[GHC] #12490: With RebindableSyntax, ApplicativeDo should eliminate return/pure

[GHC]

#12490: With RebindableSyntax, ApplicativeDo should eliminate return/pure
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        Reporter:  AaronFriel        |                Owner:  simonmar
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.0.1
      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 AaronFriel):

 Please permit me some additional questions, I would like to know more
 about the compiler internals.

 I think I see why you might want to do this in the type checker, to make
 sure that `return` or `pure` has the right type before performing the
 transformation? If so, I don't know how important that is, but with
 `RebindableSyntax` and `ApplicativeDo` I would assume the user is saying,
 "I know what I'm doing and I want the transformation to be assumed to be
 lawful." And, as the `ApplicativeDo` desugaring will still contain `join`
 or `return` in many cases, incorrectly typed `pure` and `return`
 definitions ought to cause a compiler error sooner or later. Does that
 line up with your thoughts?

 When you say "`Name` of `return`", do you mean in case `return` has been
 aliased by the user? Does `Name` then correspond to what I would call the
 fully qualified name, e.g.: `Control.Monad.foo`?

 If I understand your solution correctly, will any top-level `return` or
 `pure` permit the desugaring I wrote? (Duplicated here.)

 {{{#!hs
 fPure m = do
   a <- m
   b <- m
   pure (a, b)

 -- to:

 fPure m = My.(<*>) (My.fmap (\a b -> (a, b)) m) m
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12490#comment:6>
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