[GHC] #14490: TTG Snags

[GHC]

#14490: TTG Snags
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        Reporter:  alanz             |                Owner:  (none)
            Type:  bug               |               Status:  new
        Priority:  normal            |            Milestone:
       Component:  Compiler          |              Version:  8.3
      Resolution:                    |             Keywords:
Operating System:  Unknown/Multiple  |         Architecture:
                                     |  Unknown/Multiple
 Type of failure:  None/Unknown      |            Test Case:
      Blocked By:                    |             Blocking:
 Related Tickets:  #14482            |  Differential Rev(s):
       Wiki Page:                    |
  ImplementingTreesThatGrow          |
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Comment (by alanz):

 My current thoughts on this

 As I understand it, the Data instances can only be generated for concrete
 types. As such the index type is a red herring, as we need the Data
 instances for the indexed type.

 Effectively, we are tailoring the AST to be a number of concrete versions.
 This is the point of TTG.

 So making a Data instance per concrete version may be the best way to go.
 Plan B.

 So, we know we can do that with GHC 8.2, and do not want to complicate
 life in terms of cherry-picking in the interim.

 So, perhaps either put this work on hold, or do it in a long-running
 branch until such time as GHC 8.2 is the minimal bootstrap compiler. Yay 6
 months cycles.

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/14490#comment:28>
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