[GHC] #10751: Implement Phase 1 of MonadFail Proposal (MFP)

[GHC]

#10751: Implement Phase 1 of MonadFail Proposal (MFP)
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        Reporter:  hvr               |                   Owner:  quchen
            Type:  task              |                  Status:  patch
        Priority:  highest           |               Milestone:  8.0.1
       Component:  Compiler          |                 Version:  7.10.2
      Resolution:                    |                Keywords:  report-
                                     |  impact
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:  Phab:D1248
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Comment (by quchen):

 Discussion of the initial proposal went pretty dead soon, and there was
 close to no response for the update, so we left it at that (considering
 the few suggestions we got and that people actually cared about) and
 fleshed it out on IRC. We modified the transitional strategy based on what
 we had a bit, and it's in
 https://github.com/quchen/articles/blob/master/monad_fail.md#transitional-
 strategy, which is what I'm implementing right now.

 Objections like the purpose of failure - such exception vs. error state -
 are orthogonal to the proposal, whose main goal by a huge margin is
 getting fail out of Monad. We can revisit such issues when MonadFail is
 established, e.g. by adding a new function to the MonadFail class that is
 called by the compiler, and one that's supposed to be used by the users
 explicitly.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10751#comment:10>
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