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<p class="MsoNormal" style="margin-left:36.0pt"><span style="font-family:"Arial",sans-serif"><CReq> must be the union of the constraints required to match the pattern, _plus_ required to build with the pattern -- if it is bidirectional.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">I think that is confusing too! How about this:
<o:p></o:p></p>
<ul type="disc">
<li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level1 lfo2">
<span style="font-family:"Cambria Math",serif">⟨</span>CReq<span style="font-family:"Cambria Math",serif">⟩</span> are the constraints
<i>required</i> to match the pattern, in a pattern match.<o:p></o:p></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level1 lfo2">
<span style="font-family:"Cambria Math",serif">⟨</span>CProv<span style="font-family:"Cambria Math",serif">⟩</span> are the constraints
<i>made available (provided)</i> by a successful pattern match.<o:p></o:p></li><li class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;mso-list:l0 level1 lfo2">
<CReq> and <CProv> are <i>both</i> <i>required</i> when P is used as a constructor in an expression.<o:p></o:p></li></ul>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto">That makes the constructor form explicit.<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:36.0pt">
<span style="font-family:"Arial",sans-serif">The only mechanism for getting the constraints needed for building is by polluting the constraints needed for matching.</span><o:p></o:p></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">Yes I agree that’s bad. It is acknowledge as such in the paper, and is the subject of accepted proposal #42.<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="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:8.0pt">PS: I am leaving Microsoft at the end of November 2021, at which point
<a href="mailto:simonpj@microsoft.com"><span style="color:#0563C1">simonpj@microsoft.com</span></a> will cease to work. Use
<a href="mailto:simon.peytonjones@gmail.com"><span style="color:#0563C1">simon.peytonjones@gmail.com</span></a> instead. (For now, it just forwards to simonpj@microsoft.com.)<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"> Anthony Clayden <anthony.d.clayden@gmail.com>
<br>
<b>Sent:</b> 06 October 2021 11:42<br>
<b>To:</b> Simon Peyton Jones <simonpj@microsoft.com><br>
<b>Cc:</b> Gergő Érdi <gergo@erdi.hu>; GHC users <glasgow-haskell-users@haskell.org><br>
<b>Subject:</b> Re: Pattern synonym constraints :: Ord a => () => ...<o:p></o:p></span></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:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<o:p> </o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:0cm">
<span style="font-family:"Arial",sans-serif">On Wed, 6 Oct 2021 at 21:24, Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com">simonpj@microsoft.com</a>> wrote:</span><o:p></o:p></p>
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<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:36.0pt">
<span style="font-family:"Arial",sans-serif"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;margin-bottom:6.0pt;margin-left:36.0pt">
<span style="font-family:"Arial",sans-serif">I suggest the User Guide needs an example where a constraint needed for matching (presumably via a View pattern) is not amongst the constraints carried inside the data constructor, nor amongst those needed for building.
