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<p class="MsoNormal"><span style="mso-fareast-language:EN-US">Yes I like the “\” too. (Or some syntax separating inputs from outputs.)<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">More specifically, I really like the possibility of specifying input values to a pattern. You can do that with view patterns – but you can’t
<b>abstract</b> over those view patterns with a pattern synonym (currently). It’d be great to be able to say<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="Code"><span style="mso-fareast-language:EN-US"> pattern P n \ x <- ((\y -> if y>2*n+1 then Just y else Nothing)<o:p></o:p></span></p>
<p class="Code"><span style="mso-fareast-language:EN-US"> -> Just x)<o:p></o:p></span></p>
<p class="Code"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal">This looks pretty impenetrable, but it’s using a view pattern to test if the argument is > 2n+1, and match if so. So ‘n’ is an input, and x is an output.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">I think we have to group all the inputs together and have a separator. No inter-mingling. Remember, we want the distinction to be syntactically obvious; and outputs must be variables while inputs can be arbitrary terms or types.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">As discussed earlier, for inputs, @ distinguishes types from terms; for outputs, @ distinguishes type binder from term binders.<o:p></o:p></p>
<p class="MsoNormal"><o:p> </o:p></p>
<p class="MsoNormal">Simon<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-steering-committee <ghc-steering-committee-bounces@haskell.org>
<b>On Behalf Of </b>Richard Eisenberg<br>
<b>Sent:</b> 27 March 2018 03:47<br>
<b>To:</b> Roman Leshchinskiy <rleshchinskiy@gmail.com><br>
<b>Cc:</b> ghc-steering-committee@haskell.org; Joachim Breitner <mail@joachim-breitner.de><br>
<b>Subject:</b> Re: [ghc-steering-committee] Proposal: Binding existential type variables (#96)<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>
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<p class="MsoNormal">On Mar 26, 2018, at 5:54 PM, Roman Leshchinskiy <<a href="mailto:rleshchinskiy@gmail.com">rleshchinskiy@gmail.com</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"> C <inputs> \ <outputs></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">I had been trying to figure out how to get \ involved, and this is the way. Thanks! It does require that all inputs come before all outputs, which might not be true for constructors:<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">data T a where<o:p></o:p></p>
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<p class="MsoNormal"> MkT :: forall b a. b -> T a<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Here, `b` is an output while `a` would be an input. And in many cases constructors designed with visible type application in mind will want to put existentials ("outputs") before universals ("inputs"). If we ever did the C <inputs> \ <outputs>
in the future, I suppose we could just say that such constructors will have their type variables reordered, perhaps issuing a warning if someone writes a mis-ordered constructor.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">In any case, I'm glad that this discussion has changed your mind about accepting this proposal.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Thanks,<o:p></o:p></p>
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<p class="MsoNormal">Richard<o:p></o:p></p>
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