[GHC] #15495: Handling Source Locations via TTG

[GHC]

#15495: Handling Source Locations via TTG
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           Reporter:  Shayan-Najd    |             Owner:  (none)
               Type:  task           |            Status:  new
           Priority:  normal         |         Milestone:  ⊥
          Component:  Compiler       |           Version:
           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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 == Problem ==
 The current implementation of
 [https://ghc.haskell.org/trac/ghc/wiki/ImplementingTreesThatGrow/TreesThatGrowGuidance
 TTG HsSyn AST] in GHC stores source locations for terms of a datatype
 `Exp` in a separate wrapper datatype `LExp` which is mutually recursive
 with `Exp` such that every recursive reference to `Exp` is done
 **indirectly**, via a reference to the wrapper datatype `LExp` (see the
 example code below). We refer to this style of storing source locations as
 the ping-pong style.

 Besides the indirection and the resulting complications of the ping-pong
 style, there are two key problems with it:

 a. It bakes-in the source locations in the base TTG AST, forcing all
 instances to store source locations, even if they don't need them.For
 example, TH AST does not carry source locations, or even within GHC, there
 are generated terms without source locations.

 b. It results in a form of conceptual redundancy: source locations are
 tree decorations and they belong in the extension points.
    (see
 https://ghc.haskell.org/trac/ghc/wiki/ImplementingTreesThatGrow/TreesThatGrowGuidance
 TTG Guidance])

 == Solution ==
 We can move the source location decorations to a wrapper constructor and
 remove the ping-pong style.
 This can be done smoothly, mechanically, and gradually by using a
 getter/setter methods for source locations.
 More details can be found at
 [https://ghc.haskell.org/trac/ghc/wiki/ImplementingTreesThatGrow/HandlingSourceLocations
 the related wiki page].

-- 
Ticket URL: <http://ghc.haskell.org/trac/ghc/ticket/15495>
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