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In addition, making pattern matching strict in this way makes it hard to mix and match strict and lazy data types (e.g. Maybe), because using a lazy data type from another module will make it appear strict in your code (hurting modularity).<o:p></o:p></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">I didn’t understand that.<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"> ghc-devs [mailto:ghc-devs-bounces@haskell.org]
<b>On Behalf Of </b>Johan Tibell<br>
<b>Sent:</b> 11 December 2015 12:22<br>
<b>To:</b> Roman Cheplyaka <roma@ro-che.info><br>
<b>Cc:</b> ghc-devs@haskell.org<br>
<b>Subject:</b> Re: -XStrict: Why some binders are not made strict?<o:p></o:p></span></p>
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I believe Scala has optional lazy values, but you could also consider in strict languages if you do manual thunking.<o:p></o:p></p>
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If we force strictness all the way down it's not really call-by-value either, because the caller doesn't know what to evaluate (I think).<o:p></o:p></p>
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In addition, making pattern matching strict in this way makes it hard to mix and match strict and lazy data types (e.g. Maybe), because using a lazy data type from another module will make it appear strict in your code (hurting modularity).<o:p></o:p></p>
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On Fri, Dec 11, 2015 at 7:54 AM, Roman Cheplyaka <<a href="mailto:roma@ro-che.info" target="_blank">roma@ro-che.info</a>> wrote:<o:p></o:p></p>
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On 12/10/2015 04:34 PM, Johan Tibell wrote:<br>
> I'm snowed under but I promise I will try to reply soon! To think about<br>
> in the mean time: what do existing strict languages with pattern<br>
> matching do?<br>
<br>
Well, strict languages do not have lazy data to force to begin with, do they?<br>
<br>
Personally, I find the simple intuition of "all patterns are strict by default" rather appealing.<br>
<br>
E.g. I wouldn't expect the expressions<br>
<br>
let (v1,v2) = a in f v2<br>
<br>
and<br>
<br>
let (v1,v2) = a; v3 = v2 in f v3<br>
<br>
to have different semantics.<br>
<br>
If we decide to adopt this semantics, we need to address the meaning of the pattern<br>
<br>
~(v1, v2)<br>
<br>
under -XStrict. Intuitively, ~ should propagate to the subpatterns. An alternative is to disallow this pattern under -XStrict and require writing all ~s explicitly, which may get tedious:<br>
<br>
~(~v1, ~v2)<br>
~(~v1, ~(~v2, ~v3))<br>
etc.<br>
<br>
We also need to ensure the consistency between this extension and the unlifted data types proposal [1], given their similarity. Interestingly, I don't see constructor patterns explained there either.<br>
<br>
[1]: <a href="https://ghc.haskell.org/trac/ghc/wiki/UnliftedDataTypes#Dynamicsemanticsofunliftedtypes" target="_blank">
https://ghc.haskell.org/trac/ghc/wiki/UnliftedDataTypes#Dynamicsemanticsofunliftedtypes</a><br>
<br>
<br>
<br>
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