[GHC] #10892: ApplicativeDo should use *> and <*

[GHC]

#10892: ApplicativeDo should use *> and <*
-------------------------------------+-------------------------------------
        Reporter:  simonmar          |                Owner:  bollu
            Type:  task              |               Status:  new
        Priority:  high              |            Milestone:  8.4.1
       Component:  Compiler          |              Version:  7.11
      Resolution:                    |             Keywords:  ApplicativeDo
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:  12143
 Related Tickets:  #13309            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by AaronFriel):

 Yes, I was wondering about that too. Edward writes: "`(*>)` is typically a
 little cheaper than `(<*)`.", which suggests using the `*>` everywhere
 except in the tail, where we would use `<*`.

 Thus:

 {{{
 do x <- foo
    bar
    y <- baz
    return ...

 ⇕

 (\x y -> ...) <$> foo <*> (bar *> baz)
 }}}

 That is, we can merge any `ApplicativeArgNil lhsExpr` with a following
 `ApplicativeArg? rhsExpr`, by omitting the `ApplicativeArgNil` and
 creating a new `ApplicativeArg? rhsExpr'`, where `rhsExpr' = lhsExpr *>
 rhsExpr`.

 In cases where there is no subsequent statement, we use `<*`.

 This rewriting has the benefit of neatly addressing adding `liftA2`, as
 `ApplicativeArgNil` statements will be folded into
 `ApplicativeArgOne`/`ApplicativeArgMany`. e.g.:

 {{{
 do foo
    x <- bar
    baz
    y <- quux
    quaffle
    return (xyzzy x y)

 ⇕

 (\x y -> xyzzy x y) <$> (foo *> bar) <*> (baz *> quux) <* quaffle

 ⇕

 liftAt (\x y -> xyzzy x y) (foo *> bar) (baz *> quux) <* quaffle
 }}}

 I'm working on the syntax and desugaring rules to address this and #13309.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10892#comment:23>
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