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<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">It’s a great start, thanks<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",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" style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif"> Simon Marlow [mailto:marlowsd@gmail.com]
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<b>Sent:</b> 22 June 2016 12:32<br>
<b>To:</b> Simon Peyton Jones <simonpj@microsoft.com><br>
<b>Cc:</b> ghc-devs@haskell.org<br>
<b>Subject:</b> Re: Require -fexternal-interpreter support for future TH changes?<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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How's this? <a href="https://ghc.haskell.org/trac/ghc/wiki/Commentary/Compiler/ExternalInterpreter">
https://ghc.haskell.org/trac/ghc/wiki/Commentary/Compiler/ExternalInterpreter</a><o:p></o:p></p>
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I don't want to go into too much detail in the wiki, because details are more likely to stay current if they're in Notes in the code. There are already a few Notes (e.g.
<a href="https://phabricator.haskell.org/diffusion/GHC/browse/master/compiler/ghci/GHCi.hs;619958832cbe11096cae3dac9a0a7a5591163a00$86">
https://phabricator.haskell.org/diffusion/GHC/browse/master/compiler/ghci/GHCi.hs;619958832cbe11096cae3dac9a0a7a5591163a00$86</a>) but if anything is confusing I'll happily add more Notes.<o:p></o:p></p>
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Cheers<o:p></o:p></p>
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Simon<o:p></o:p></p>
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<o:p> </o:p></p>
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On 22 June 2016 at 11:50, Simon Marlow <<a href="mailto:marlowsd@gmail.com" target="_blank">marlowsd@gmail.com</a>> wrote:<o:p></o:p></p>
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On 22 June 2016 at 11:37, Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com" target="_blank">simonpj@microsoft.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Calibri",sans-serif">I’m ok with this.
<i>It would certainly be great not to support TWO mechanisms indefinitely</i>.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Calibri",sans-serif"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Calibri",sans-serif">What are the disadvantages to committing to this path? Would anyone even notice?</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Yes, people who are making changes to TH will need to ensure that their changes work with -fexternal-interpreter. In some cases that might mean extra work, e.g. if we do
<a href="https://ghc.haskell.org/trac/ghc/wiki/TemplateHaskell/Introspective" target="_blank">
TemplateHaskell/Introspective</a> then potentially the whole of HsSyn needs to be binary serializable. That worries me quite a lot - THSyn is big but tractable, Generic deriving handled the generation of the binary instances easily enough, but HsSyn is another
matter entirely.<o:p></o:p></p>
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<p class="MsoNormal"> <o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Calibri",sans-serif">There are a lot of moving parts to the implementation, and I for one am utterly ignorant of how it all works. I would love to see
an implementation overview, either somewhere in the code or on a wiki page. Things like:</span><o:p></o:p></p>
<p><span style="font-family:Symbol">·</span><span style="font-size:7.0pt">
</span><span style="font-family:"Calibri",sans-serif">How, when, and where in the compiler is the separate process started?</span><o:p></o:p></p>
<p><span style="font-family:Symbol">·</span><span style="font-size:7.0pt">
</span><span style="font-family:"Calibri",sans-serif">How do the compiler and server communicate? Unix pipes? Is it the same on Windows and Unix?</span><o:p></o:p></p>
<p><span style="font-family:Symbol">·</span><span style="font-size:7.0pt">
</span><span style="font-family:"Calibri",sans-serif">What is serialised, when, and how? For example, GHC has some TH code to run. Do we send a syntax tree? Or compile to bytecode and send that? Or what?</span><o:p></o:p></p>
<p><span style="font-family:Symbol">·</span><span style="font-size:7.0pt">
</span><span style="font-family:"Calibri",sans-serif">How are external references managed. E.g. if the code to be run refers to ‘map’ I’m sure we don’t serialise the code for ‘map’.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Calibri",sans-serif">I’m sure there is a lot more. E.g the [wiki:RemoteGHCi wiki page] refers to “a library implementing a message type…” but I don’t know
what that library is called or where it lives.</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">Yes, we should really have a page in the commentary with an overview of the pieces and the main implementation strategy. I'll write one.<o:p></o:p></p>
