
<html><head><meta http-equiv="Content-Type" content="text/html; charset=utf-8"></head><body style="word-wrap: break-word; -webkit-nbsp-mode: space; line-break: after-white-space;" class="">I'm in strong support. This will be a nice win for GHC.<div class=""><br class=""></div><div class="">Richard<br class=""><div><br class=""><blockquote type="cite" class=""><div class="">On Jun 3, 2020, at 10:03 AM, Spiwack, Arnaud <<a href="mailto:arnaud.spiwack@tweag.io" class="">arnaud.spiwack@tweag.io</a>> wrote:</div><br class="Apple-interchange-newline"><div class=""><div dir="ltr" class=""><div class="markdown-here-wrapper" style=""><p style="margin:0px 0px 1.2em!important" class="">Dear all,</p><p style="margin:0px 0px 1.2em!important" class="">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!important" class="">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" class="">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" class="">-XImpredicativeTypes</code>. It’s useful, and an implementation already exists (<a href="https://gitlab.haskell.org/ghc/ghc/-/merge_requests/3220" class="">!3220</a>).</p><p style="margin:0px 0px 1.2em!important" class="">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" class="">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" class="">[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" class="">a = forall b.t</code>).</p><p style="margin:0px 0px 1.2em!important" class="">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" class="">($)</code>, which currently has an ad hoc typing rule baked in the type checker. The proposal suggests removing this baked in rule, and replacing it with making <code style="font-size:0.85em;font-family:Consolas,Inconsolata,Courier,monospace;margin:0px 0.15em;padding:0px 0.3em;white-space:pre-wrap;border:1px solid rgb(234,234,234);background-color:rgb(248,248,248);border-radius:3px;display:inline" class="">($)</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" class="">-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" class="">($)</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" class="">-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" class="">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!important" class="">In summary, I recommend accepting this proposal in its current state.</p>

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