[GHC] #12490: ApplicativeDo and RebindableSyntax do not desugar as expected

[GHC]

#12490: ApplicativeDo and RebindableSyntax do not desugar as expected
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           Reporter:  AaronFriel     |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.0.1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
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 In a module with -XApplicativeDo, -XRebindableSyntax, and local
 definitions for everything in the Functor-Applicative-Monad hierarchy, do-
 notation always desugars to "join (... (return ...))" (or
 /s/return/pure/). This forces the result to have at least the constraints
 of join, which in my case is "IxMonad m".

 {{{#!hs
 {-# LANGUAGE RebindableSyntax, ApplicativeDo #-}

 module My where

 -- straightforward definitions of fmap, pure, (<*>), join, return, (>>=),
 (>>) and fail in terms of IxFunctor, IxPointed, IxApplicative, IxMonad

 fPure m = do
   a <- m
   b <- m
   pure (a, b)

 fReturn m = do
   a <- m
   b <- m
   return (a, b)
 }}}

 According to -ddump-ds, these desugar to:

 {{{#!hs
 fPure :: IxMonad m => m k1 k1 a -> m k1 k1 (a, a)
 fPure m = My.join ( My.(<*>) (My.fmap (\a b -> My.pure (a, b)) m) m )

 fReturn :: IxMonad m => m k1 k1 a -> m k1 k1 (a, a)
 fReturn m = My.join ( My.(<*>) (My.fmap (\a b -> My.return (a, b)) m) m )
 }}}

 But I would expect:

 {{{#!hs
 fPure m = My.(<*>) (My.fmap (\a b -> (a, b)) m) m

 fReturn m = -- same
 }}}

 It appears that when "return" is not from base, ApplicativeDo only
 partially desugars to the specification.

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