<div dir="ltr">Hi, <div><br></div><div>I support the proposal.</div><div><br></div><div>Vitaly</div></div><br><div class="gmail_quote"><div dir="ltr" class="gmail_attr">ср, 3 июн. 2020 г. в 12:04, Spiwack, Arnaud <<a href="mailto:arnaud.spiwack@tweag.io">arnaud.spiwack@tweag.io</a>>:<br></div><blockquote class="gmail_quote" style="margin:0px 0px 0px 0.8ex;border-left:1px solid rgb(204,204,204);padding-left:1ex"><div dir="ltr"><div><p style="margin:0px 0px 1.2em">Dear all,</p>
<p style="margin:0px 0px 1.2em">Last time around that we had a discussion about this proposal, the authors asked to have some more time to revise a few things. Since then, the proposal has been simplified, and the paper accepted (congratulations!).</p>
<p style="margin:0px 0px 1.2em">Last time around, those of us who opined were mostly in agreement with accepting the proposal, though there were some distracting complications. Which have since been forked (and accepted) as <a href="https://github.com/ghc-proposals/ghc-proposals/blob/master/proposals/0287-simplify-subsumption.rst" target="_blank">proposal #287</a>. I do believe that this iteration of the proposal is a no-brainer. It simply gives a clear semantics to the unspecified <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">-XImpredicativeTypes</code>. It’s useful, and an implementation already exists (<a href="https://gitlab.haskell.org/ghc/ghc/-/merge_requests/3220" target="_blank">!3220</a>).</p>
<p style="margin:0px 0px 1.2em">The summary of the proposed specification is: to instantiate a type variable to something of the form <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">forall a. …</code>, go through arguments and look for an absolutely necessary such instantiation (which will happen when there is a data constructor: <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">[a] ~ [forall b.t]</code> iif <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">a = forall b.t</code>).</p>
<p style="margin:0px 0px 1.2em">The only point which we may want to discuss is <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">($)</code>, which currently has an ad hoc typing rule baked in the type checker. The proposal suggests removing this baked in rule, and replacing it with making <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">($)</code> always use the <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">-XImpredicativeTypes</code> typing rule. An alternative, of course, is to simply not treat <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">($)</code> any specially, and require <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">-XImpredicativeTypes</code> to be explicitly turned on when one wants <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline">runST $ do …</code>. But, as the proposal points out, this would break a lot of existing programs. So I agree with the proposal’s choice.</p>
<p style="margin:0px 0px 1.2em">In summary, I recommend accepting this proposal in its current state.</p>
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</blockquote></div>