've looked closer, I'm noticing that the spurious warnings
 happen when I'm performing an Acid-State update.

 The full warning is:
 {{{

 src/Yesod/Foundation.hs:476:16: Warning:
     A do-notation statement discarded a result of type EventResult
                                                          AddUser.
     Suppress this warning by saying "_ <- lift
                                             (($)
                                                acidUpdate
                                                Brilliance.Query.AddUser
                                                  (Brilliance.User.User
                                                     ((^.) auth
 Brilliance.userId) f l))"
 }}}

 EventResult AddUser ''happens'' to be (), under a type family definition.
 That is, I have code of the form

 {{{
   instance SomeClass AddUser
     type EventResult AddUser = ()
 }}}

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/8470#comment:3>
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