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<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:36.0pt">
Note though, it doesn't mean the same thing to say (Foo a, Bar a b) => ... as it does to say<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:36.0pt">
Foo a => Bar a b => ...<o:p></o:p></p>
<p class="MsoNormal" style="mso-margin-top-alt:6.0pt;margin-right:0cm;margin-bottom:6.0pt;margin-left:36.0pt">
The latter can use Foo a when working on Bar a b, but not Bar a b to discharge Foo a, which makes a difference when you have functional dependencies.<o:p></o:p></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US">I disagree. Can you offer a concrete example, and show that one typechecks when the other does not?<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>
<p class="MsoNormal"><span style="font-size:11.0pt;font-family:"Calibri","sans-serif";color:#1F497D;mso-fareast-language:EN-US"><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""> Edward Kmett [mailto:ekmett@gmail.com]
<br>
<b>Sent:</b> 10 November 2014 15:46<br>
<b>To:</b> Richard Eisenberg<br>
<b>Cc:</b> Simon Peyton Jones; GHC Devs<br>
<b>Subject:</b> Re: Concrete syntax for pattern synonym type signatures<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">
Note though, it doesn't mean the same thing to say (Foo a, Bar a b) => ... as it does to say<o:p></o:p></p>
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<o:p> </o:p></p>
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Foo a => Bar a b => ...<o:p></o:p></p>
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<o:p> </o:p></p>
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The latter can use Foo a when working on Bar a b, but not Bar a b to discharge Foo a, which makes a difference when you have functional dependencies.<o:p></o:p></p>
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<o:p> </o:p></p>
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So in some sense the 'pattern requires/supplies' split is just that.<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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<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">
That said, Richard's other option<o:p></o:p></p>
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<o:p> </o:p></p>
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pattern Foo a => P :: Bar a => a<o:p></o:p></p>
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<o:p> </o:p></p>
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has the benefit that it looks a bit like the old datatype contexts (but here applied to the constructor/pattern).<o:p></o:p></p>
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<o:p> </o:p></p>
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If we expect the left hand side or the right hand side to be most often trivial then that may be worth considering.<o:p></o:p></p>
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<o:p> </o:p></p>
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You'd occasionally have things like<o:p></o:p></p>
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<o:p> </o:p></p>
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pattern (Num a, Eq a) => Foo :: a<o:p></o:p></p>
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<o:p> </o:p></p>
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for <o:p></o:p></p>
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<o:p> </o:p></p>
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pattern Foo = 8<o:p></o:p></p>
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<o:p> </o:p></p>
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but most of the time they'd wind up just looking like a GADT constructor.<o:p></o:p></p>
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<o:p> </o:p></p>
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-Edward<o:p></o:p></p>
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<o:p> </o:p></p>
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On Sun, Nov 9, 2014 at 10:02 PM, Richard Eisenberg <<a href="mailto:eir@cis.upenn.edu" target="_blank">eir@cis.upenn.edu</a>> wrote:<o:p></o:p></p>
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<br>
On Nov 9, 2014, at 2:11 PM, Simon Peyton Jones <<a href="mailto:simonpj@microsoft.com">simonpj@microsoft.com</a>> wrote:<br>
><br>
> * One other possibility would be two => thus<br>
> pattern P :: (Eq b) => (Num a, Eq a) => ...blha...<br>
><br>
<br>
I should note that I can say this in 7.8.3:<br>
<br>
foo :: Show a => Eq a => a -> String<br>
foo x = show x ++ show (x == x)<br>
<br>
Note that I've separated the two constraints with a =>, not a comma. This syntax does what you might expect. (I actually believe that this is an improvement over the conventional syntax, but that's a story for another day.) For better or worse, this trick does
not work for GADT constructors (which is a weird incongruence with function type signatures), so adding the extra arrow does not really steal syntax from GADT pattern synonyms.<br>
<span style="color:#888888"><br>
<span class="hoenzb">Richard</span></span><o:p></o:p></p>
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