[GHC] #15016: Referencing a do-bound variable in a rec block with ApplicativeDo results in variable not in scope during type checking

[GHC]

#15016: Referencing a do-bound variable in a rec block with ApplicativeDo results
in variable not in scope during type checking
-------------------------------------+-------------------------------------
        Reporter:  rjmk              |                Owner:  sighingnow
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:  8.4.3
       Component:  Compiler          |              Version:  8.2.2
      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:                    |
-------------------------------------+-------------------------------------
Changes (by sighingnow):

 * owner:  (none) => sighingnow


Comment:

 I think the "not is scope" is by design. See more discussion under
 ticket:4148. When we use `rec` in `do` blocks, rather than `mdo` blocks,
 the outer variables won't be included into the scope of `rec` block.

 The real problem is when there's no binding for `a` inside `rec` block,
 the rhs `a` in `let b = a` will have the same name with the outer `a`.

 For code:

 {{{#!hs
 }}}

 The rename produces:

 {{{#!hs
 Lib.f :: MonadFix m_auv => m_auv ()
 Lib.f
   = do a_auw <- return () |
        () <- do rec let b_aux = a_auw
                 return ()
        return ()
 }}}

 But for code:

 {{{#!hs
 f :: MonadFix m => m ()
 f = do
   a <- return ()
   rec
     let b = a
         a = ()
   return ()
 }}}

 The rename produces:

 {{{#!hs
 Lib.f :: MonadFix m_auv => m_auv ()
 Lib.f
   = do a_auw <- return () |
        () <- do rec let b_aux = a_auy
                         a_auy = ()
                 return ()
        return ()
 }}}

 Indeed it's a bug. I'm optimistic to assign that to myself :) We also
 should note this behavior of `rec` in documentation.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15016#comment:1>
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