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<p class="MsoNormal">Cheers<o:p></o:p></p>
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<p class="MsoNormal"><span style="color:#888888">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" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Calibri",sans-serif"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Calibri",sans-serif">Thanks</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Calibri",sans-serif"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Calibri",sans-serif">Simon</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Calibri",sans-serif"> </span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif">From:</span></b><span lang="EN-US" style="font-size:11.0pt;font-family:"Calibri",sans-serif"> ghc-devs
[mailto:<a href="mailto:ghc-devs-bounces@haskell.org" target="_blank">ghc-devs-bounces@haskell.org</a>]
<b>On Behalf Of </b>Simon Marlow<br>
<b>Sent:</b> 22 June 2016 09:51<br>
<b>To:</b> <a href="mailto:ghc-devs@haskell.org" target="_blank">ghc-devs@haskell.org</a><br>
<b>Subject:</b> Require -fexternal-interpreter support for future TH changes?</span><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>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:12.0pt"><b>Background</b><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt">A few months ago I added -fexternal-interpreter to GHC:
<o:p></o:p></p>
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<li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l1 level1 lfo1">
docs: <a href="https://na01.safelinks.protection.outlook.com/?url=http:%2f%2fdownloads.haskell.org%2f~ghc%2flatest%2fdocs%2fhtml%2fusers_guide%2fghci.html%23ghc-flag--fexternal-interpreter&data=01%7C01%7Csimonpj%40064d.mgd.microsoft.com%7C5ca29759b5c9483e03fb08d39a7a60ea%7C72f988bf86f141af91ab2d7cd011db47%7C1&sdata=6t6FA3nUkP5FOi%2fqCBSr3GxMH2rwWNk89je1qBH9GfI%3d" target="_blank">
http://downloads.haskell.org/~ghc/latest/docs/html/users_guide/ghci.html#ghc-flag--fexternal-interpreter</a><o:p></o:p></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l1 level1 lfo1">
wiki, rationale: <a href="https://ghc.haskell.org/trac/ghc/wiki/RemoteGHCi" target="_blank">
https://ghc.haskell.org/trac/ghc/wiki/RemoteGHCi</a><o:p></o:p></li></ul>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:12.0pt">When -fexternal-interpreter is used, GHC runs interpreted code in a separate subprocess, and communicates with it using binary messages over a pipe.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:12.0pt">-fexternal-interpreter currently implements all of TH, quasi-quoting, annotations, and all the GHCi features except for some features of the debugger. It is also now implemented on Windows,
thanks to Tamar Christina.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:12.0pt"><b>Proposal</b><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:12.0pt">I'd like to propose that going forward we commit to maintaining full support for -fexternal-interpreter, with a view to making it the default.<o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Why?<o:p></o:p></p>
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<li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level1 lfo2">
-fexternal-interpreter will be a prerequisite for GHCJS support, so maintaining full support for TH in -fexternal-interpreter will ensure that everything that works with GHC works with GHCJS.<o:p></o:p></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level1 lfo2">
We will be able to make simplifications in GHC and the build system once -fexternal-interpreter is the default, because when compiling with -prof or -dynamic we won't have to compile things twice any more.<o:p></o:p></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level1 lfo2">
Ultimately we don't want to have two ways of doing everything, because that's harder to maintain.<o:p></o:p></li></ul>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">How?<o:p></o:p></p>
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I'll make all the TH and quasi-quoting tests run with and without -fexternal-interpreter, so it will break validate if one of these fails.<o:p></o:p></li></ul>
<p><b>Why now?</b><o:p></o:p></p>
<p>There are some TH changes in the pipeline that will need special attention to work with -fexternal-interpreter. e.g.
<a href="https://phabricator.haskell.org/D2286" target="_blank">https://phabricator.haskell.org/D2286</a> and
<a href="https://ghc.haskell.org/trac/ghc/wiki/TemplateHaskell/Introspective" target="_blank">
https://ghc.haskell.org/trac/ghc/wiki/TemplateHaskell/Introspective</a>, so I'd like to raise it now so we can keep the issue in mind.<o:p></o:p></p>
<p> <o:p></o:p></p>
<p>Cheers<o:p></o:p></p>
<p>Simon<o:p></o:p></p>
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