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<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri",sans-serif;color:#1F497D;mso-fareast-language:EN-US">Best in the ticket
<a href="https://ghc.haskell.org/trac/ghc/ticket/11418#comment:27">https://ghc.haskell.org/trac/ghc/ticket/11418#comment:27</a>, I transferred your comment there<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"> Glasgow-haskell-users [mailto:glasgow-haskell-users-bounces@haskell.org]
<b>On Behalf Of </b>Sven Panne<br>
<b>Sent:</b> 16 February 2016 18:03<br>
<b>To:</b> glasgow-haskell-users@haskell.org<br>
<b>Subject:</b> Re: [Feedback requested]: -fhelpful-import-errors<o:p></o:p></span></p>
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2016-02-16 18:12 GMT+01:00 Tom Sydney Kerckhove <<a href="mailto:syd.kerckhove@gmail.com" target="_blank">syd.kerckhove@gmail.com</a>>:<o:p></o:p></p>
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[...] As suggested by `thomie`, I created a design proposal at<br>
<a href="https://ghc.haskell.org/trac/ghc/wiki/Proposal/HelpfulImportError" target="_blank">https://ghc.haskell.org/trac/ghc/wiki/Proposal/HelpfulImportError</a><br>
and am now looking for feedback.<o:p></o:p></p>
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[ Not sure if the feedback should be submitted here on in the corresponding ticket... ]<o:p></o:p></p>
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Just a few quick remarks:<o:p></o:p></p>
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* Whatever you do, never walk the file system tree up or down in an uncontrolled way, this will kill basically all benefits and is a show-stopper. File systems like NFS, NTFS, stuff on USB sticks etc. are so *horribly* slow when used that way that the walks
will probably dominate your compilation time. And even under Linux it's not uncommon to have a few dozen directory levels and hundreds of thousands of files below our cwd: Just check out a few repositories, have some leftovers from compilations, tons of documentations
in small HTML files etc., and this sums up quickly. Git walks up the tree, but only looking for a specific directory and will e.g. not cross mount points under normal circumstances. This is probably the limit of what you can do.<o:p></o:p></p>
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* Caching between runs will be tricky: How will you invalidate the cache? People can (and will :-) do all kinds of evil things between runs, so how can you (in-)validate the cache quicker than re-scanning the file system again?<o:p></o:p></p>
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* As a general rule to keep in mind during the design: Successful compiler runs should not pay a price. It's OK if things are a little bit slower when an error occurs, but the main use case is successful compilation. This is a bit like exceptions in most
programming language implementations: They are more or less for free when you don't use them (yes, they have a cost even then because they complicate/invalidate some compiler optimizations, but let's forget that for now), and are often costly when you actually
raise them.<o:p></o:p></p>
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Just my 2c,<o:p></o:p></p>
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S.<o:p></o:p></p>
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