[GHC] #11661: Missing MonadFail instance for Q monad from TemplateHaskell

[GHC]

#11661: Missing MonadFail instance for Q monad from TemplateHaskell
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        Reporter:  PeterTrsko        |                Owner:
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Template Haskell  |              Version:  8.0.1-rc2
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:                    |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by hvr):

 Yes... I have been wondering for some time already (which is the reason
 there is no `MonadFail` instance yet) how to properly define one given :

 {{{#!hs
 class Monad m => Quasi m where
    -- ...

 newtype Q a = Q { unQ :: forall m. Quasi m => m a }

 instance Monad Q where
   -- ...
   fail s     = report True s >> Q (fail "Q monad failure")
 }}}

 Now obviously we can't

 {{{#!hs
 instance Fail.MonadFail Q where
   -- ...
   fail = Control.Monad.fail
 }}}

 as that would call `Q`'s inner `Monad(fail)`, whereas we'd need it to call
 `Q`'s inner `MonadFail(fail)` which would require changing `Quasi`'s
 superclass, and I'm not sure that's appropriate.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/11661#comment:1>
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