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<p class="MsoNormal"><span style="mso-fareast-language:EN-US">I’m a bit late to this debate, but here are a few thoughts.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<ol style="margin-top:0cm" start="1" type="1">
<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level1 lfo1">“Right now, there is one rule: if the type of any variable bound in the pattern is unlifted, then the binding is strict.” Not only is this rule simple and consistent, it is also
necessary: we cannot bind a variable of unlifted type to a thunk. So any new rule must include this, and maybe add something extra, complicating the language.<span style="mso-fareast-language:EN-US"><o:p></o:p></span></li></ol>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<ol style="margin-top:0cm" start="2" type="1">
<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level1 lfo1"><b><span style="mso-fareast-language:EN-US">All</span></b><span style="mso-fareast-language:EN-US"> lazy pattern bindings go via an intermediate tuple. If you write<o:p></o:p></span></li></ol>
<p class="MsoNormal" style="margin-left:72.0pt"><span style="mso-fareast-language:EN-US">(Just x, [y]) = e<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span style="mso-fareast-language:EN-US">then you get<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:72.0pt"><span style="mso-fareast-language:EN-US">t = case e of (Just x, [y]) -> (x,y)<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:72.0pt"><span style="mso-fareast-language:EN-US">x = fst t<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:72.0pt"><span style="mso-fareast-language:EN-US">y = snd t<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span style="mso-fareast-language:EN-US">The pattern can be complex and we don’t want to unpack it many times. Moreover, the semantics says that it must be fully matched, so even if you just want ‘x’, you must
check that the second component of the pair is indeed a singleton list.<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span style="mso-fareast-language:EN-US">So the “going via a tuple” part is nothing special about unboxed tuples – it simply holds for all lazy pattern bindings.<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<ol style="margin-top:0cm" start="3" type="1">
<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level1 lfo1"><span style="mso-fareast-language:EN-US">I don’t understand the details of Iavor’s proposal to add that “unlifted patterns are strict”, in addition to (1). Do you mean “any sub-pattern
of the LHS has an unlifted type”? So<o:p></o:p></span></li></ol>
<p class="MsoNormal" style="margin-left:72.0pt"><span style="mso-fareast-language:EN-US">Just (# a,b #) = e<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span style="mso-fareast-language:EN-US">would be strict. And even<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:72.0pt"><span style="mso-fareast-language:EN-US">MkT _ = e<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span style="mso-fareast-language:EN-US">would be strict if data T = MkT (# Int,Int #)<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<ol style="margin-top:0cm" start="4" type="1">
<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level1 lfo1"><span style="mso-fareast-language:EN-US">Anything we do *<b>must</b>* scale to when we have user-defined unlifted data types (a current GHC Proposal)<o:p></o:p></span></li></ol>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">TL;DR. At the moment a change does not look attractive to me. I could live with a warning for *<b>outermost</b>* unboxed tuples (or in general a constructor of an unlifted data type), suggesting
to add a “!” or a “~” to make the intent clear.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">Simon<o:p></o:p></span></p>
<p class="MsoListParagraph"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="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>Richard Eisenberg<br>
<b>Sent:</b> 02 September 2020 14:47<br>
<b>To:</b> Spiwack, Arnaud <arnaud.spiwack@tweag.io><br>
<b>Cc:</b> GHC developers <ghc-devs@haskell.org><br>
<b>Subject:</b> Re: Implicit reboxing of unboxed tuple in let-patterns<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
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<br>
<o:p></o:p></p>
<blockquote style="margin-top:5.0pt;margin-bottom:5.0pt">
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<p class="MsoNormal">On Sep 2, 2020, at 9:39 AM, Spiwack, Arnaud <<a href="mailto:arnaud.spiwack@tweag.io">arnaud.spiwack@tweag.io</a>> wrote:<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-size:9.0pt;font-family:"Helvetica",sans-serif">Ooh… pattern synonyms for unboxed tuple. I must confess that I don't know what the semantics of these ought to be. It does look like an interesting can of worms. How do they
currently desugar?</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">Right now, there is one rule: if the type of any variable bound in the pattern is unlifted, then the pattern is an unlifter-var pattern and is strict. The pattern must be banged, unless the bound variable is not nested. This rule is consistent
across all features.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">This thread is suggesting to add a special case -- one that seems to match intuition, but it's still a special case. And my question is: should the special case be for unboxed tuples? or should the special case be for any pattern whose
overall type is unlifted?<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Richard<o:p></o:p></p>
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