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<p class="MsoNormal"><span style="mso-fareast-language:EN-US">I really am on the fence. Good things:<o:p></o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<ul style="margin-top:0cm" type="disc">
<li class="MsoListParagraph" style="margin-left:2.5pt;mso-list:l0 level1 lfo1"><span style="mso-fareast-language:EN-US">Richard’s first motivating example, where we still need Proxy, is quite convincing.<o:p></o:p></span></li></ul>
<p class="MsoListParagraph" style="margin-left:38.5pt"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<ul style="margin-top:0cm" type="disc">
<li class="MsoListParagraph" style="margin-left:2.5pt;mso-list:l0 level1 lfo1"><span style="mso-fareast-language:EN-US">It fills out an obvious gap, with the right sort of intro/elim duality with visible type application.<o:p></o:p></span></li></ul>
<p class="MsoListParagraph" style="margin-left:38.5pt"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<ul style="margin-top:0cm" type="disc">
<li class="MsoListParagraph" style="margin-left:2.5pt;mso-list:l0 level1 lfo1"><span style="mso-fareast-language:EN-US">And I like that it gives us a
<b>language</b> in which to talk about System F elaboration of the program, if and when we want to. E.g. we can say: if you write<o:p></o:p></span></li></ul>
<p class="MsoListParagraph"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="Code" style="margin-left:72.0pt"><span style="mso-fareast-language:EN-US">f x = Just x<o:p></o:p></span></p>
<p class="MsoListParagraph" style="margin-left:38.5pt"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoListParagraph" style="margin-left:38.5pt"><span style="mso-fareast-language:EN-US">it is as if you had written<o:p></o:p></span></p>
<p class="MsoListParagraph" style="margin-left:38.5pt"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="Code" style="margin-left:72.0pt"><span style="mso-fareast-language:EN-US">f :: forall a. a -> Maybe a<o:p></o:p></span></p>
<p class="Code" style="margin-left:72.0pt"><span style="mso-fareast-language:EN-US">f = \@a \(x::a). Just x<o:p></o:p></span></p>
<p class="MsoListParagraph" style="margin-left:38.5pt"><span style="mso-fareast-language:EN-US"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="mso-fareast-language:EN-US">Less good:<o:p></o:p></span></p>
<ul style="margin-top:0cm" type="disc">
<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level1 lfo3"><span style="mso-fareast-language:EN-US">It’s still incomplete concerning (B) because we can’t talk about dictionary bindings.<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level1 lfo3"><span style="mso-fareast-language:EN-US">It adds more complexity.
<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level1 lfo3"><span style="mso-fareast-language:EN-US">We are not under real pressure to do this now.<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’d like to hear from a broader range of opinion.<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>
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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>Iavor Diatchki<br>
<b>Sent:</b> 03 April 2019 17:46<br>
<b>To:</b> Eric Seidel <eric@seidel.io><br>
<b>Cc:</b> ghc-steering-committee <ghc-steering-committee@haskell.org><br>
<b>Subject:</b> Re: [ghc-steering-committee] Discussion on #155 Type Variable in Labmdas<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">Hello,<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">perhaps it is time to come up with some sort of decision here. Based on the replies to this thread we seem to have the following opinions:<o:p></o:p></p>
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<p class="MsoNormal"> 1. Eric and Richard seem to be quite keen on the feature<o:p></o:p></p>
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<p class="MsoNormal"> 2. Simon is on the fence, but likes it because it introduces System F vocabulary to Haskell<o:p></o:p></p>
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<p class="MsoNormal"> 3. I am skeptical of the proposal as is, as I think it adds additional complexity to the language (more non-orthogonal features) without significant payoff.<o:p></o:p></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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<p class="MsoNormal">Does anyone else have anything else to add?<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"><o:p> </o:p></p>
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<p class="MsoNormal">On Tue, Mar 26, 2019 at 6:48 PM Eric Seidel <<a href="mailto:eric@seidel.io">eric@seidel.io</a>> wrote:<o:p></o:p></p>
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<p class="MsoNormal">On Tue, Mar 26, 2019, at 13:17, Iavor Diatchki wrote:<br>
> My concern is that the notation certainly suggests that you are binding <br>
> types with the @ syntax, but in really it is still the type signature <br>
> that guides the binding of the variables and the @ parameters just <br>
> duplicate the information from the type signature.<br>
<br>
But you are binding types with the @ syntax. The proposal gives a number of examples where the @-bound type variable is bound by a different name (or not at all) in the type signature. Many are contrived, to demonstrate where the binders are allowed, but the
higher-rank and proxy-eliding examples look compelling to me.<br>
<br>
We also already allow repeated value binders in Haskell. When I write a function in equational style, I have to rebind each argument in each alternate equation. Sometimes this is noisy and I'll prefer a single equation with an explicit `case`. But for functions
where the body is sizable, I find the repeated binders to be quite helpful because the scopes are smaller. I can easily see the same benefit applying to type binders. Ultimately, I think this comes down to a matter of style, and I favor letting Haskell programmers
pick the style that works best for them.<br>
<br>
Eric<br>
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