[GHC] #12249: Template Haskell top level scoping error

30 Jun 2016

      #12249: Template Haskell top level scoping error
-------------------------------------+-------------------------------------
           Reporter:  simonpj        |             Owner:
               Type:  bug            |            Status:  new
           Priority:  normal         |         Milestone:
          Component:  Template       |           Version:  8.0.1
  Haskell                            |
           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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 Consider this
 {{{
 module TH where

   $( [d| x = True; f1 = x |])

   $( [d| x = 'c'; f2 = x |] )
 }}}
 I think this should compile fine.  There are two bindings of `x`, so I
 would not expect to be able to name `x` in the export list, or indeed
 anywhere else.  But `x` may just be a local helper-function to the other
 bindings (`f1` and `f2` resp) so that's fine.

 Alas I get
 {{{
 ~/5builds/HEAD-4/inplace/bin/ghc-stage2 -c TH.hs -ddump-splices -ddump-tc
 TH.hs:5:4-26: Splicing declarations
     [d| x_apA = True
         f1_apB = x_apA |]
   ======>
     x_a3tN = True
     f1_a3tO = x_a3tN
 TH.hs:7:4-25: Splicing declarations
     [d| x_a3u4 = 'c'
         f2_a3u5 = x_a3u4 |]
   ======>
     x_a3uW = 'c'
     f2_a3uX = x_a3uW

 TH.hs:7:4: error:
     Multiple declarations of ‘x’
     Declared at: TH.hs:5:4
                  TH.hs:7:4

 TH.hs:7:4: error:
     The exact Name ‘x_a3uW’ is not in scope
       Probable cause: you used a unique Template Haskell name (NameU),
       perhaps via newName, but did not bind it
       If that's it, then -ddump-splices might be useful
 }}}
 So:

 * Perhaps we should not complain about multiple top-level bindings for `x`
 if they are all `Exact` RdrNames.

 * And the "not in scope" message is clearly bogus.

 Mind you, would you expect this to work?
 {{{
 x = True

 $( [d| x = 'c'; f2 = x |] )

 foo = x  -- Which 'x' do we get?
 }}}
 Maybe the occurrence of `x` in `foo` is ambiguous.  If you want to splice
 a hidden helper binding, which can't clash with some earlier or later
 user-supplied thing, then use an occurrence like `$x`.

--
Ticket URL: http://ghc.haskell.org/trac/ghc/ticket/12249
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