Handling source locations in HsSyn via TTG

[Simon Peyton Jones]

Friends
As you know

  *   We are trying to use "Trees That Grow<https://gitlab.haskell.org/ghc/ghc/wikis/implementing-trees-that-grow>" (TTG) to move HsSyn towards a situation in which GHC is merely a client of a generic HsSyn data type that can be used by clients other than GHC.
  *   One sticking point has been the question of attaching source locations.  We used to have a "ping-pong" style, in which very node is decorated with a source location, but that's a bit more awkward in TTG.
  *   This wiki page<https://gitlab.haskell.org/ghc/ghc/wikis/implementing-trees-that-grow/handling-source-locations> outlines some choices, while ticket #15495<https://gitlab.haskell.org/ghc/ghc/issues/15495> has a lot of discussion.
  *   HEAD embodies Solution A.  But it has the disadvantage that the type system doesn't enforce locations to be present at all.   That has undesirable consequences (eg ticket #17330<https://gitlab.haskell.org/ghc/ghc/issues/17330>)
  *   The proposal is to move to Solution D on that page; you can see how it plays out in MR !1970<https://gitlab.haskell.org/ghc/ghc/merge_requests/1970>.
  *   (I think Solutions B and C are non-starters by comparison.)
If you care, please check out the design and the MR.   We can change later, of course, but doing so changes a lot of code, including client code, so we'd prefer not to.
Let's try to converge by the end of the week.
Thanks
Simon


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