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<p class="MsoNormal">Friends<o:p></o:p></p>
<p class="MsoNormal">As you know<o:p></o:p></p>
<ul style="margin-top:0cm" type="disc">
<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo1">We are trying to use “<a href="https://gitlab.haskell.org/ghc/ghc/wikis/implementing-trees-that-grow">Trees That Grow</a>” (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.<o:p></o:p></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo1">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.<o:p></o:p></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo1"><a href="https://gitlab.haskell.org/ghc/ghc/wikis/implementing-trees-that-grow/handling-source-locations">This wiki page</a> outlines some choices, while
<a href="https://gitlab.haskell.org/ghc/ghc/issues/15495">ticket #15495</a> has a lot of discussion.<o:p></o:p></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo1">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
<a href="https://gitlab.haskell.org/ghc/ghc/issues/17330">ticket #17330</a>)<o:p></o:p></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo1">The proposal is to move to Solution D on that page; you can see how it plays out in
<a href="https://gitlab.haskell.org/ghc/ghc/merge_requests/1970">MR !1970</a>.<o:p></o:p></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l0 level1 lfo1">(I think Solutions B and C are non-starters by comparison.)<o:p></o:p></li></ul>
<p class="MsoNormal">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.<o:p></o:p></p>
<p class="MsoNormal">Let’s try to converge by the end of the week.<o:p></o:p></p>
<p class="MsoNormal">Thanks<o:p></o:p></p>
<p class="MsoNormal">Simon<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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