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<p class="MsoNormal" style="margin-left:36.0pt">I think this is the consistent way to interpret your rule (1) that unlifted bindings are always strict<o:p></o:p></p>
<p class="MsoNormal" style="margin-left:36.0pt"><o:p> </o:p></p>
<p class="MsoNormal">But that’s not what rule (1) says. It says that <b>a pattern binding is strict iff it binds a variable of unlifted type</b>.
<o:p></o:p></p>
<p class="MsoNormal" style="margin-left:5.25pt"><o:p> </o:p></p>
<p class="MsoNormal">Now I think we agree that your proposal says that <b>a pattern binding is strict iff it or any of its sub-patterns has unlifted type,
</b>including wild-cards, variables, and constructor patterns; in fact <i>any</i> sub-pattern. Call that (2).<o:p></o:p></p>
<p class="MsoNormal" style="margin-left:5.25pt"><o:p> </o:p></p>
<p class="MsoNormal" style="margin-left:5.25pt"><span style="mso-fareast-language:EN-US">So
<o:p></o:p></span></p>
<ul style="margin-top:0cm" type="disc">
<li class="MsoListParagraph" style="margin-left:5.25pt;mso-list:l0 level1 lfo2"><span style="mso-fareast-language:EN-US">(1) is
<i>necessary</i>. <o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:5.25pt;mso-list:l0 level1 lfo2"><span style="mso-fareast-language:EN-US">(2) is strictly stronger, and will make fewer program defined. But is perhaps less surprising.<o:p></o:p></span></li></ul>
<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">I think you could make a proposal out of that if you wanted. I can’t decide if I like it, myself, but I think that it, too, is simple and consistent.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><br>
Simon<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"> Iavor Diatchki <iavor.diatchki@gmail.com>
<br>
<b>Sent:</b> 07 September 2020 20:46<br>
<b>To:</b> Simon Peyton Jones <simonpj@microsoft.com><br>
<b>Cc:</b> Richard Eisenberg <rae@richarde.dev>; Spiwack, Arnaud <arnaud.spiwack@tweag.io>; GHC developers <ghc-devs@haskell.org><br>
<b>Subject:</b> Re: !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>
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<o:p> </o:p></p>
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<o:p> </o:p></p>
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On Mon, Sep 7, 2020 at 5:12 AM Simon Peyton Jones via ghc-devs <<a href="mailto:ghc-devs@haskell.org" target="_blank">ghc-devs@haskell.org</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"><o:p> </o:p></p>
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<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">
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”? I think this is fully compatible with unlifted user defined data<o:p></o:p></li></ol>
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Just (# a,b #) = e<o:p></o:p></p>
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would be strict. And even<o:p></o:p></p>
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MkT _ = e<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:36.0pt">
would be strict if data T = MkT (# Int,Int #)<o:p></o:p></p>
<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"><o:p> </o:p></p>
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<p class="MsoNormal">Yes, the first one would be strict up to the tuple, and the second one would also be strict. I think this is the consistent way to interpret your rule (1) that unlifted bindings are always strict, and it shouldn't really matter if you
used a variable pattern, or a wild card pattern. I don't think there's any other part of the language where replacing a `_` with an unused name changes the semantics of the program, and I don't think it should in this case either.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Just to be fully explicit, the thing I find odd with GHC's current behavior is that these two are different:<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">let MkT x = undefined in () --> undefined<o:p></o:p></p>
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<p class="MsoNormal">let MkT _ = undefined in () --> ()<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Even more explicitly:<o:p></o:p></p>
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<p class="MsoNormal">let (_ :: Int#) = undefined in () --> () -- the value `undefined` is not representable in type `Int#` but GHC is happy to proceed because it doesn't need to represent it<o:p></o:p></p>
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<p class="MsoNormal">let (x :: Int#) = undefined in () --> () -- same situation, but now GHC is strict, even though it still doesn't need to represent the value.<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 think that the consistent behavior is for all of these to diverge, because laziness does not mix with unlfited values, at least in the presence of non-termination.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">-Iavor<o:p></o:p></p>
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<p class="MsoNormal"><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"> <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">From:</span></b><span lang="EN-US"> ghc-devs <<a href="mailto:ghc-devs-bounces@haskell.org" target="_blank">ghc-devs-bounces@haskell.org</a>>
<b>On Behalf Of </b>Richard Eisenberg<br>
<b>Sent:</b> 02 September 2020 14:47<br>
<b>To:</b> Spiwack, Arnaud <<a href="mailto:arnaud.spiwack@tweag.io" target="_blank">arnaud.spiwack@tweag.io</a>><br>
<b>Cc:</b> GHC developers <<a href="mailto:ghc-devs@haskell.org" target="_blank">ghc-devs@haskell.org</a>><br>
<b>Subject:</b> Re: Implicit reboxing of unboxed tuple in let-patterns</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>
<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"><o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">On Sep 2, 2020, at 9:39 AM, Spiwack, Arnaud <<a href="mailto:arnaud.spiwack@tweag.io" target="_blank">arnaud.spiwack@tweag.io</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-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" 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;mso-margin-bottom-alt:auto">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" 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;mso-margin-bottom-alt:auto">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" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">Richard<o:p></o:p></p>
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<p class="MsoNormal">_______________________________________________<br>
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