[GHC] #12003: Improve error message about closed variables

[GHC]

#12003: Improve error message about closed variables
-------------------------------------+-------------------------------------
           Reporter:                 |             Owner:
  facundo.dominguez                  |
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Compiler       |           Version:  8.1
           Keywords:                 |  Operating System:  Unknown/Multiple
  StaticPointers                     |
       Architecture:                 |   Type of failure:  None/Unknown
  Unknown/Multiple                   |
          Test Case:                 |        Blocked By:
           Blocking:                 |   Related Tickets:  #11656
Differential Rev(s):                 |         Wiki Page:
-------------------------------------+-------------------------------------
 After allowing local bindings in static definitions the following program
 {{{
 {-# LANGUAGE StaticPointers #-}
 module M where

 f x = static g
   where
     g = h
     h = x
 }}}
 yields the error
 {{{
 ../tmp/M.hs:4:7: error:
     • ‘g’ is used in a static form but it is not closed.
     • In the expression: static g
       In an equation for ‘f’:
           f x
             = static g
             where
                 g = h
                 h = x
 }}}
 Where it would be more helpful to get a message like
 {{{
 ../tmp/M.hs:4:7: error:
     • ‘g’ is used in a static form but it is not closed because it
       uses ‘h’ which uses ‘x’ which is not let-bound.
     • In the expression: static g
       In an equation for ‘f’:
           f x
             = static g
             where
                 g = h
                 h = x
 }}}

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/12003>
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