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<p class="msipheader7557e74b" style="margin:0in"><span style="font-size:9.0pt;font-family:"Arial",sans-serif;color:#317100">PUBLIC</span><o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif;mso-fareast-language:EN-US"><a href="https://gitlab.haskell.org/ghc/ghc/-/issues/23647">https://gitlab.haskell.org/ghc/ghc/-/issues/23647</a><o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:10.0pt;font-family:"Arial",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
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<p class="MsoNormal"><b><span lang="EN-US">From:</span></b><span lang="EN-US"> ghc-devs <ghc-devs-bounces@haskell.org>
<b>On Behalf Of </b>Simon Peyton Jones<br>
<b>Sent:</b> Wednesday, July 12, 2023 7:25 PM<br>
<b>To:</b> Gergõ Érdi <gergo@erdi.hu><br>
<b>Cc:</b> GHC Devs <ghc-devs@haskell.org><br>
<b>Subject:</b> [External] Re: Usage of Template Haskell quotes in GHC source tree vs. usage of GHC as a library<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal" style="line-height:12.0pt"><span style="font-size:10.0pt;color:red">ATTENTION: This email came from an external source. Do not open attachments or click on links from unknown senders or unexpected emails.<strong><span style="font-family:"Calibri",sans-serif">
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Tahoma",sans-serif">Gergo<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Tahoma",sans-serif"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Tahoma",sans-serif">I'm not close enough to this to have a well-formed opinion, but it looks like a good question to me, esp if the new dependence on TH is optional.<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Tahoma",sans-serif"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Tahoma",sans-serif">Would you like to transfer the text into a GHC ticket?<o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Tahoma",sans-serif"><o:p> </o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Tahoma",sans-serif">Simon<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Wed, 12 Jul 2023 at 04:40, Gergõ Érdi <<a href="mailto:gergo@erdi.hu">gergo@erdi.hu</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">Hi,<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">A recent commit 983ce55815f2dd57f84ee86eee97febf7d80b470 starts using TemplateHaskellQuotes in the GHC codebase. It seems this is at odds with using GHC as a library, a la ghc-lib.
<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">The `ghc-lib` approach is to basically take the module hierarchy from the `compiler/` subtree, and compile it as a completely vanilla Haskell library, with no direct attachment
to the host GHC version. This enables using e.g. GHC 9.4 to compile a program using the GHC 9.6 API, and so on. In particular, it also makes it very easy to apply patches to the version of GHC used as a library, since in this setup it doesn’t need to be able
to bootstrap. <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">So what is the problem with using TemplateHaskellQuotes? The problem is the dependency on the template-haskell package. When a module inside GHC-as-a-library containing TH quotes
is compiled, the quotes are translated into applications of the constructors defined by the *host* GHC’s TH package. But because GHC is tightly coupled to the TH support library, GHC-as-a-library needs to ship with its own internal version of the library.
So the code that tries to process the results of these quotes is using the *target* GHC’s TH definitions. And that leads to a conflict: code like<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">leftName :: Namel<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">leftName = ‘Left<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><o:p> </o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">is now a type mismatch between the type of `’Left` being template-haskell-2.19.0.0:<a href="https://clicktime.symantec.com/15tStcGbBJvRFSPaKjQ96?h=rR_XOsXvthSWoiYbkJKbEwF8CkM-pT30ZpS5jSTU3fM=&u=http://Language.Haskell.TH.Syntax.Name" target="_blank">Language.Haskell.TH.Syntax.Name</a>
(example when using GHC 9.4.5 as the host) and the type of `leftName` being ghc-lib-9.9.20230712:<a href="https://clicktime.symantec.com/15tStcGbBJvRFSPaKjQ96?h=rR_XOsXvthSWoiYbkJKbEwF8CkM-pT30ZpS5jSTU3fM=&u=http://Language.Haskell.TH.Syntax.Name" target="_blank">Language.Haskell.TH.Syntax.Name</a>
(example when the target version is built from recent `master`).<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Currently, `ghc-lib-gen` has a pre-processing step on the GHC source tree that replaces these quotations with applications containing direct references to the target TH constructors:
<a href="https://clicktime.symantec.com/15tT4GfA6YHc5L3RQrCSL?h=MyTYmIJeEpi9THujD4dSHUfucPEFbsgY8PPm_sRRYV0=&u=https://github.com/digital-asset/ghc-lib/blob/ab01fb2b4d1e3a9338390e9c10ccd769bbf37aeb/ghc-lib-gen/src/Ghclibgen.hs%23L419-L467" target="_blank">
https://github.com/digital-asset/ghc-lib/blob/ab01fb2b4d1e3a9338390e9c10ccd769bbf37aeb/ghc-lib-gen/src/Ghclibgen.hs#L419-L467</a> but I am worried that this is very fragile.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">So any ideas on how to tackle this situation better?<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> <o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Thanks,<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"> Gergo<o:p></o:p></p>
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<p class="MsoNormal">_______________________________________________<br>
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<a href="https://clicktime.symantec.com/15tSySTsdvc1fPDVsHoHi?h=KfVPr9-8vQwSTBkGFfs6gvc-09ifE7PEQr-9HSL9LQo=&u=http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs" target="_blank">http://mail.haskell.org/cgi-bin/mailman/listinfo/ghc-devs</a><o:p></o:p></p>
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