[GHC] #9628: Add Annotations to the AST to simplify source to source conversions

[GHC]

#9628: Add Annotations to the AST to simplify source to source conversions
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              Reporter:  alanz       |            Owner:  alanz
                  Type:  feature     |           Status:  new
  request                            |        Milestone:
              Priority:  normal      |          Version:  7.9
             Component:  Compiler    |         Keywords:
            Resolution:              |     Architecture:  Unknown/Multiple
      Operating System:              |       Difficulty:  Unknown
  Unknown/Multiple                   |       Blocked By:
       Type of failure:              |  Related Tickets:
  None/Unknown                       |
             Test Case:              |
              Blocking:              |
Differential Revisions:  D297        |
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Changes (by alanz):

 * differential:  D246 => D297
 * version:  7.8.3 => 7.9


Old description:

> Two different kinds of annotation are proposed to be added to HsSyn
>
> 1. Additional elements to explictly track the location of all syntactic
> elements not currently tracked. In particular, locations of reserved
> words and punctuation.
>
> 2. A top-level type parameter that allows a tool-writer to thread an
> application-specific annotation through the syntax tree.
>
> These two could potentially be merged, if the default annotation
> parameter is capable of tracking the first category of information.
>
> Discussion of the feature is at GhcAstAnnotations

New description:

 Add Annotations to HsSyn to explicitly track the locations of all non-
 blank source code elements to allow tools to parse a Haskell file, modify
 the AST and then produce an updated version of the source preserving the
 layout for unchanged parts.

 Discussion of the feature is at GhcAstAnnotations

 Note: an early effort was at Phab:D297, but this was abandoned as
 unworkable.

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Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/9628#comment:13>
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