[GHC] #13648: ApplicativeDo selects "GHC.Base.Monad.return" when actions are used without patterns.

5 May 2017

      #13648: ApplicativeDo selects "GHC.Base.Monad.return" when actions are used without
patterns.
-------------------------------------+-------------------------------------
           Reporter:  AaronFriel     |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.2.1-rc1
           Keywords:                 |  Operating System:  Unknown/Multiple
       Architecture:                 |   Type of failure:  GHC rejects
  Unknown/Multiple                   |  valid program
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 GHC 8.0.2 and 8.2.1-rc1 (rc2 not checked) have a bug where -XApplicativeDo
 causes "GHC.Base.Monad.return" to be used instead of the locally available
 "return", and a spurious "return ()" shows up. This desugaring is not
 adhering to the -XRebindableSyntax spec (see: #12490).

 Example:

 {{{#!hs
 {-# LANGUAGE NoImplicitPrelude #-}
 {-# LANGUAGE RebindableSyntax  #-}
 -- Bug vanishes if this next line is removed:
 {-# LANGUAGE ApplicativeDo  #-}

 module Main where

 import Prelude (String, print)

 class MyFunctor f where
     fmap :: (a -> b) -> f a -> f b

 class MyApplicative f where
     pure :: a -> f a
     (<*>) :: f (a -> b) -> f a -> f b

 class MyMonad m where
     return :: a -> m a
     (>>) :: m a -> m b -> m b
     (>>=) :: m a -> (a -> m b) -> m b
     fail :: String -> m a
     join :: m (m a) -> m a

 testCase1 m1 m2 = do
     m1
     m2
     return ()

 testCase2 m1 m2 = do
     _ <- m1
     _ <- m2
     return ()

 main = print "42"
 }}}

 {{{
 :t testCase1
 testCase1
   :: (MyFunctor f, MyApplicative f, MyMonad f, Monad f) =>
      f a2 -> f a1 -> f ()

 :t testCase2
   :: testCase2 :: (MyFunctor f, MyApplicative f) => f t -> f a -> f ()
 }}}

 The desugaring for testCase1 shows the issue:

 {{{#!hs
 testCase1' m1 m2 =
       (<*>)
         (fmap
            (\() () -> ())
            (m1 >> (GHC.Base.Monad.return ())))
         (m2 >> (GHC.Base.Monad.return ()))
 -- or:
 testCase1' m1 m2 = (fmap (\() () -> () ) (m1 >> (return ()))) <*> (m2 >>
 (return ()))
 }}}

 I would be able to work on this if someone pointed me in the right
 direction. It looks like it would be in `compiler/rename/RnEnv` and
 `compiler/rename/RnExpr`, as with #12490?

 As a proposed fix, I would want to implement a limited-scope fix before
 the 8.2.1 release which would not address the thornier issue of #10892.
 The patch would:

 1. Replace `GHC.Base.Monad.return` with local `pure`, removing the `Monad`
 constraint.
 2. Replace `>>` with `*>`, removing the `MyMonad` constraint.

 This isn't a _complete_ fix, as this would still induce an unnecessary use
 of the local `fmap`, but it would reduce the desugaring bug in `testCase1`
 to only that local `fmap`.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/13648
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