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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">Yes; but we should jolly well figure out a way to do better for guards. These guards are no more than patterns really!<o:p></o:p></span></p>
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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">Simon<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"> Richard Eisenberg [mailto:eir@cis.upenn.edu]
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<b>Sent:</b> 11 December 2015 15:59<br>
<b>To:</b> George Karachalias <george.karachalias@gmail.com><br>
<b>Cc:</b> Ben Gamari <ben@well-typed.com>; Simon Peyton Jones <simonpj@microsoft.com>; GHC developers <ghc-devs@haskell.org><br>
<b>Subject:</b> Re: Performance of pattern checker on OptCoercions<o:p></o:p></span></p>
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<p class="MsoNormal">If I understand correctly, you're saying that the problem is due to my use of guards, not anything with fancy types. If that's correct, then this sort of thing might be a trap anyone could stumble into. (The code in OptCoercion is fairly
routine for Haskell.) It seems like disabling checking guards by default may be best, but I'd love a flag (not with -Wall) to enable it optionally.<o:p></o:p></p>
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<p class="MsoNormal">Richard<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Dec 11, 2015, at 6:27 AM, George Karachalias <<a href="mailto:george.karachalias@gmail.com">george.karachalias@gmail.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">Hello Ben,<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">On Fri, Dec 11, 2015 at 11:58 AM, Ben Gamari <<a href="mailto:ben@well-typed.com" target="_blank">ben@well-typed.com</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal"><br>
Hi George,<br>
<br>
Richard has encountered a bit of a performance cliff when merging his<br>
no-kinds work. In particular OptCoercions now results in multiple<br>
gigabytes of memory consumption during compilation due to the pattern<br>
checker. The problem seems to be the opt_trans_rule binding, which has<br>
numerous equations, each of which has patterns of various complexities<br>
and guards. Might this be another case where disabling the pattern<br>
checker is unavoidable?<o:p></o:p></p>
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<p class="MsoNormal">I am afraid so. I have just responded to the ticket about it. The essence is the<o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">difference between `f` and `g` below:<o:p></o:p></p>
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<p class="MsoNormal">f x = case x of<o:p></o:p></p>
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<p class="MsoNormal"> [] -> ...<br>
(_:_) -> ...<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">g y | [] <- y = ...<o:p></o:p></p>
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<p class="MsoNormal"> | (_:_) <- y = ...<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">`f` will generate an empty uncovered set while g will generate:<o:p></o:p></p>
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<p class="MsoNormal">uncovered =<o:p></o:p></p>
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<p class="MsoNormal"> { x |> { x ~ [], x ~ (_:_) }<o:p></o:p></p>
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<p class="MsoNormal"> , x |> { x ~ (_:_), x ~ [] } }<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">which is also semantically empty but this cannot be detected until we call the<o:p></o:p></p>
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<p class="MsoNormal">term oracle on it to see the inconsistency. Since pattern guards can pattern match<o:p></o:p></p>
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<p class="MsoNormal">against <b><i>any</i></b> variable whilst case expressions match a single expression (`x` above)<o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt">I can not make the check treat them the same.<o:p></o:p></p>
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<p class="MsoNormal">From what I see, until now the pattern guards in <b><i>opt_trans_co</i></b> involved mostly<br>
pattern matching with Maybe which has only two constructors. I can easily assume<o:p></o:p></p>
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<p class="MsoNormal">that this is the reason we did not have such a problem until now. So many guards<o:p></o:p></p>
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<p class="MsoNormal">are already a challenge for the pattern match checker but maybe changing this (by<br>
this I mean to <b>not</b> use pattern guards on types with many constructors because this<br>
is the most expensive thing for the whole check) is enough to make GHC bootstrap.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">I hope this helps, I am really confident that this is the cause of the problem.<br>
Nevertheless, I will look into it more to see if I can find another source.<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">George<o:p></o:p></p>
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<p class="MsoNormal">-- <o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif;color:#660000">things you own end up owning you</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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