[GHC] #10853: Refine addTopDecls

[GHC]

#10853: Refine addTopDecls
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        Reporter:  goldfire          |                   Owner:
            Type:  bug               |                  Status:  new
        Priority:  normal            |               Milestone:  8.0.1
       Component:  Template Haskell  |                 Version:  7.10.2
      Resolution:                    |                Keywords:
Operating System:  Unknown/Multiple  |            Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |               Test Case:
      Blocked By:                    |                Blocking:
 Related Tickets:                    |  Differential Revisions:
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Comment (by osa1):

 Sorry if this is completely unrelated, but I realized this weird behavior
 while implementing #10891. Let's say I have this:

 {{{#!haskell
 {-# LANGUAGE TemplateHaskell #-}

 module Main where

 import Language.Haskell.TH
 import System.IO

 data D = A Int | B Bool

 test = $(... reify ''D ...)

 main :: IO ()
 main = return ()
 }}}

 This fails with:

 {{{
 Main.hs:11:5:
     ‘D’ is not in the type environment at a reify
 }}}

 But if I add this line between the splice and `D`:

 {{{
 $(return [])
 }}}

 Suddenly it works.

 I couldn't find anything about regions in the wiki, but how do we split
 this program into regions and how do we explain this seemingly weird
 behavior? It seems like we can improve the way we evaluate TH splices.

--
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/10853#comment:2>
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