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

9 Apr 2018

      #15016: Referencing a do-bound variable in a rec block with ApplicativeDo results
in variable not in scope during type checking
-------------------------------------+-------------------------------------
           Reporter:  rjmk           |             Owner:  (none)
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:  8.4.3
          Component:  Compiler       |           Version:  8.2.2
           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:
-------------------------------------+-------------------------------------
 I searched + hope this isn't a dupe.

 When using both ApplicativeDo and RecursiveDo, referring to a do-bound
 variable from outside of a rec block causes a GHC internal error.

 Here's a minimal example:

 {{{#!hs
 {-# LANGUAGE ApplicativeDo #-}
 {-# LANGUAGE RecursiveDo #-}

 module Lib where

 import Control.Monad.Fix

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

 The error message I get is

 {{{
 src/Lib.hs:12:13: error:
     • GHC internal error: ‘a’ is not in scope during type checking, but it
 passed the renamer
       tcl_env of environment: [a1pF :-> Type variable ‘m’ = m :: * -> *,
                                r1mX :-> Identifier[f::forall (m :: * ->
 *).
                                                       MonadFix m =>
                                                       m (), TopLevelLet []
 True]]
     • In the expression: a
       In an equation for ‘b’: b = a
       In a stmt of a 'do' block: rec let b = a
    |
 12 |     let b = a
    |             ^
 }}}

 I have reproduced it in 8.2.2 and 8.4.1

-- 
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/15016
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