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<p class="MsoNormal"><span style="font-size:12.0pt">[Redirecting to the steering committee]<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">I argue for acceptance.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<ul style="margin-top:0cm" type="disc">
<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level1 lfo1"><span style="font-size:12.0pt">For me a compelling motivation is this: at the moment
<i>we can infer types and kinds that we cannot write down with an explicitly-quantified type signature</i>. That seems all wrong to me: we should be able to write down any type you can infer.<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level1 lfo1"><span style="font-size:12.0pt">The proposal adds
<i>notation</i>. It does <i>not</i> add semantics. We already have the distinction between “inferred” and “specified” type variables. You might not like it, but it’s there in GHC.
<o:p></o:p></span>
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<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level2 lfo1"><span style="font-size:12.0pt">If it wasn’t there already, I’d be happy to debate not adding it.
<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level2 lfo1"><span style="font-size:12.0pt">If you want to argue for removing the distinction, that would certainly render the current proposal moot. But that would take a GHC proposal – and there
are good reasons for the status quo!<o:p></o:p></span></li><li class="MsoListParagraph" style="margin-left:0cm;mso-list:l1 level2 lfo1"><span style="font-size:12.0pt">What is bad is to maintain the semantic distinction, but be unable to express it. That’s what we have right now, and it’s a bad thing.<o:p></o:p></span></li></ul>
</li></ul>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">What is the distinction between “specified” and “inferred”?<o:p></o:p></span></p>
<ul style="margin-top:0cm" type="disc">
<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l2 level1 lfo3"><b><span style="font-size:12.0pt">Specified. f :: forall a. blah, or f :: a -> a.</span></b><span style="font-size:12.0pt">
<o:p></o:p></span></li></ul>
<p class="MsoListParagraph"><span style="font-size:12.0pt">You may give a visible type argument at a call of f, but you do not have to. Thus (f x) or (f @type x).<o:p></o:p></span></p>
<p class="MsoListParagraph"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
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<li class="MsoListParagraph" style="margin-left:0cm;mso-list:l2 level1 lfo3"><b><span style="font-size:12.0pt">Inferred</span></b><span style="font-size:12.0pt">.
<b>f :: t a -> t a.</b> The type of f is really<o:p></o:p></span></li></ul>
<p class="MsoListParagraph" style="margin-left:72.0pt"><span style="font-size:12.0pt">g :: forall {k} (t :: k -> *) (a :: k). t a -> t a<o:p></o:p></span></p>
<p class="MsoNormal" style="margin-left:36.0pt"><span style="font-size:12.0pt">Note that k does not appear at all in the signature; that’s why it is “inferred”. In GHC today you cannot supply an explicit kind argument for g, but you can supply explicit argument
for t and a.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">So by all means make the case for abolishing the distinction, to render the present proposal moot. But I think it’s unreasonably simply to reject on the grounds of “please think of something better”.<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><o:p> </o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt">Simon<o:p></o:p></span></p>
<p class="MsoNormal"><span style="font-size:12.0pt"><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-devs <ghc-devs-bounces@haskell.org>
<b>On Behalf Of </b>Iavor Diatchki<br>
<b>Sent:</b> 30 April 2018 17:12<br>
<b>To:</b> ghc-devs@haskell.org<br>
<b>Subject:</b> Discussion on proposal #99: forall {k}<o:p></o:p></span></p>
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<p class="MsoNormal"><o:p> </o:p></p>
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Hello,<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">
As a shepherd for proposal #99, I'd like to kick off the discussion. The full proposal is available here: <a href="https://github.com/goldfirere/ghc-proposals/blob/explicit-specificity/proposals/0000-explicit-specificity.rst">https://github.com/goldfirere/ghc-proposals/blob/explicit-specificity/proposals/0000-explicit-specificity.rst</a><o:p></o:p></p>
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<o:p> </o:p></p>
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Summary: allows programmers to write `forall {x}` instead of `forall x` in type signatures. The meaning of the braces is that this parameter cannot be instantiated with an explicit type application and will always be inferred. The motivation is to shorten
explicit type applications by skipping parameters that are known to be inferable, the common example being omitting the kinds in signatures with poly kinds.<o:p></o:p></p>
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<o:p> </o:p></p>
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As I understand it, the main motivation for this proposals is to give programmers more flexibility when instantiating type variables, with a less noisy syntax. While the proposed solution might work in some situations, I am unconvinced that it is the best
way to address the issue in general, for the following reasons:<o:p></o:p></p>
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<o:p> </o:p></p>
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1. It requires that programmers commit at declaration time about which arguments will be inferred, and which may be inferred or specified. While in some cases this may be an easy decision to make, in many cases this really is a decision which should be
made at the use site of a function (e.g., I'd like to provide argument X, but would like GHC to infer argument Y).<o:p></o:p></p>
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<o:p> </o:p></p>
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2. It still requires that programmers instantiate arguments in a fixed order, which is sometimes dictated by the structure of the type itslef. Here is, for example, what the proposal suggests to do if you want to provide type before a kind:<o:p></o:p></p>
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<o:p> </o:p></p>
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typeRep4 :: forall {k} (a :: k) k'. (k ~ k', Typeable a) => TypeRep a<o:p></o:p></p>
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<o:p> </o:p></p>
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While technically this is not wrong, it is not exactly elegant.<o:p></o:p></p>
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<o:p> </o:p></p>
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I think that we should reject this proposal, and try to come up with a more comprehensive solution to the problem.<o:p></o:p></p>
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<o:p> </o:p></p>
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One alternative design is to allow programmers to instantiate type variables by name. This is much more flexible as it allows programmers to instantiate whichever variables they want, and in whatever order. We've been doing this in Cryptol for a long time,
and it seems to work really well.<o:p></o:p></p>
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<o:p> </o:p></p>
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-Iavor<o:p></o:p></p>
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