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<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">I’m ok with this.
<i>It would certainly be great not to support TWO mechanisms indefinitely</i>.<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">What are the disadvantages to committing to this path? Would anyone even notice?<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">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:<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo4"><![if !supportLists]><span style="font-family:Symbol;mso-fareast-language:EN-US"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">How, when, and where in the compiler is the separate process started?<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo4"><![if !supportLists]><span style="font-family:Symbol;mso-fareast-language:EN-US"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">How do the compiler and server communicate? Unix pipes? Is it the same on Windows and Unix?<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo4"><![if !supportLists]><span style="font-family:Symbol;mso-fareast-language:EN-US"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">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?<o:p></o:p></span></p>
<p class="MsoListParagraph" style="text-indent:-18.0pt;mso-list:l0 level1 lfo4"><![if !supportLists]><span style="font-family:Symbol;mso-fareast-language:EN-US"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">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’.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-family:"Calibri",sans-serif;mso-fareast-language:EN-US">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.<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">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"> ghc-devs [mailto:ghc-devs-bounces@haskell.org]
<b>On Behalf Of </b>Simon Marlow<br>
<b>Sent:</b> 22 June 2016 09:51<br>
<b>To:</b> ghc-devs@haskell.org<br>
<b>Subject:</b> 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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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:12.0pt;margin-left:0cm">
<b>Background</b><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
A few months ago I added -fexternal-interpreter to GHC: <o:p></o:p></p>
<ul type="disc">
<li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l3 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">
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:l3 level1 lfo1">
wiki, rationale: <a href="https://ghc.haskell.org/trac/ghc/wiki/RemoteGHCi">https://ghc.haskell.org/trac/ghc/wiki/RemoteGHCi</a><o:p></o:p></li></ul>
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<p class="MsoNormal" style="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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<div>
<p class="MsoNormal" style="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="margin-bottom:12.0pt"><b>Proposal</b><o:p></o:p></p>
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<div>
<p class="MsoNormal" style="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">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:l2 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:l2 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:l2 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">How?<o:p></o:p></p>
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<ul type="disc">
<li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l1 level1 lfo3">
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">https://phabricator.haskell.org/D2286</a> and
<a href="https://ghc.haskell.org/trac/ghc/wiki/TemplateHaskell/Introspective">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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