Then the limitations in the current design would be more apparent for users.</span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto"><span style="font-family:"Arial",sans-serif">The
<a href="https://nam06.safelinks.protection.outlook.com/?url=https%3A%2F%2Fghc.gitlab.haskell.org%2Fghc%2Fdoc%2Fusers_guide%2Fexts%2Fpattern_synonyms.html%3Fhighlight%3Dpattern%2520syn%23typing-of-pattern-synonyms&data=04%7C01%7Csimonpj%40microsoft.com%7C941ec01fb46743eefb4d08d988b5e64a%7C72f988bf86f141af91ab2d7cd011db47%7C1%7C0%7C637691137569565464%7CUnknown%7CTWFpbGZsb3d8eyJWIjoiMC4wLjAwMDAiLCJQIjoiV2luMzIiLCJBTiI6Ik1haWwiLCJXVCI6Mn0%3D%7C2000&sdata=B7ZHX1fB5hRZkVuvId1OhnC6j5oYoqeSaD2hByBAPRM%3D&reserved=0" target="_blank">
user manual</a> does already speak about the type of a builder, here:</span><o:p></o:p></p>
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<p class="gmail-m-2913048578943876266first" style="margin-left:36.0pt"><span style="font-family:"Arial",sans-serif">...<br>
How could we make that clearer?</span><o:p></o:p></p>
<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"><span style="font-family:"Arial",sans-serif">This point in that section of the Guide is wrong/misleading:</span><o:p></o:p></p>
<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"><span style="font-family:"Arial",sans-serif"> </span><o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:auto;mso-margin-bottom-alt:auto;margin-left:54.0pt;text-indent:-18.0pt;line-height:18.0pt;mso-list:l1 level1 lfo1">
<![if !supportLists]><span style="font-size:10.0pt;font-family:Symbol;color:#404040"><span style="mso-list:Ignore">·<span style="font:7.0pt "Times New Roman"">
</span></span></span><![endif]><span style="font-family:"Cambria Math",serif;color:#404040;background:#FCFCFC">⟨</span><span style="font-family:"Arial",sans-serif;color:#404040;background:#FCFCFC">CReq</span><span style="font-family:"Cambria Math",serif;color:#404040;background:#FCFCFC">⟩</span><span style="font-family:"Arial",sans-serif;color:#404040;background:#FCFCFC">
are the constraints <em><span style="font-family:"Arial",sans-serif">required</span></em> to ma</span><span style="font-family:"Arial",sans-serif;color:#404040;background:white">tch the <span class="gmail-highlighted">pattern</span>.</span><span style="color:#404040"><o:p></o:p></span></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif"><CReq> must be the union of the constraints required to match the pattern, _plus_ required to build with the pattern -- if it is bidirectional.</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"><o:p> </o:p></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">Then thank you Simon, but it's the type of the _matcher_ that's problematic. The only mechanism for getting the constraints needed for building is by polluting the constraints needed for matching.
Here's a (crude, daft) example, using guards to 'raise' a required-for-failing-to-build that isn't required-for-successful-building nor for-matching</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"><span style="font-family:"Arial",sans-serif">> pattern TwoNode :: (Show a, Ord a) => () => a -> a -> Tree a -- GHC insists on both constraints as Req'd</span><o:p></o:p></p>
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<p class="MsoNormal" style="margin-bottom:12.0pt"><span style="font-family:"Arial",sans-serif">><br>
> pattern TwoNode x y <- Node Empty x (Leaf y) where<br>
> TwoNode x y | x > y = Node Empty x (Leaf y)<br>
> | otherwise = error (show x ++ " not greater " ++ show y)</span><o:p></o:p></p>
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<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">To quote you from May 1999</span><o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<pre><span style="font-family:"Arial",sans-serif;color:black">> But when you take a constructor *apart*, the invariant must hold<o:p></o:p></span></pre>
<pre><span style="font-family:"Arial",sans-serif;color:black">> by construction: you couldn't have built the thing you are taking<o:p></o:p></span></pre>
<pre><span style="font-family:"Arial",sans-serif;color:black">> apart unless invariant held. So enforcing the invariant again is<o:p></o:p></span></pre>
<pre><span style="font-family:"Arial",sans-serif;color:black">> redundant; and in addition it pollutes the type of selectors.<o:p></o:p></span></pre>
<pre><span style="font-family:"Arial",sans-serif;color:black"><o:p> </o:p></span></pre>
<p class="MsoNormal"><span style="font-family:"Arial",sans-serif">`Show a` must have "held by construction" of the `Node`. But the PatSyn's constraints are requiring more than that was true in some distant line of code: </span> it wants
<i>evidence</i> in the form of a dictionary at the point of deconstructing; since the build was successful, I ipso facto don't want to `show` anything in consuming it. An `instance Foldable Tree` has no mechanism to pass in any such dictionaries (which'll
anyway be redundant, as you say).<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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