[GHC] #15777: Ordering of code in file affects compilation

[GHC]

#15777: Ordering of code in file affects compilation
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        Reporter:  chessai           |                Owner:  (none)
            Type:  bug               |               Status:  closed
        Priority:  normal            |            Milestone:  8.6.2
       Component:  Compiler          |              Version:  8.6.1
      Resolution:  duplicate         |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
 Type of failure:  GHC rejects       |  Unknown/Multiple
  valid program                      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #12088            |  Differential Rev(s):
       Wiki Page:                    |
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Comment (by RyanGlScott):

 When kind-checking, GHC performs strongly-connected component (SCC)
 analysis to determine which declarations depend on the presence of other
 declarations. In your example, the `type Unlifted Stuff = Stuff#` instance
 declaration depends on the `type instance Rep Stuff = TupleRep '[ 'IntRep,
 'IntRep ]` instance declaration in order to kind-check, so a proper SCC
 analysis should put the former declaration //after// the latter one.
 Because of #12088, however, this does not happen correctly, and these
 declarations get processed out of dependency order.

 The use of a Template Haskell splice (such as `$(pure [])`) is a gruesome
 hack which forces the declarations following the splice to be processed
 //after// the declarations preceding the splice. This is often an annoying
 weakness of Template Haskell, but in this particular case, it happens to
 work to our advantage :)

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15777#comment:3>
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